Einstein College of Engineering

UNIT I
PN DIODE AND ITS APPLICATIONS
INTRODUCTION
The current-voltage characteristics is of prime concern in the study of semiconductor devices
with light entering as a third variable in optoelectronics devices.The external characteristics of
the device is determined by the interplay of the following internal variables:
1. Electron and hole currents
2. Potential
3. Electron and hole density
4. Doping
5. Temperature
Semiconductor equations

The semiconductor equations relating these variables are given below:

Carrier density:


where is the electron quasi Fermi level and is the hole quasi Fermi level. These two
equations lead to

In equilibrium = = Constant

Current:
There are two components of current; electron current density and hole current density .
There are several mechanisms of current flow:
(i) Drift
(ii) Diffusion
(iii) thermionic emission
(iv) tunneling
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The last two mechanisms are important often only at the interface of two different materials such
as a metal-semiconductor junction or a semiconductor-semiconductor junction where the two
semiconductors are of different materials. Tunneling is also important in the case of PN junctions
where both sides are heavily doped.

In the bulk of semiconductor , the dominant conduction mechanisms involve drift and diffusion.

The current densities due to these two mechanisms can be written as

where are electron and hole mobilities respectively and are their diffusion
constants.

Potential:

The potential and electric field within a semiconductor can be defined in the following ways:


All these definitions are equivalent and one or the other may be chosen on the basis of
convenience.The potential is related to the carrier densities by the Poisson equation: -

where the last two terms represent the ionized donor and acceptor density.

Continuity equations

These equations are basically particle conservation equations:
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Where G and R represent carrier generation and recombination rates.Equations (1-8) will form
the basis of most of the device analysis that shall be discussed later on. These equations require
models for mobility and recombination along with models of contacts and boundaries.

Analysis Flow

Like most subjects, the analysis of semiconductor devices is also carried out by starting from
simpler problems and gradually progressing to more complex ones as described below:
(i) Analysis under zero excitation i.e. equilibrium.
(ii) Analysis under constant excitation: in other words dc or static characteristics.
(iii) Analysis under time varying excitation but with quasi-static approximation dynamic
characteristics.
(iv) Analysis under time varying excitation: non quasi-static dynamic characteristics.



Even though there is zero external current and voltage in equilibrium, the situation inside the
device is not so trivial. In general, voltages, charges and drift-diffusion current components at
any given point within the semiconductor may not be zero.


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Equilibrium in semiconductors implies the following:

(i) steady state:
Where Z is any physical quantity such as charge, voltage electric field etc

(ii) no net electrical current and thermal currents:

Since current can be carried by both electrons and holes, equilibrium implies zero values for both
net electron current and net hole current. The drift and diffusion components of electron and hole
currents need not be zero.

(iii) Constant Fermi energy:

The only equations that are relevant (others being zero!) for analysis in equilibrium are:
Poisson Eq:



In equilibrium, there is only one independent variable out of the three variables :
If one of them is known, all the rest can be computed from the equations listed above. We shall
take this independent variable to be potential.

The analysis problem in equilibrium is therefore determination of potential or equivalently,
energy band diagram of the semiconductor device.

This is the reason why we begin discussions of all semiconductor devices with a sketch of its
energy band diagram in equilibrium.


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Energy Band Diagram

This diagram in qualitative form is sketched by following the following procedure:
1. The semiconductor device is imagined to be formed by bringing together the various
distinct semiconductor layers, metals or insulators of which it is composed. The starting
point is therefore the energy band diagram of all the constituent layers.
2. The band diagram of the composite device is sketched using the fact that after
equilibrium, the Fermi energy is the same everywhere in the system. The equalization of
the Fermi energy is accompanied with transfer of electrons from regions of higher Fermi
energy to region of lower Fermi energy and viceversa for holes.
3. The redistribution of charges results in electric field and creation of potential barriers in
the system. These effects however are confined only close to the interface between the
layers. The regions which are far from the interface remain as they were before the
equilibrium

Analysis in equilibrium: Solution of Poissons Equation with appropriate boundary conditions -

Non-equilibrium analysis:
- The electron and hole densities are no longer related together by the inverse relationship
of Eq. (5) but through complex relationships involving all three variables Y , , p
- The three variables are in general independent of each other in the sense that a knowledge
of two of them does not lead automatically to a knowledge of the third.
- The concept of Fermi energy is no longer valid but new quantities called the quasi-Fermi
levels are used and these are not in general constant.
- For static or dc analysis, the continuity equation becomes time independent so that only
ordinary differential equations need to be solved.
- For dynamic analysis however, the partial differential equations have to be solved
increasing the complexity of the analysis.
Analysis of Semiconductor Devices

There are two complementary ways of studying semiconductor devices:

(i) Through numerical simulation of the semiconductor equations.
(ii) Through analytical solution of semiconductor equations.
- There are a variety of techniques used for device simulation with some of them starting
from the drift diffusion formalism outlined earlier, while others take a more fundamental
approach starting from the Boltzmann transport equation instead.
- In general, the numerical approach gives highly accurate results but requires heavy
computational effort also.
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- The output of device simulation in the form of numerical values for all internal variables
requires relatively larger effort to understand and extract important relationships among
the device characteristics.
The electrons in the valence band are not capable of gaining energy from external electric field
and hence do not contribute to the current. This band is never empty but may be partially or
completely with electrons. On the contrary in the conduction band, electrons are rarely present.
But it is possible for electrons to gain energy from external field and so the electrons in these
bands contribute to the electric current. The forbidden energy gap is devoid of any electrons and
this much energy is required by electrons to jump from valence band to the conduction band.
In other words, in the case of conductors and semiconductors, as the temperature increases, the
valence electrons in the valence energy move from the valence band to conductance band. As the
electron (negatively charged) jumps from valence band to conductance band, in the valence band
there is a left out deficiency of electron that is called Hole (positively charged).
Depending on the value of E
gap
, i.e., energy gap solids can be classified as metals (conductors),
insulators and semi conductors.
Semiconductors
- Conductivity in between those of metals and insulators.
- Conductivity can be varied over orders of magnitude by changes in temperature, optical
excitation, and impurity content (doping).
- Generally found in column IV and neighboring columns of the periodic table.
- Elemental semiconductors: Si, Ge.
- Compound semiconductors:

Binary :

GaAs, AlAs, GaP,
etc. (III-V).

ZnS, ZnTe, CdSe (II-
VI).

SiC, SiGe (IV
compounds).
- Ternary : GaAsP.
Quaternary : InGaAsP.
- Si widely used for rectifiers, transistors, and ICs.
- III-V compounds widely used in optoelectronic and high-speed applications.

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Applications
- Integrated circuits (ICs) SSI, MSI, LSI, and VLSI.
- Fluorescent materials used in TV screens II-VI (ZnS).
- Light detectors InSb, CdSe, PbTe, HgCdTe.
- Infrared and nuclear radiation detectors Si and Ge.
- Gunn diode (microwave device) GaAs, InP.
- Semiconductor LEDs GaAs, GaP.
- Semiconductor LASERs GaAs, AlGaAs.
Energy Gap
- Distinguishing feature among metals, insulators, and semiconductors.
- Determines the absorption/emission spectra, the leakage current, and the intrinsic
conductivity.
- Unique value for each semiconductor (e.g. 1.12 eV for Si, 1.42 eV for GaAs) function of
temperature.
Impurities
- Can be added in precisely controlled amounts.
- Can change the electronic and optical properties.
- Used to vary conductivity over wide ranges.
- Can even change conduction process from conduction by negative charge carriers to
positive charge carriers and vice versa.
- Controlled addition of impurities doping.
Energy Bands and Charge Carriers in Semiconductors
Bonding Forces and Energy Bands in Solids
- Electrons are restricted to sets of discrete energy levels within atoms, with large gaps
among them where no energy state is available for the electron to occupy.
- Electrons in solids also are restricted to certain energies and are not allowed at other
energies.
- Difference in the solid, the electron has a range (or band) of available energies.
- The discrete energy levels of the isolated atom spread into bands of energies in the solid
because
i) in the solid, the wave functions of electrons in neighboring atoms overlap, thus, it
affects the potential energy term and the boundary conditions in the
equation, and different energies are obtained in the solution, and
ii) an electron is not necessarily localized at a particular atom.
- The influence of neighboring atoms on the energy levels of a particular atom can be
treated as a small perturbation, giving rise to shifting and splitting of energy states into
energy bands.
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Bonding Forces in Solids
Ionic Bonding
- Example: NaCl.
- Na (Z = 11) gives up its outermost shell electron to Cl (Z=17) atom, thus the crystal is
made up of ions with the electronic structures of the inert atoms Ne and Ar.
- Note: the ions have net electric charges after the electron exchange ion has a net
positive charge, having lost an electron, and ion has a net negative charge, having
acquired an electron.
- Thus, an electrostatic attractive force is established, and the balance is reached when this
equals the net repulsive force.
- Note: all the electrons are tightly bound to the atom.
- Since there are no loosely bound electrons to participate in current flow NaCl is a good
insulator.
Metallic Bonding
- In metals, the outer shell is filled by no more than three electrons (loosely bound and
given up easily) great chemical activity and high electrical conductivity.
- Outer electron(s) contributed to the crystal as a whole solid made up of ions with
closed shells immersed in a sea of free electrons, which are free to move about the crystal
under the influence of an electric field.
- Coulomb attraction force between the ions and the electrons hold the lattice together.
Covalent Bonding
- Exhibited by the diamond lattice semiconductors.
- Each atom surrounded by four nearest neighbors, each having four electrons in the
outermost orbit.
- Each atom shares its valence electrons with its four nearest neighbors.
- Bonding forces arise from a quantum mechanical interaction between the shared
electrons.
- Both electrons belong to each bond, are indistinguishable, and have opposite spins.
- No free electrons available at 0 K, however, by thermal or optical excitation, electrons
can be excited out of a covalent bond and can participate in current conduction
important feature of semiconductors.
Mixed Bonding
- Shown by III-V compounds bonding partly ionic and partly covalent.
- Ionic character of bonding becomes more prominent as the constituent atoms move
further away in the periodic table, e.g., II-VI compounds.

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Energy Bands
- As isolated atoms are brought together to form a solid, the electron wave functions begin
to overlap.
- Various interactions occur, and, at the proper interatomic spacing for the crystal, the
forces of attraction and repulsion find a balance.
- Due to Pauli exclusion principle, the discrete energy levels of individual atoms split into
bands belonging to the pair instead of to individual atoms.
- In a solid, due to large number of atoms, the split energy levels for essentially continuous
bands of energy.


Fig.2.1 Splitting of individual energy levels to energy bands as atoms are brought closer
together.
- Imaginary formation of a diamond crystal from isolated carbon atoms .
- Each atom has two 1s states, two 2s states, six 2p states, and higher states.
- For N atoms, the numbers of states are 2N, 2N, and 6N of type 1s, 2s, and 2p
respectively.
- With a reduction in the interatomic spacing, these energy levels split into bands, and the
2s and 2p bands merge into a single band having 8N available states.
- As the interatomic spacing approaches the equilibrium spacing of diamond crystal, this
band splits into two bands separated by an energy gap , where no allowed energy
states for electrons exist forbidden gap.
- The upper band (called the conduction band) and the lower band (called the valence
band) contain 4N states each.
- The lower 1s band is filled with 2N electrons, however, the 4N electrons residing in the
original n = 2 state will now occupy states either in the valence band or in the conduction
band.
- At 0 K, the electrons will occupy the lowest energy states available to them thus, the
4N states in the valence band will be completely filled, and the 4N states in the
conduction band will be completely empty.
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Metals, Semiconductors, and Insulators
- For electrons to move under an applied electric field, there must be states available to
them.
- A completely filled band cannot contribute to current transport; neither can a completely
empty band.
- Thus, semiconductors at 0 K are perfect insulators.
- With thermal or optical excitation, some of these electrons can be excited from the
valence band to the conduction band, and then they can contribute to the current transport
process.
- At temperatures other than 0 K, the magnitude of the band gap separates an insulator
from a semiconductor, e.g., at 300 K, (diamond) = 5 eV (insulator), and (Silicon) =
1.12 eV (semiconductor).
- Number of electrons available for conduction can be increased greatly in semiconductors
by reasonable amount of thermal or optical energy.
- In metals, the bands are either partially filled or they overlap thus, electrons and
empty states coexist great electrical conductivity.
Direct and Indirect Semiconductors
- In a typical quantitative calculation of band structures, the wave function of a single
electron traveling through a perfectly periodic lattice is assumed to be in the form of a
plane wave moving in the x-direction (say) with propagation constant k, also called a
wave vector.
- In quantum mechanics, the electron momentum can be given by
- The space dependent wave function for the electron is
(2.1)
where the function modulates the wave function according to the periodicity of the
lattice.
- Allowed values of energy, while plotted as a function of k, gives the E-k diagram.
- Since the periodicity of most lattices is different in various directions, the E-k diagram is
a complex surface, which is to be visualized in three dimensions.
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Fig.2.2 Direct and indirect transition of electrons from the conduction band to the valence band:
(a) direct - with accompanying photon emission, (b) indirect via defect level.
- Direct band gap semiconductor: the minima of the conduction band and the maxima of
the valence band occur at the same value of k an electron making the smallest energy
transition from the conduction band to the valence band can do so without a change in k
(and, the momentum).
- Indirect band gap semiconductor: the minima of the conduction band and the maxima of
the valence band occur for different values of k, thus, the smallest energy transition for an
electron requires a change in momentum.
- Electron falling from conduction band to an empty state in valence band
recombination.
- Recombination probability for direct band gap semiconductors is much higher than that
for indirect band gap semiconductors.
- Direct band gap semiconductors give up the energy released during this transition (= )
in the form of light used for optoelectronic applications (e.g., LEDs and LASERs).
- Recombination in indirect band gap semiconductors occurs through some defect states
within the band gap, and the energy is released in the form of heat given to the lattice.
Variation of Energy Bands with Alloy Composition
- The band structures of III-V ternary and quaternary compounds change as their
composition is varied.
- There are three valleys in the conduction band: (at k = 0), L, and X.
- In GaAs, the valley has the minimum energy (direct with = 1.43 eV) with very few
electrons residing in L and X valleys (except for high field excitations).
- In AlAs, the X valley has minimum energy (indirect with = 2.16 eV).
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Fig.2.3 The E-k diagram of (a) GaAs and (b) AlAs, showing the three valleys (L, , and X) in
the conduction band.

Charge Carriers in Semiconductors
- In a metal, the atoms are imbedded in a "sea" of free electrons, and these electrons can
move as a group under the influence of an applied electric field.
- In semiconductors at 0 K, all states in the valence band are full, and all states in the
conduction band are empty.
- At T > 0 K, electrons get thermally excited from the valence band to the conduction band,
and contribute to the conduction process in the conduction band.
- The empty states left in the valence band can also contribute to current conduction.
- Also, introduction of impurities has an important effect on the availability of the charge
carriers.
- Considerable flexibility in controlling the electrical properties of semiconductors.
Electrons and Holes
- For T> 0 K, there would be some electrons in the otherwise empty conduction band, and
some empty states in the otherwise filled valence band.
- The empty states in the valence band are referred to as holes.
- If the conduction band electron and the valence band hole are created by thermal
excitation of a valence band electron to the conduction band, then they are called
electron-hole pair (EHP).
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- After excitation to the conduction band, an electron is surrounded by a large number of
empty states, e.g., the equilibrium number of EHPs at 300 K in Si is , whereas
the Si atom density is .
- Thus, the electrons in the conduction band are free to move about via the many available
empty states.
- Corresponding problem of charge transport in the valence band is slightly more complex.
- Current transport in the valence band can be accounted for by keeping track of the holes
themselves.
- In a filled band, all available energy states are occupied.
- For every electron moving with a given velocity, there is an equal and opposite electron
motion somewhere else in the band.
- Under an applied electric field, the net current is zero, since for every electron j moving
with a velocity , there is a corresponding electron moving with a velocity - .
- In a unit volume, the current density J can be given by

(filled band) (2.2)
where N is the number of in the band, and q is the electronic charge.
- Now, if the electron is removed and a hole is created in the valence band, then the net
current density


- Thus, the current contribution of the empty state (hole), obtained by removing the jth
electron, is equivalent to that of a positively charged particle with velocity .
- Note that actually this transport is accounted for by the motion of the uncompensated
electron having a charge of q and moving with a velocity .
- Its current contribution (- q)(- ) is equivalent to that of a positively charged particle
with velocity + .
- For simplicity, therefore, the empty states in the valence band are called holes, and they
are assigned positive charge and positive mass.
- The electron energy increases as one moves up the conduction band, and electrons
gravitate downward towards the bottom of the conduction band.
- On the other hand, hole energy increases as one moves down the valence band (since
holes have positive charges), and holes gravitate upwards towards the top of the valence
band.


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Effective Mass
- The "wave-particle" motion of electrons in a lattice is not the same as that for a free
electron, because of the interaction with the periodic potential of the lattice.
- To still be able to treat these particles as "free", the rest mass has to be altered to take into
account the influence of the lattice.
- The calculation of effective mass takes into account the shape of the energy bands in
three-dimensional k-space, taking appropriate averages over the various energy bands.
- The effective mass of an electron in a band with a given (E,k) relation is given by
(2.4)


EXAMPLE 2.1: Find the dispersion relation for a free electron, and, thus, observe the relation
between its rest mass and effective mass.

SOLUTION: For a free electron, the electron momentum is . Thus,
. Therefore, the dispersion relation, i.e., the E-k relation is
parabolic. Hence, . This is a very interesting relation, which states that for
a free electron, the rest mass and the effective mass are one and the same, which is due to the
parabolic band structure. Most materials have non-parabolic E-k relation, and, thus, they have
quite different rest mass and effective mass for electrons.

Note: for severely non-parabolic band structures, the effective mass may become a function of
energy, however, near the minima of the conduction band and towards the maxima of the
valence band, the band structure can be taken to be parabolic, and, thus, an effective mass, which
is independent of energy, may be obtained.
- Thus, the effective mass is an inverse function of the curvature of the E-k diagram: weak
curvature gives large mass, and strong curvature gives small mass.
- Note that in general, the effective mass is a tensor quantity, however, for parabolic bands,
it is a constant.
- Another interesting feature is that the curvature is positive at the conduction band
minima, however, it is negative at the valence band maxima.
- Thus, the electrons near the top of the valence band have negative effective mass.
- Valence band electrons with negative charge and negative mass move in an electric field
in the same direction as holes with positive charge and positive mass.
- Thus, the charge transport in the valence band can be fully accounted for by considering
hole motion alone.
- The electron and hole effective masses are denoted by and respectively.
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Intrinsic Material
- A perfect semiconductor crystal with no impurities or lattice defects.
- No carriers at 0 K, since the valence band is completely full and the conduction band is
completely empty.
- For T > 0 K, electrons are thermally excited from the valence band to the conduction
band (EHP generation).
- EHP generation takes place due to breaking of covalent bonds required energy = .
- The excited electron becomes free and leaves behind an empty state (hole).
- Since these carriers are created in pairs, the electron concentration ( ) is always
equal to the hole concentration ( ), and each of these is commonly referred to as the
intrinsic carrier concentration ( ).
- Thus, for intrinsic material n = p = .
- These carriers are not localized in the lattice; instead they spread out over several lattice
spacings, and are given by quantum mechanical probability distributions.
- Note: ni = f(T).
- To maintain a steady-state carrier concentration, the carriers must also recombine at the
same rate at which they are generated.
- Recombination occurs when an electron from the conduction band makes a transition
(direct or indirect) to an empty state in the valence band, thus annihilating the pair.
- At equilibrium, = , where and are the generation and recombination rates
respectively, and both of these are temperature dependent.
- (T) increases with temperature, and a new carrier concentration ni is established, such
that the higher recombination rate (T) just balances generation.
- At any temperature, the rate of recombination is proportional to the equilibrium
concentration of electrons and holes, and can be given by (2.5)
where is a constant of proportionality (depends on the mechanism by which
recombination takes place).
Extrinsic Material
- In addition to thermally generated carriers, it is possible to create carriers in the
semiconductor by purposely introducing impurities into the crystal doping.
- Most common technique for varying the conductivity of semiconductors.
- By doping, the crystal can be made to have predominantly electrons (n-type) or holes (p-
type).
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- When a crystal is doped such that the equilibrium concentrations of electrons (n0) and
holes (p0) are different from the intrinsic carrier concentration (ni), the material is said to
be extrinsic.
- Doping creates additional levels within the band gap.
- In Si, column V elements of the periodic table (e.g., P, As, Sb) introduce energy levels
very near (typically 0.03-0.06 eV) the conduction band.
- At 0 K, these levels are filled with electrons, and very little thermal energy (50 K to 100
K) is required for these electrons to get excited to the conduction band.
- Since these levels donate electrons to the conduction band, they are referred to as the
donor levels.
- Thus, Si doped with donor impurities can have a significant number of electrons in the
conduction band even when the temperature is not sufficiently high enough for the
intrinsic carriers to dominate, i.e., >> , n-type material, with electrons as
majority carriers and holes as minority carriers.
- In Si, column III elements of the periodic table (e.g., B, Al, Ga, In) introduce energy
levels very near (typically 0.03-0.06 eV) the valence band.
- At 0 K, these levels are empty, and very little thermal energy (50 K to 100 K) is required
for electrons in the valence band to get excited to these levels, and leave behind holes in
the valence band.
- Since these levels accept electrons from the valence band, they are referred to as the
acceptor levels.
- Thus, Si doped with acceptor impurities can have a significant number of holes in the
valence band even at a very low temperature, i.e., >> , p-type material, with
holes as majority carriers and electrons as minority carriers.
- The extra electron for column V elements is loosely bound and it can be liberated very
easily ionization; thus, it is free to participate in current conduction.
- Similarly, column III elements create holes in the valence band, and they can also
participate in current conduction.
- Rough calculation of the ionization energy can be made based on the Bohr's model for
atoms, considering the loosely bound electron orbiting around the tightly bound core
electrons. Thus,

(2.6)where is the relative permittivity of Si.


EXAMPLE2.2: Calculate the approximate donor binding energy for Si ( r = 11.7, = 1.18
).

SOLUTION: From Eq.(2.6), we have
= 1.867 x J = 0.117 eV.

Note: The effective mass used here is an average of the effective mass in different
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crystallographic directions, and is called the "conductivity effective mass" with values of 1.28
(at 600 K), 1.18 (at 300 K), 1.08 (at 77 K), and 1.026 (at 4.2 K).
- In III-V compounds, column VI impurities (e.g., S, Se, Te) occupying column V sites act
as donors. Similarly, column II impurities (e.g., Be, Zn, Cd) occupying column III sites
act as acceptors.
- When a column IV material (e.g., Si, Ge) is used to dope III-V compounds, then they
may substitute column III elements (and act as donors), or substitute column V elements
(and act as acceptors) amphoteric dopants.
- Doping creates a large change in the electrical conductivity, e.g., with a doping of
, the resistivity of Si changes from 2 x -cm to 5 -cm.


Carrier Concentrations
- For the calculation of semiconductor electrical properties and analyzing device behavior,
it is necessary to know the number of charge carriers/cm3 in the material.
- The majority carrier concentration in a heavily doped material is obvious, since for each
impurity atom, one majority carrier is obtained.
- However, the minority carrier concentration and the dependence of carrier concentrations
on temperature are not obvious.
- To obtain the carrier concentrations, their distribution over the available energy states is
required.
- These distributions are calculated using statistical methods.
The Fermi Level
- Electrons in solids obey Fermi-Dirac (FD) statistics.
- This statistics accounts for the indistinguishability of the electrons, their wave nature, and
the Pauli exclusion principle.
- The Fermi-Dirac distribution function f(E) of electrons over a range of allowed energy
levels at thermal equilibrium can be given by

(2.7)where k is Boltzmann's constant (= 8.62 x eV/K = 1.38 x
J/K).
- This gives the probability that an available energy state at E will be occupied by an
electron at an absolute temperature T.
- is called the Fermi level and is a measure of the average energy of the electrons in the
lattice an extremely important quantity for analysis of device behavior.
- Note: for (E - ) > 3kT (known as Boltzmann approximation), f(E) exp[- (E- )/kT]
this is referred to as the Maxwell-Boltzmann (MB) distribution (followed by gas
atoms).
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- The probability that an energy state at will be occupied by an electron is 1/2 at all
temperatures.
- At 0 K, the distribution takes a simple rectangular form, with all states below
occupied, and all states above empty.
- At T > 0 K, there is a finite probability of states above to be occupied and states
below to be empty.
- The F-D distribution function is highly symmetric, i.e., the probability f( + ) that a
state E above is filled is the same as the probability [1- f( - )] that a state E
below is empty.
- This symmetry about EF makes the Fermi level a natural reference point for the
calculation of electron and hole concentrations in the semiconductor.
- Note: f(E) is the probability of occupancy of an available state at energy E, thus, if there
is no available state at E (e.g., within the band gap of a semiconductor), there is no
possibility of finding an electron there.
- For intrinsic materials, the Fermi level lies close to the middle of the band gap (the
difference between the effective masses of electrons and holes accounts for this small
deviation from the mid gap).
- In n-type material, the electrons in the conduction band outnumber the holes in the
valence band, thus, the Fermi level lies closer to the conduction band.
- Similarly, in p-type material, the holes in the valence band outnumber the electrons in the
conduction band, thus, the Fermi level lies closer to the valence band.
- The probability of occupation f(E) in the conduction band and the probability of vacancy
[1- f(E)] in the valence band are quite small, however, the densities of available states in
these bands are very large, thus a small change in f(E) can cause large changes in the
carrier concentrations.


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Fig.2.4 The density of states N(E), the Fermi-Dirac distribution function f(E), and the carrier
concentration as functions of energy for (a) intrinsic, (b) n-type, and (c) p-type semiconductors at
thermal equilibrium.
- Note: since the function f(E) is symmetrical about , a large electron concentration
implies a small hole concentration, and vice versa.
- In n-type material, the electron concentration in the conduction band increases as
moves closer to ; thus, ( - ) gives a measure of n.
- Similarly, in p-type material, the hole concentration in the valence band increases as
moves closer to ; thus, ( - ) gives a measure of p.

Electron and Hole Concentrations at Equilibrium
- The F-D distribution function can be used to calculate the electron and hole
concentrations in semiconductors, if the densities of available states in the conduction
and valence bands are known.
- In equilibrium, the concentration of electrons in the conduction band can be given by

(2.8)

where N(E)dE is the density of available states/cm3 in the energy range dE.
- Note: the upper limit of is theoretically not proper, since the conduction band does not
extend to infinite energies; however, since f(E) decreases rapidly with increasing E, the
contribution to this integral for higher energies is negligible.
- Using the solution of 's wave equation under periodic boundary conditions, it
can be shown that

(2.9)
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- Thus, N(E) increases with E, however, f(E) decreases rapidly with E, thus, the product
f(E)N(E) decreases rapidly with E, and very few electrons occupy states far above the
conduction band edge, i.e., most electrons occupy a narrow energy band near the
conduction band edge.
- Similarly, the probability of finding an empty state in the valence band [1 - f(E)]
decreases rapidly below , and most holes occupy states near the top of the valence
band.
- Thus, a mathematical simplification can be made assuming that all available states in the
conduction band can be represented by an effective density of states NC located at the
conduction band edge and using Boltzmann approximation.

Thus, (2.10)

where .
- Note: as ( - ) decreases, i.e., the Fermi level moves closer to the conduction band,
the electron concentration increases.
- By similar arguments,

(2.11)

where is the effective density of states located at the valence band edge .
- Note: the only terms separating the expressions for and are the effective masses of
electrons ( ) and holes ( ) respectively, and since , hence, .
- Thus, as ( - ) decreases, i.e., the Fermi level moves closer to the valence band
edge, and the hole concentration increases.
- These equations for and are valid in equilibrium, irrespective of the material being
intrinsic or doped.
- For intrinsic material lies at an intrinsic level (very near the middle of the band
gap), and the intrinsic electron and hole concentrations are given by
and (2.12)
- Note: At equilibrium, the product is a constant for a particular material and
temperature, even though the doping is varied,

i.e., (2.13)
- This equation gives an expression for the intrinsic carrier concentration ni as a function of
, , and temperature:

(2.14)
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- These relations are extremely important, and are frequently used for calculations.
- Note: if were to be equal to , then would have been exactly at mid gap (i.e., -
= - = /2).
- However, since , is displaced slightly from mid gap (more for GaAs than that
for Si).
- Alternate expressions for and :
and (2.15)
- Note: the electron concentration is equal to ni when is at , and n0 increases
exponentially as moves away from towards the conduction band.
- Similarly, the hole concentration varies from to larger values as moves from
towards the valence band.


EXAMPLE 2.3: A Si sample is doped with B . What is the equilibrium electron
concentration n0 at 300 K? Where is relative to ? Assume for Si at 300 K = 1.5 x


SOLUTION: Since B (trivalent) is a p-type dopant in Si, hence, the material will be
predominantly p-type, and since >> , therefore, will be approximately equal to , and
= . Also,
. The resulting band diagram is:



Temperature Dependence of Carrier Concentrations
- The intrinsic carrier concentration has a strong temperature dependence, given by
(2.16)
- Thus, explicitly, ni is proportional to T3/2 and to e 1/T, however, Eg also has a
temperature dependence (decreasing with increasing temperature, since the interatomic
spacing changes with temperature).
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Fig.2.5 The intrinsic carrier concentration as a function of inverse temperature for Si, Ge,
and GaAs.
- As changes with temperature, so do and .
- With and T given, the unknowns are the carrier concentrations and the Fermi level
position with respect to one of these quantities must be given in order to calculate the
other.
- Example: Si doped with donors ( ).
- At very low temperature, negligible intrinsic EHPs exist, and all the donor electrons are
bound to the donor atoms.
- As temperature is raised, these electrons are gradually donated to the conduction band,
and at about 100 K (1000/T = 10), almost all these electrons are donated this
temperature range is called the ionization region.
Once all the donor atoms are ionized, the electron concentration , since for each donor
atom, one electron is obtained.

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Fig.2.6 Variation of carrier concentration with inverse temperature clearly showing the three
regions: ionization, extrinsic, and intrinsic.
- Thus, remains virtually constant with temperature for a wide range of temperature
(called the extrinsic region), until the intrinsic carrier concentration ni starts to become
comparable to .
- For high temperatures, >> , and the material loses its extrinsic property (called the
intrinsic region).
- Note: in the intrinsic region, the device loses its usefulness => determines the maximum
operable temperature range.
Compensation and Space Charge Neutrality
- Semiconductors can be doped with both donors ( ) and acceptors ( ) simultaneously.
- Assume a material doped with > predominantly n-type lies above
acceptor level Ea completely full, however, with above , the hole concentration
cannot be equal to .
- Mechanism:
o Electrons are donated to the conduction band from the donor level
o An acceptor state gets filled by a valence band electron, thus creating a hole in the
valence band.
o An electron from the conduction band recombines with this hole.
o Extending this logic, it is expected that the resultant concentration of electrons in
the conduction band would be instead of .
o This process is called compensation.
- By compensation, an n-type material can be made intrinsic (by making = ) or even
p-type (for > ).

Note: a semiconductor is neutral to start with, and, even after doping, it remains neutral
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(since for all donated electrons, there are positively charged ions ( ); and for all
accepted electrons (or holes in the valence band), there are negatively charged ions ( ).
- Therefore, the sum of positive charges must equal the sum of negative charges, and this
governing relation,
given by (2.17) is referred to as the equation for space charge neutrality.
- This equation, solved simultaneously with the law of mass action (given by )
gives the information about the carrier concentrations.
Note: for ,
Drift of Carriers in Electric and Magnetic Fields
- In addition to the knowledge of carrier concentrations, the collisions of the charge
carriers with the lattice and with the impurity atoms (or ions) under electric and/or
magnetic fields must be accounted for, in order to compute the current flow through the
device.
- These processes will affect the ease (mobility) with which carriers move within a lattice.
- These collision and scattering processes depend on temperature, which affects the
thermal motion of the lattice atoms and the velocity of the carriers.
Conductivity and Mobility
- Even at thermal equilibrium, the carriers are in a constant motion within the lattice.
- At room temperature, the thermal motion of an individual electron may be visualized as
random scattering from lattice atoms, impurities, other electrons, and defects.
- There is no net motion of the group of n electrons/cm3 over any period of
time, since the scattering is random, and there is no preferred direction of motion for the
group of electrons and no net current flow.
- However, for an individual electron, this is not true the probability of an electron
returning to its starting point after time t is negligibly small.
- Now, if an electric field is applied in the x-direction, each electron experiences a net
force q from the field.
- This will create a net motion of group in the x-direction, even though the force may be
insufficient to appreciably alter the random path of an individual electron.
- If is the x-component of the total momentum of the group, then the force of the field on
the n is
(2.18)

Note: this expression indicates a constant acceleration in the x-direction, which
realistically cannot happen.
- In steady state, this acceleration is just balanced by the deceleration due to the collisions.
- Thus, while the steady field does produce a net momentum , for steady state
current flow, the net rate of change of momentum must be zero when collisions are
included.
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- Note: the collision processes are totally random, thus, there is a constant probability of
collision at any time for each electron.
- Consider a group of electrons at time t = 0, and define N(t) as the number of electrons
that have not undergone a collision by time t


Fig.2.7 The random thermal motion of an individual electron, undergoing random
scattering.
- The rate of decrease of N(t) at any time t is proportional to the number left unscattered at
t, i.e.

(2.19)

where is the constant of proportionality.
- The solution is an exponential function
(2.20)

and represents the mean time between scattering events, called the mean free time.
- The probability that any electron has a collision in time interval dt is dt/ , thus, the
differential change in due to collisions in time dt is
(2.21)
- Thus, the rate of change of due to the decelerating effect of collisions is
(2.22)
- For steady state, the sum of acceleration and deceleration effects must be zero, thus,
(2.23)
- The average momentum per electron (averaged over the entire group of electrons) is
(2.24)
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- Thus, as expected for steady state, the electrons would have on the average a constant net
velocity in the -x-direction
(2.25)
- This speed is referred to as the drift speed, and, in general, it is usually much smaller than
the random speed due to thermal motion .
- The current density resulting from this drift
(2.26)
- This is the familiar Ohm's law with being the conductivity of the sample, which
can also be written as , with is defined as the electron mobility
(in ), and it describes the ease with which electrons drift in the material.

- The mobility can also be expressed as the average drift velocity per unit electric field,
thus with the negative sign denoting a positive value for mobility since
electrons drift opposite to the direction of the electric field.
- The total current density can be given by (2.27) when both electrons
and holes contribute to the current conduction; on the other hand, for predominantly n-
type or p-type samples, respectively the first or the second term of the above equation
dominates.

Note: both electron and hole drift currents are in the same direction, since holes (with
positive charges) move along the direction of the electric field, and electrons (with
negative charges) drift opposite to the direction of the electric field.
- Since GaAs has a strong curvature of the E-k diagram at the bottom of the conduction
band, the electron effective mass in GaAs is very small the electron mobility in GaAs
is very high since is inversely proportional to .
- The other parameter in the mobility expression, i.e., (the mean free time between
collisions) is a function of temperature and the impurity concentration in the
semiconductor.
- For a uniformly doped semiconductor bar of length L, width w, and thickness t, the
resistance R of the bar can be given by where is the resistivity.

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Effects of Temperature and Doping on Mobility
- The two main scattering events that influence electron and hole motion (and, thus,
mobility) are the lattice scattering and the impurity scattering.
- All lattice atoms vibrate due to temperature and can scatter carriers due to collisions.
- These collective vibrations are called phonons, thus lattice scattering is also known as
phonon scattering.
- With increasing temperature, lattice vibrations increase, and the mean free time between
collisions decreases mobility decreases (typical dependence ).
- Scattering from crystal defects and ionized impurities dominate at low temperatures.
- Since carriers moving with low velocity (at low temperature) can get scattered more
easily by ionized impurities, this kind of scattering causes a decrease in carrier mobility
with decreasing temperature (typical dependence ).
- Note: the scattering probability is inversely proportional to the mean free time (and to
mobility), hence, the mobilities due to two or more scattering events add inversely:
(2.28)
- Thus, the mechanism causing the lowest mobility value dominates.
- Mobility also decreases with increasing doping, since the ionized impurities scatter
carriers more (e.g., for intrinsic Si is 1350 at 300 K, whereas with a donor
doping of , n drops to 700 ).
High Field Effects
- For small electric fields, the drift current increases linearly with the electric field, since
is a constant.
- However, for large electric fields (typically > ), the current starts to show a
sublinear dependence on the electric field and eventually saturates for very high fields.
- Thus, becomes a function of the electric field, and this is known as the hot carrier effect,
when the carrier drift velocity becomes comparable to its thermal velocity.
- The maximum carrier drift velocity is limited to its mean thermal velocity (typically
), beyond which the added energy imparted by the electric field is absorbed by
the lattice (thus generating heat) instead of a corresponding increase in the drift velocity.
2.4.4 The Hall Effect
- An extremely important measurement procedure for determining the majority carrier
concentration and mobility.
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Fig.2.8 The experimental setup for the Hall Effect measurement.
- If a magnetic field is applied perpendicular to the direction of carrier flow, the path of the
carriers get deflected due to the Lorentz force experienced by the carriers, which can be
given by
F = q(E + v x B) (2.29)
- Thus, the holes will get deflected towards the -y-direction, and establish an electric field
along the y-direction, such that in steady state
- The establishment of this electric field is known as the Hall effect, and the resulting
voltage is called the Hall voltage.
- Using the expression for the drift current, is called the
Hall coefficient.
- A measurement of the Hall voltage along with the information for magnetic field and
current density gives the majority carrier concentration
- Also, the majority carrier mobility can be obtained from a measurement of
the resistivity
- This experiment can be performed to obtain the variation of majority carrier
concentration and mobility as a function of temperature.
- For n-type samples, the Hall voltage and the Hall coefficient are negative a common
diagnostic tool for obtaining the sample type.
- Note: caution should be exercised for near intrinsic samples.


EXAMPLE 2.4: A sample of Si is doped with In . What will be the measured
value of its resistivity? What is the expected Hall voltage in a 150 m thick sample if
?

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SOLUTION:



Equilibrium Condition
- In equilibrium, there is no external excitation except a constant temperature, no net
transfer of energy, no net carrier motion, and no net current transport.
- An important condition for equilibrium is that no discontinuity or gradient can arise in the
equilibrium Fermi level EF.
- Assume two materials 1 and 2 (e.g., n- and p-type regions, dissimilar semiconductors,
metal and semiconductor, two adjacent regions in a nonuniformly doped semiconductor)
in intimate contact such that electron can move between them.
- Assume materials 1 and 2 have densities of state N1(E) and N2(E), and F-D distribution
functions f1(E) and f2(E) respectively at any energy E.


- The rate of electron motion from 1 to 2 can be given byrate from 1 to 2 N1(E)f1(E) .
N2(E)[1 f2(E)] (2.30)and the rate of electron motion from 2 to 1 can be given byrate from
2 to 1 N2(E)f2(E) . N1(E)[1 f1(E)] (2.31)" At equilibrium, these two rates must be equal,
which gives f1(E) = f2(E) => EF1 = EF2 => dEF/dx = 0; thus, the Fermi level is constant
at equilibrium, or, in other words, there cannot be any discontinuity or gradient in the
Fermi level at equilibrium.

Practice Problems
2.1 Electrons move in a crystal as wave packets with a group velocity where
is the angular frequency. Show that in a given electric field, these wave packets obey
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Newton's second law of motion, i.e., the force F = m*a, where m* is the effective mass
and a is the acceleration.
2.2 Some semiconductors of interest have the dependence of its energy E with respect to
the wave vector k, given by is the effective mass for E =
0, k is the wave vector, and is a constant. Calculate the dependence of the effective
mass on energy.
2.3 Determine the equilibrium recombination constant r for Si and GaAs, having
equilibrium thermal generation rates of
respectively, and intrinsic carrier concentrations of
respectively. Comment on the answers. Will change with doping at equilibrium?
2.4 The relative dielectric constant for GaP is 10.2 and the electron effective mass is
Calculate the approximate ionization energy of a donor atom in GaP.
2.5 Show that the probability that a state above the Fermi level is occupied is the
same as the probability that a state below is empty.
2.6 Derive an expression relating the intrinsic level to the center of the band gap
and compute the magnitude of this displacement for Si and GaAs at 300 K. Assume
respectively.
2.7 Show that in order to obtain maximum resistivity in a GaAs sample
it has to be
doped slightly p-type. Determine this doping concentration. Also, determine the ratio of
the maximum resistivity to the intrinsic resistivity.
2.8 A GaAs sample (use the date given in Problem 2.7) is doped uniformly with
out of which 70% occupy Ga sites, and the rest 30% occupy As sites.
Assume 100% ionization and T = 300 K.
a) Calculate the equilibrium electron and hole concentrations
b) Clearly draw the equilibrium band diagram, showing the position of the Fermi level
with respect to the intrinsic level , assuming that lies exactly at midgap.
c) Calculate the percentage change in conductivity after doping as compared to the
intrinsic case.
2.9 A Si sample is doped with donor atoms. Determine the minimum
temperature at which the sample becomes intrinsic. Assume that at this minimum
temperature, the free electron concentration does not exceed by more than 1% of the
donor concentration (beyond its extrinsic value). For

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2.10 Since the event of collision of an electron in a lattice is a truly random process, thus
having a constant probability of collision at any given time, the number of particles left
unscattered at time t,
Hence, show that if there are a total of i number of scattering events, each with a mean
free time of then the net electron mobility can be given by where
is the mobility due to the ith scattering event.
2.11 A Ge sample is oriented in a magnetic field (refer to
Fig.2.8). The current is 4 mA, and the sample dimensions are w = 0.25 mm,
t = 50 m, and L = 2.5 mm. The following data are taken:
Find the type and concentration of the majority
carrier, and its mobility. Hence, compute the net relaxation time for the various scattering
events, assuming
2.12 In the Hall effect experiment, there is a chance that the Hall Probes A and B (refer to
Fig.2.8) are not perfectly aligned, which may give erroneous Hall voltage readings. Show
that the true Hall voltage can be obtained from two measurements of with the
magnetic field first in the +z-direction, and then in the z-direction.


Metals

or

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In metals, either the conduction band is partially filled or overlaps with valence band. There is no
forbidden energy gap in between. Even if a small electric field is applied, free electrons start
moving in a direction opposite to field and hence a good conductor of electricity.
Insulators
Here the valency bands are completely filled and conduction band is empty and the forbidden
gap is quite large. For example in diamond E
gap
is 6eV. Even if an electric field is applied, no
electron is able to go from valence band to conduction band.
Semiconductors
The valence band is completely filled and conduction band is empty. The E
gap
is also less i.e., of
the order of few eV. At zero kelvin, electrons are not able to cross this forbidden gap and so
behave like insulators. But as temperature is increased, electrons in valence band (VB) gain
thermal energy and jump to conduction band (CB) and acquire small conductivity at room
temperature and so behave like conductors. Hence they are called semiconductors.

Charge carriers in semiconductors
At high temperature, electrons move from valance band to conduction band and as a result a
vacancy is created in the valence band at a place where an electron was present before shifting to
conduction band. The valency is a hole and is seat of positive charge having the same value of
electron. Therefore the electrical conduction in semiconductors is due to motion of electrons in
conduction band and also due to motion of holes in valence band.


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Semiconductor Basics
If Resistors are the most basic passive component in electrical or electronic circuits, then we
have to consider the Signal Diode as being the most basic "Active" component. However, unlike
a resistor, a diode does not behave linearly with respect to the applied voltage as it has an
exponential I-V relationship and therefore can not be described simply by using Ohm's law as we
do for resistors. Diodes are unidirectional semiconductor devices that will only allow current to
flow through them in one direction only, acting more like a one way electrical valve, (Forward
Biased Condition). But, before we have a look at how signal or power diodes work we first need
to understand their basic construction and concept.
Diodes are made from a single piece of Semiconductor material which has a positive "P-region"
at one end and a negative "N-region" at the other, and which has a resistivity value somewhere
between that of a conductor and an insulator. But what is a "Semiconductor" material?, firstly
let's look at what makes something either a Conductor or an Insulator.
Resistivity
The electrical Resistance of an electrical or electronic component or device is generally defined
as being the ratio of the voltage difference across it to the current flowing through it, basic
Ohms Law principals. The problem with using resistance as a measurement is that it depends
very much on the physical size of the material being measured as well as the material out of
which it is made. For example, If we were to increase the length of the material (making it
longer) its resistance would also increase. Likewise, if we increased its diameter (making it
fatter) its resistance would then decrease. So we want to be able to define the material in such a
way as to indicate its ability to either conduct or oppose the flow of electrical current through it
no matter what its size or shape happens to be. The quantity that is used to indicate this specific
resistance is called Resistivity and is given the Greek symbol of , (Rho). Resistivity is
measured in Ohm-metres, ( -m ) and is the inverse to conductivity.
If the resistivity of various materials is compared, they can be classified into three main groups,
Conductors, Insulators and Semi-conductors as shown below.




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Resistivity Chart


Notice also that there is a very small
margin between the resistivity of the
conductors such as silver and gold,
compared to a much larger margin for
the resistivity of the insulators
between glass and quartz. The
resistivity of all the materials at any
one time also depends upon their
temperature.
Conductors
From above we now know that Conductors are materials that have a low value of resistivity
allowing them to easily pass an electrical current due to there being plenty of free electrons
floating about within their basic atom structure. When a positive voltage potential is applied to
the material these "free electrons" leave their parent atom and travel together through the
material forming an electron drift. Examples of good conductors are generally metals such as
Copper, Aluminium, Silver or non metals such as Carbon because these materials have very few
electrons in their outer "Valence Shell" or ring, resulting in them being easily knocked out of the
atom's orbit. This allows them to flow freely through the material until they join up with other
atoms, producing a "Domino Effect" through the material thereby creating an electrical current.
Generally speaking, most metals are good conductors of electricity, as they have very small
resistance values, usually in the region of micro-ohms per metre with the resistivity of
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conductors increasing with temperature because metals are also generally good conductors of
heat.
Insulators
Insulators on the other hand are the exact opposite of conductors. They are made of materials,
generally non-metals, that have very few or no "free electrons" floating about within their basic
atom structure because the electrons in the outer valence shell are strongly attracted by the
positively charged inner nucleus. So if a potential voltage is applied to the material no current
will flow as there are no electrons to move and which gives these materials their insulating
properties. Insulators also have very high resistances, millions of ohms per metre, and are
generally not affected by normal temperature changes (although at very high temperatures wood
becomes charcoal and changes from an insulator to a conductor). Examples of good insulators
are marble, fused quartz, p.v.c. plastics, rubber etc.
Insulators play a very important role within electrical and electronic circuits, because without
them electrical circuits would short together and not work. For example, insulators made of glass
or porcelain are used for insulating and supporting overhead transmission cables while epoxy-
glass resin materials are used to make printed circuit boards, PCB's etc.
Semiconductor Basics
Semiconductors materials such as silicon (Si), germanium (Ge) and gallium arsenide (GaAs),
have electrical properties somewhere in the middle, between those of a "conductor" and an
"insulator". They are not good conductors nor good insulators (hence their name "semi"-
conductors). They have very few "fee electrons" because their atoms are closely grouped
together in a crystalline pattern called a "crystal lattice". However, their ability to conduct
electricity can be greatly improved by adding certain "impurities" to this crystalline structure
thereby, producing more free electrons than holes or vice versa. By controlling the amount of
impurities added to the semiconductor material it is possible to control its conductivity. These
impurities are called donors or acceptors depending on whether they produce electrons or holes.
This process of adding impurity atoms to semiconductor atoms (the order of 1 impurity atom per
10 million (or more) atoms of the semiconductor) is called Doping.
The most commonly used semiconductor material by far is silicon. It has four valence electrons
in its outer most shell which it shares with its adjacent atoms in forming covalent bonds. The
structure of the bond between two silicon atoms is such that each atom shares one electron with
its neighbour making the bond very stable. As there are very few free electrons available to move
from place to place producing an electrical current, crystals of pure silicon (or germanium) are
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therefore good insulators, or at the very least very high value resistors. Silicon atoms are
arranged in a definite symmetrical pattern making them a crystalline solid structure. A crystal of
pure silicon (silicon dioxide or glass) is generally said to be an intrinsic crystal (it has no
impurities).


The diagram above shows the structure and lattice of a 'normal' pure crystal
of Silicon.
N-type Semiconductor Basics
In order for our silicon crystal to conduct electricity, we need to introduce an impurity atom such
as Arsenic, Antimony or Phosphorus into the crystalline structure making it extrinsic (impurities
are added). These atoms have five outer electrons in their outermost co-valent bond to share with
other atoms and are commonly called "Pentavalent" impurities. This allows four of the five
electrons to bond with its neighbouring silicon atoms leaving one "free electron" to move about
when an electrical voltage is applied (electron flow). As each impurity atom "donates" one
electron, pentavalent atoms are generally known as "donors".
Antimony (symbol Sb) is frequently used as a pentavalent additive as it has 51 electrons
arranged in 5 shells around the nucleus. The resulting semiconductor material has an excess of
current-carrying electrons, each with a negative charge, and is therefore referred to as "N-type"
material with the electrons called "Majority Carriers" and the resultant holes "Minority Carriers".
Then a semiconductor material is N-type when its donor density is greater than its acceptor
density. Therefore, a N-type semiconductor has more electrons than holes.
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The diagram above shows the structure and lattice of the donor impurity atom
Antimony.
P-Type Semiconductor Basics
If we go the other way, and introduce a "Trivalent" (3-electron) impurity into the crystal
structure, such as Aluminium, Boron or Indium, only three valence electrons are available in the
outermost covalent bond meaning that the fourth bond cannot be formed. Therefore, a complete
connection is not possible, giving the semiconductor material an abundance of positively charged
carriers known as "holes" in the structure of the crystal. As there is a hole an adjoining free
electron is attracted to it and will try to move into the hole to fill it. However, the electron filling
the hole leaves another hole behind it as it moves. This in turn attracts another electron which in
turn creates another hole behind, and so forth giving the appearance that the holes are moving as
a positive charge through the crystal structure (conventional current flow). As each impurity
atom generates a hole, trivalent impurities are generally known as "Acceptors" as they are
continually "accepting" extra electrons.
Boron (symbol B) is frequently used as a trivalent additive as it has only 5 electrons arranged in
3 shells around the nucleus. Addition of Boron causes conduction to consist mainly of positive
charge carriers results in a "P-type" material and the positive holes are called "Majority Carriers"
while the free electrons are called "Minority Carriers". Then a semiconductors is P-type when its
acceptor density is greater than its donor density. Therefore, a P-type semiconductor has more
holes than electrons.
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The diagram above shows the structure and lattice of the acceptor impurity
atom Boron.
Semiconductor Basics Summary
N-type (e.g. add Antimony)
These are materials which have Pentavalent impurity atoms (Donors) added and conduct by
"electron" movement and are called, N-type Semiconductors.
In these types of materials are:
- 1. The Donors are positively charged.
- 2. There are a large number of free electrons.
- 3. A small number of holes in relation to the number of free electrons.
- 4. Doping gives:
o positively charged donors.
o negatively charged free electrons.
- 5. Supply of energy gives:
o negatively charged free electrons.
o positively charged holes.
P-type (e.g. add Boron)
These are materials which have Trivalent impurity atoms (Acceptors) added and conduct by
"hole" movement and are called, P-type Semiconductors.
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In these types of materials are:
- 1. The Acceptors are negatively charged.
- 2. There are a large number of holes.
- 3. A small number of free electrons in relation to the number of holes.
- 4. Doping gives:
o negatively charged acceptors.
o positively charged holes.
- 5. Supply of energy gives:
o positively charged holes.
o negatively charged free electrons.
and both P and N-types as a whole, are electrically neutral.
In the next tutorial about semiconductors and diodes, we will look at joining the two
semiconductor materials, the P-type and the N-type materials to form a PN Junction which can
be used to produce diodes.
Pn junction
The PN junction
In the previous tutorial we saw how to make an N-type semiconductor material by doping it with
Antimony and also how to make a P-type semiconductor material by doping that with Boron.
This is all well and good, but these semiconductor N and P-type materials do very little on their
own as they are electrically neutral, but when we join (or fuse) them together these two materials
behave in a very different way producing what is generally known as a PN Junction.
When the N and P-type semiconductor materials are first joined together a very large density
gradient exists between both sides of the junction so some of the free electrons from the donor
impurity atoms begin to migrate across this newly formed junction to fill up the holes in the P-
type material producing negative ions. However, because the electrons have moved across the
junction from the N-type silicon to the P-type silicon, they leave behind positively charged donor
ions (N
D
) on the negative side and now the holes from the acceptor impurity migrate across the
junction in the opposite direction into the region were there are large numbers of free electrons.
As a result, the charge density of the P-type along the junction is filled with negatively charged
acceptor ions (N
A
), and the charge density of the N-type along the junction becomes positive.
This charge transfer of electrons and holes across the junction is known as diffusion.
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This process continues back and forth until the number of electrons which have crossed the
junction have a large enough electrical charge to repel or prevent any more carriers from
crossing the junction. The regions on both sides of the junction become depleted of any free
carriers in comparison to the N and P type materials away from the junction. Eventually a state
of equilibrium (electrically neutral situation) will occur producing a "potential barrier" zone
around the area of the junction as the donor atoms repel the holes and the acceptor atoms repel
the electrons. Since no free charge carriers can rest in a position where there is a potential barrier
the regions on both sides of the junction become depleted of any more free carriers in
comparison to the N and P type materials away from the junction. This area around the junction
is now called the Depletion Layer.
The PN junction


The total charge on each side of the junction must be equal and opposite to maintain a neutral
charge condition around the junction. If the depletion layer region has a distance D, it therefore
must therefore penetrate into the silicon by a distance of Dp for the positive side, and a distance
of Dn for the negative side giving a relationship between the two of Dp.N
A
= Dn.N
D
in order to
maintain charge neutrality also called equilibrium.
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PN junction Distance

As the N-type material has lost electrons and the P-type has lost holes, the N-type material has
become positive with respect to the P-type. Then the presence of impurity ions on both sides of
the junction cause an electric field to be established across this region with the N-side at a
positive voltage relative to the P-side. The problem now is that a free charge requires some extra
energy to overcome the barrier that now exists for it to be able to cross the depletion region
junction.

This electric field created by the diffusion process has created a "built-in potential difference"
across the junction with an open-circuit (zero bias) potential of:

Where: E
o
is the zero bias junction voltage, V
T
the thermal voltage of 26mV at room
temperature, N
D
and N
A
are the impurity concentrations and n
i
is the intrinsic concentration.
A suitable positive voltage (forward bias) applied between the two ends of the PN junction can
supply the free electrons and holes with the extra energy. The external voltage required to
overcome this potential barrier that now exists is very much dependent upon the type of
semiconductor material used and its actual temperature. Typically at room temperature the
voltage across the depletion layer for silicon is about 0.6 - 0.7 volts and for germanium is about
0.3 - 0.35 volts. This potential barrier will always exist even if the device is not connected to any
external power source.
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The significance of this built-in potential across the junction, is that it opposes both the flow of
holes and electrons across the junction and is why it is called the potential barrier. In practice, a
PN junction is formed within a single crystal of material rather than just simply joining or fusing
together two separate pieces. Electrical contacts are also fused onto either side of the crystal to
enable an electrical connection to be made to an external circuit. Then the resulting device that
has been made is called a PN junction Diode or Signal Diode.
In the next tutorial about the PN junction, we will look at one of the most interesting aspects of
the PN junction is its use in circuits as a diode. By adding connections to each end of the P-type
and the N-type materials we can produce a two terminal device called a PN Junction Diode
which can be biased by an external voltage to either block or allow the flow of current through it.

Introduction to depletion layer pn junction:
If one side of crystal pure semiconductor Si(silicon) or Ge(Germanium) is doped with acceptor
impurity atoms and the other side is doped with donor impurity atoms , a PN junction is formed
as shown in figure.P region has high concentration of holes and N region contains large number
of electrons.
What is Depletion Layer of Pn Junction?
As soon as the junction is formed, free electrons and holes cross through the junction by the
process of diffusion.During this process , the electrons crossing the junction from N- region into
P-region , recombine with holes in the P-region very close to the junction.Similarly holes
crossing the junction from the P-region into the N-region, recombine with electrons in the N-
region very close to the junction. Thus a region is formed, which does not have any mobile
charge very close to the junction. This region is called the depletion layer of pn junction.

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In this region, on the left side of the junction, the acceptor atoms become negative ions and on
the right side of the junction, the donor atoms become positive ions as shown in figure.
Function of Depletion Layer of Pn Junction :
An electric field is set up, between the donor and acceptor ions in the depletion layer of the pn
junction .The potential at the N-side is higher than the potential at P-side.Therefore electrons in
the N- side are prevented to go to the lower potential of P-side. Similarly, holes in the P-side find
themselves at a lower potential and are prevented to cross to the N-side. Thus, there is a barrier at
the junction which opposes the movement of the majority charge carriers. The difference of
potential from one side of the barrier to the other side of the barrier is called potential
barrier.The potential barrier is approximately 0.7V for a silicon PN junction and 0.3V for
germanium PN junction. The distance from one side of the barrier to the other side is called the
width of the barrier, which depends on the nature of the material.
Introduction to Diodes and Transistors:
Diodes are known as p-n junction in the physics or in most of the basic sciences but in the
electronics and engineering sciences a p-n junction has the abbreviated name i.e. diode. Diodes
are the electrical component of any electric circuit which prevents the circuit from the high
electric current because diodes are get break i.e. they get burn when a large current is flowing
through them and hence prevents the electrical circuits.
Transistors are the electrical device which mainly consists of two junctions thus they are called
as the junction transistors. The transistors have three terminals instead of the two terminals and
each terminal has its specific characteristics. In electronic circuits two transistors namely n-p-n
and p-n-p are most preferably used.
More about Diodes and Transistors:
When a p-type semiconductor is brought into a close contact with an n-type semiconductor, then
the resultant arrangement is called a p-n junction or a junction diode or simply a diode.
A junction transistor is obtained by growing a thin layer of one semiconductor in between two
thick layers of other similar type semiconductor. Thus a junction transistor is a semiconductor
device having two junctions and three terminals.
Example of Diodes and Transistors:
Junction diodes are of many types. Important among them are:
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(a) Zener diode: A zener diode is specially designed junction diode which can operate
continuously without being damaged in the region of reverse break down voltage shown in Fig.1.
(b) Photo diode: Its working is based on the electric conduction from light shown in Fig.2.
(c) Light emitting diode (LED): Its working is based on the production of light from electric
current shown in Fig.3.

Fig.1 Zener Diode Fig.2 Photo Diode Fig.3 LED
The transistors are also of several types but most important among them are listed below:
(a) Junction Transistors: These are of two kind p-n-p and n-p-n and they are basic transistors
which are used in the electronic circuitry and other electronic equipment very rapidly because of
there low cost and high reliability.
(b) Field effect transistors: In present days most of the electronic integrated circuits are using
these kinds of transistors because they are highly conductive and easy to prepare then the
junction transistor.
The Junction Diode

The Junction Diode
The effect described in the previous tutorial is achieved without any external voltage being
applied to the actual PN junction resulting in the junction being in a state of equilibrium.
However, if we were to make electrical connections at the ends of both the N-type and the P-type
materials and then connect them to a battery source, an additional energy source now exists to
overcome the barrier resulting in free charges being able to cross the depletion region from one
side to the other. The behaviour of the PN junction with regards to the potential barrier width
produces an asymmetrical conducting two terminal device, better known as the Junction Diode.
A diode is one of the simplest semiconductor devices, which has the characteristic of passing
current in one direction only. However, unlike a resistor, a diode does not behave linearly with
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respect to the applied voltage as the diode has an exponential I-V relationship and therefore we
can not described its operation by simply using an equation such as Ohm's law.
If a suitable positive voltage (forward bias) is applied between the two ends of the PN junction, it
can supply free electrons and holes with the extra energy they require to cross the junction as the
width of the depletion layer around the PN junction is decreased. By applying a negative voltage
(reverse bias) results in the free charges being pulled away from the junction resulting in the
depletion layer width being increased. This has the effect of increasing or decreasing the
effective resistance of the junction itself allowing or blocking current flow through the diode.
Then the depletion layer widens with an increase in the application of a reverse voltage and
narrows with an increase in the application of a forward voltage. This is due to the differences in
the electrical properties on the two sides of the PN junction resulting in physical changes taking
place. One of the results produces rectification as seen in the PN junction diodes static I-V
(current-voltage) characteristics. Rectification is shown by an asymmetrical current flow when
the polarity of bias voltage is altered as shown below.
Junction Diode Symbol and Static I-V Characteristics.

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But before we can use the PN junction as a practical device or as a rectifying device we need to
firstly bias the junction, ie connect a voltage potential across it. On the voltage axis above,
"Reverse Bias" refers to an external voltage potential which increases the potential barrier. An
external voltage which decreases the potential barrier is said to act in the "Forward Bias"
direction.
There are two operating regions and three possible "biasing" conditions for the standard
Junction Diode and these are:
- 1. Zero Bias - No external voltage potential is applied to the PN-junction.
-
2. Reverse Bias - The voltage potential is connected negative, (-ve) to the P-type material and
positive, (+ve) to the N-type material across the diode which has the effect of Increasing the
PN-junction width.
-
3. Forward Bias - The voltage potential is connected positive, (+ve) to the P-type material and
negative, (-ve) to the N-type material across the diode which has the effect of Decreasing the
PN-junction width.
Zero Biased Junction Diode
When a diode is connected in a Zero Bias condition, no external potential energy is applied to
the PN junction. However if the diodes terminals are shorted together, a few holes (majority
carriers) in the P-type material with enough energy to overcome the potential barrier will move
across the junction against this barrier potential. This is known as the "Forward Current" and is
referenced as I
F

Likewise, holes generated in the N-type material (minority carriers), find this situation
favourable and move across the junction in the opposite direction. This is known as the "Reverse
Current" and is referenced as I
R
. This transfer of electrons and holes back and forth across the
PN junction is known as diffusion, as shown below.




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Zero Biased Junction Diode


The potential barrier that now exists discourages the diffusion of any more majority carriers
across the junction. However, the potential barrier helps minority carriers (few free electrons in
the P-region and few holes in the N-region) to drift across the junction. Then an "Equilibrium" or
balance will be established when the majority carriers are equal and both moving in opposite
directions, so that the net result is zero current flowing in the circuit. When this occurs the
junction is said to be in a state of "Dynamic Equilibrium".
The minority carriers are constantly generated due to thermal energy so this state of equilibrium
can be broken by raising the temperature of the PN junction causing an increase in the generation
of minority carriers, thereby resulting in an increase in leakage current but an electric current
cannot flow since no circuit has been connected to the PN junction.
Reverse Biased Junction Diode
When a diode is connected in a Reverse Bias condition, a positive voltage is applied to the N-
type material and a negative voltage is applied to the P-type material. The positive voltage
applied to the N-type material attracts electrons towards the positive electrode and away from the
junction, while the holes in the P-type end are also attracted away from the junction towards the
negative electrode. The net result is that the depletion layer grows wider due to a lack of
electrons and holes and presents a high impedance path, almost an insulator. The result is that a
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high potential barrier is created thus preventing current from flowing through the semiconductor
material.
Reverse Biased Junction Diode showing an Increase in the Depletion Layer

This condition represents a high resistance value to the PN junction and practically zero current
flows through the junction diode with an increase in bias voltage. However, a very small leakage
current does flow through the junction which can be measured in microamperes, (A). One final
point, if the reverse bias voltage Vr applied to the diode is increased to a sufficiently high enough
value, it will cause the PN junction to overheat and fail due to the avalanche effect around the
junction. This may cause the diode to become shorted and will result in the flow of maximum
circuit current, and this shown as a step downward slope in the reverse static characteristics
curve below.
Reverse Characteristics Curve for a Junction Diode
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Sometimes this avalanche effect has practical applications in voltage stabilising circuits where a
series limiting resistor is used with the diode to limit this reverse breakdown current to a preset
maximum value thereby producing a fixed voltage output across the diode. These types of diodes
are commonly known as Zener Diodes and are discussed in a later tutorial.
Forward Biased Junction Diode
When a diode is connected in a Forward Bias condition, a negative voltage is applied to the N-
type material and a positive voltage is applied to the P-type material. If this external voltage
becomes greater than the value of the potential barrier, approx. 0.7 volts for silicon and 0.3 volts
for germanium, the potential barriers opposition will be overcome and current will start to flow.
This is because the negative voltage pushes or repels electrons towards the junction giving them
the energy to cross over and combine with the holes being pushed in the opposite direction
towards the junction by the positive voltage. This results in a characteristics curve of zero current
flowing up to this voltage point, called the "knee" on the static curves and then a high current
flow through the diode with little increase in the external voltage as shown below.
Forward Characteristics Curve for a Junction Diode
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The application of a forward biasing voltage on the junction diode results in the depletion layer
becoming very thin and narrow which represents a low impedance path through the junction
thereby allowing high currents to flow. The point at which this sudden increase in current takes
place is represented on the static I-V characteristics curve above as the "knee" point.
Forward Biased Junction Diode showing a Reduction in the Depletion Layer

This condition represents the low resistance path through the PN junction allowing very large
currents to flow through the diode with only a small increase in bias voltage. The actual potential
difference across the junction or diode is kept constant by the action of the depletion layer at
approximately 0.3v for germanium and approximately 0.7v for silicon junction diodes. Since the
diode can conduct "infinite" current above this knee point as it effectively becomes a short
circuit, therefore resistors are used in series with the diode to limit its current flow. Exceeding its
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maximum forward current specification causes the device to dissipate more power in the form of
heat than it was designed for resulting in a very quick failure of the device.
Junction Diode Summary
The PN junction region of a Junction Diode has the following important characteristics:
- 1). Semiconductors contain two types of mobile charge carriers, Holes and Electrons.
-
- 2). The holes are positively charged while the electrons negatively charged.
-
- 3). A semiconductor may be doped with donor impurities such as Antimony (N-type doping), so
that it contains mobile charges which are primarily electrons.
-
- 4). A semiconductor may be doped with acceptor impurities such as Boron (P-type doping), so
that it contains mobile charges which are mainly holes.
-
- 5). The junction region itself has no charge carriers and is known as the depletion region.
-
- 6). The junction (depletion) region has a physical thickness that varies with the applied voltage.
-
- 7).When a diode is Zero Biased no external energy source is applied and a natural Potential
Barrier is developed across a depletion layer which is approximately 0.5 to 0.7v for silicon
diodes and approximately 0.3 of a volt for germanium diodes.
-
- 8). When a junction diode is Forward Biased the thickness of the depletion region reduces and
the diode acts like a short circuit allowing full current to flow.
-
- 9). When a junction diode is Reverse Biased the thickness of the depletion region increases and
the diode acts like an open circuit blocking any current flow, (only a very small leakage current).
In the next tutorial about diodes, we will look at the small signal diode sometimes called a
switching diode that are used in general electronic circuits. A signal diode is designed for low-
voltage or high frequency signal applications such as in radio or digital switching circuits as
opposed to the high-current mains rectification diodes in which silicon diodes are usually used,
and examine the Signal Diode static current-voltage characteristics curve and parameters.




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Signal Diode
The Signal Diode
The semiconductor Signal Diode is a small non-linear semiconductor devices generally used in
electronic circuits, where small currents or high frequencies are involved such as in radio,
television and digital logic circuits. The signal diode which is also sometimes known by its older
name of the Point Contact Diode or the Glass Passivated Diode, are physically very small in
size compared to their larger Power Diode cousins.
Generally, the PN junction of a small signal diode is encapsulated in glass to protect the PN
junction, and usually have a red or black band at one end of their body to help identify which end
is the cathode terminal. The most widely used of all the glass encapsulated signal diodes is the
very common 1N4148 and its equivalent 1N914 signal diode. Small signal and switching diodes
have much lower power and current ratings, around 150mA, 500mW maximum compared to
rectifier diodes, but they can function better in high frequency applications or in clipping and
switching applications that deal with short-duration pulse waveforms.
The characteristics of a signal point contact diode are different for both germanium and silicon
types and are given as:
- Germanium Signal Diodes - These have a low reverse resistance value giving a lower forward
volt drop across the junction, typically only about 0.2-0.3v, but have a higher forward resistance
value because of their small junction area.
-
- Silicon Signal Diodes - These have a very high value of reverse resistance and give a forward
volt drop of about 0.6-0.7v across the junction. They have fairly low values of forward resistance
giving them high peak values of forward current and reverse voltage.
The electronic symbol given for any type of diode is that of an arrow with a bar or line at its end
and this is illustrated below along with the Steady State V-I Characteristics Curve.




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Silicon Diode V-I Characteristic Curve


The arrow points in the direction of conventional current flow through the diode meaning that
the diode will only conduct if a positive supply is connected to the Anode (a) terminal and a
negative supply is connected to the Cathode (k) terminal thus only allowing current to flow
through it in one direction only, acting more like a one way electrical valve, (Forward Biased
Condition). However, we know from the previous tutorial that if we connect the external energy
source in the other direction the diode will block any current flowing through it and instead will
act like an open switch, (Reversed Biased Condition) as shown below.
Forward and Reversed Biased Diode

Then we can say that an ideal small signal diode conducts current in one direction (forward-
conducting) and blocks current in the other direction (reverse-blocking). Signal Diodes are used
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in a wide variety of applications such as a switch in rectifiers, limiters, snubbers or in wave-
shaping circuits.
Signal Diode Parameters
Signal Diodes are manufactured in a range of voltage and current ratings and care must be taken
when choosing a diode for a certain application. There are a bewildering array of static
characteristics associated with the humble signal diode but the more important ones are.
1. Maximum Forward Current
The Maximum Forward Current (I
F(max)
) is as its name implies the maximum forward current
allowed to flow through the device. When the diode is conducting in the forward bias condition,
it has a very small "ON" resistance across the PN junction and therefore, power is dissipated
across this junction (Ohms Law) in the form of heat. Then, exceeding its (I
F(max)
) value will
cause more heat to be generated across the junction and the diode will fail due to thermal
overload, usually with destructive consequences. When operating diodes around their maximum
current ratings it is always best to provide additional cooling to dissipate the heat produced by
the diode.
For example, our small 1N4148 signal diode has a maximum current rating of about 150mA with
a power dissipation of 500mW at 25
o
C. Then a resistor must be used in series with the diode to
limit the forward current, (I
F(max)
) through it to below this value.
2. Peak Inverse Voltage
The Peak Inverse Voltage (PIV) or Maximum Reverse Voltage (V
R(max)
), is the maximum
allowable Reverse operating voltage that can be applied across the diode without reverse
breakdown and damage occurring to the device. This rating therefore, is usually less than the
"avalanche breakdown" level on the reverse bias characteristic curve. Typical values of V
R(max)

range from a few volts to thousands of volts and must be considered when replacing a diode.
The peak inverse voltage is an important parameter and is mainly used for rectifying diodes in
AC rectifier circuits with reference to the amplitude of the voltage were the sinusoidal waveform
changes from a positive to a negative value on each and every cycle.
3. Forward Power Dissipation
Signal diodes have a Forward Power Dissipation, (P
D(max)
) rating. This rating is the maximum
possible power dissipation of the diode when it is forward biased (conducting). When current
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flows through the signal diode the biasing of the PN junction is not perfect and offers some
resistance to the flow of current resulting in power being dissipated (lost) in the diode in the form
of heat. As small signal diodes are nonlinear devices the resistance of the PN junction is not
constant, it is a dynamic property then we cannot use Ohms Law to define the power in terms of
current and resistance or voltage and resistance as we can for resistors. Then to find the power
that will be dissipated by the diode we must multiply the voltage drop across it times the current
flowing through it: P
D
= VxI
4. Maximum Operating Temperature
The Maximum Operating Temperature actually relates to the Junction Temperature (T
J
) of
the diode and is related to maximum power dissipation. It is the maximum temperature allowable
before the structure of the diode deteriorates and is expressed in units of degrees centigrade per
Watt, (
o
C/W ). This value is linked closely to the maximum forward current of the device so that
at this value the temperature of the junction is not exceeded. However, the maximum forward
current will also depend upon the ambient temperature in which the device is operating so the
maximum forward current is usually quoted for two or more ambient temperature values such as
25
o
C or 70
o
C.
Then there are three main parameters that must be considered when either selecting or replacing
a signal diode and these are:
- The Reverse Voltage Rating
- The Forward Current Rating
- The Forward Power Dissipation Rating
Signal Diode Arrays
When space is limited, or matching pairs of switching signal diodes are required, diode arrays
can be very useful. They generally consist of low capacitance high speed silicon diodes such as
the 1N4148 connected together in multiple diode packages called an array for use in switching
and clamping in digital circuits. They are encased in single inline packages (SIP) containing 4 or
more diodes connected internally to give either an individual isolated array, common cathode,
(CC), or a common anode, (CA) configuration as shown.



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Signal Diode Arrays

Signal diode arrays can also be used in digital and computer circuits to protect high speed data
lines or other input/output parallel ports against electrostatic discharge, (ESD) and voltage
transients. By connecting two diodes in series across the supply rails with the data line connected
to their junction as shown, any unwanted transients are quickly dissipated and as the signal
diodes are available in 8-fold arrays they can protect eight data lines in a single package.

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CPU Data Line Protection
Signal diode arrays can also be used to connect together diodes in either series or parallel
combinations to form voltage regulator or voltage reducing type circuits or to produce a known
fixed voltage. We know that the forward volt drop across a silicon diode is about 0.7v and by
connecting together a number of diodes in series the total voltage drop will be the sum of the
individual voltage drops of each diode. However, when signal diodes are connected together in
series, the current will be the same for each diode so the maximum forward current must not be
exceeded.
Connecting Signal Diodes in Series
Another application for the small signal diode is to create a regulated voltage supply. Diodes are
connected together in series to provide a constant DC voltage across the diode combination. The
output voltage across the diodes remains constant in spite of changes in the load current drawn
from the series combination or changes in the DC power supply voltage that feeds them.
Consider the circuit below.
Signal Diodes in Series

As the forward voltage drop across a silicon diode is almost constant at about 0.7v, while the
current through it varies by relatively large amounts, a forward-biased signal diode can make a
simple voltage regulating circuit. The individual voltage drops across each diode are subtracted
from the supply voltage to leave a certain voltage potential across the load resistor, and in our
simple example above this is given as 10v - (3 x 0.7v) = 7.9v. This is because each diode has a
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junction resistance relating to the small signal current flowing through it and the three signal
diodes in series will have three times the value of this resistance, along with the load resistance
R, forms a voltage divider across the supply.
By adding more diodes in series a greater voltage reduction will occur. Also series connected
diodes can be placed in parallel with the load resistor to act as a voltage regulating circuit. Here
the voltage applied to the load resistor will be 3 x 0.7v = 2.1v. We can of course produce the
same constant voltage source using a single Zener Diode. Resistor, R
D
is used to prevent
excessive current flowing through the diodes if the load is removed.
Freewheel Diodes
Signal diodes can also be used in a variety of clamping, protection and wave shaping circuits
with the most common form of clamping diode circuit being one which uses a diode connected
in parallel with a coil or inductive load to prevent damage to the delicate switching circuit by
suppressing the voltage spikes and/or transients that are generated when the load is suddenly
turned "OFF". This type of diode is generally known as a "Free-wheeling Diode" or Freewheel
diode as it is more commonly called.
The Freewheel diode is used to protect solid state switches such as power transistors and
MOSFET's from damage by reverse battery protection as well as protection from highly
inductive loads such as relay coils or motors, and an example of its connection is shown below.
Use of the Freewheel Diode

Modern fast switching, power semiconductor devices require fast switching diodes such as free
wheeling diodes to protect them form inductive loads such as motor coils or relay windings.
Every time the switching device above is turned "ON", the freewheel diode changes from a
conducting state to a blocking state as it becomes reversed biased. However, when the device
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rapidly turns "OFF", the diode becomes forward biased and the collapse of the energy stored in
the coil causes a current to flow through the freewheel diode. Without the protection of the
freewheel diode high di/dt currents would occur causing a high voltage spike or transient to flow
around the circuit possibly damaging the switching device.
Previously, the operating speed of the semiconductor switching device, either transistor,
MOSFET, IGBT or digital has been impaired by the addition of a freewheel diode across the
inductive load with Schottky and Zener diodes being used instead in some applications. But
during the past few years however, freewheel diodes had regained importance due mainly to
their improved reverse-recovery characteristics and the use of super fast semiconductor materials
capable at operating at high switching frequencies.
Other types of specialized diodes not included here are Photo-Diodes, PIN Diodes, Tunnel
Diodes and Schottky Barrier Diodes. By adding more PN junctions to the basic two layer diode
structure other types of semiconductor devices can be made. For example a three layer
semiconductor device becomes a Transistor, a four layer semiconductor device becomes a
Thyristor or Silicon Controlled Rectifier and five layer devices known as Triacs are also
available.
In the next tutorial about diodes, we will look at the large signal diode sometimes called the
Power Diode. Power diodes are silicon diodes designed for use in high-voltage, high-current
mains rectification circuits.

Power Diodes and Rectifiers

The Power Diode
In the previous tutorials we saw that a semiconductor signal diode will only conduct current in
one direction from its anode to its cathode (forward direction), but not in the reverse direction
acting a bit like an electrical one way valve. A widely used application of this feature is in the
conversion of an alternating voltage (AC) into a continuous voltage (DC). In other words,
Rectification. Small signal diodes can be used as rectifiers in low-power, low current (less than
1-amp) rectifiers or applications, but were larger forward bias currents or higher reverse bias
blocking voltages are involved the PN junction of a small signal diode would eventually overheat
and melt so larger more robust Power Diodes are used instead.
The power semiconductor diode, known simply as the Power Diode, has a much larger PN
junction area compared to its smaller signal diode cousin, resulting in a high forward current
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capability of up to several hundred amps (KA) and a reverse blocking voltage of up to several
thousand volts (KV). Since the power diode has a large PN junction, it is not suitable for high
frequency applications above 1MHz, but special and expensive high frequency, high current
diodes are available. For high frequency rectifier applications Schottky Diodes are generally
used because of their short reverse recovery time and low voltage drop in their forward bias
condition.
Power diodes provide uncontrolled rectification of power and are used in applications such as
battery charging and DC power supplies as well as AC rectifiers and inverters. Due to their high
current and voltage characteristics they can also be used as freewheeling diodes and snubber
networks. Power diodes are designed to have a forward "ON" resistance of fractions of an Ohm
while their reverse blocking resistance is in the mega-Ohms range. Some of the larger value
power diodes are designed to be "stud mounted" onto heatsinks reducing their thermal resistance
to between 0.1 to 1
o
C/Watt.
If an alternating voltage is applied across a power diode, during the positive half cycle the diode
will conduct passing current and during the negative half cycle the diode will not conduct
blocking the flow of current. Then conduction through the power diode only occurs during the
positive half cycle and is therefore unidirectional i.e. DC as shown.
Power Diode Rectifier


Power diodes can be used individually as above or connected together to produce a variety of
rectifier circuits such as "Half-Wave", "Full-Wave" or as "Bridge Rectifiers". Each type of
rectifier circuit can be classed as either uncontrolled, half-controlled or fully controlled were an
uncontrolled rectifier uses only power diodes, a fully controlled rectifier uses thyristors (SCRs)
and a half controlled rectifier is a mixture of both diodes and thyristors. The most commonly
used individual power diode for basic electronics applications is the general purpose 1N400x
Series Glass Passivated type rectifying diode with standard ratings of continuous forward
rectified current of 1.0 amp and reverse blocking voltage ratings from 50v for the 1N4001 up to
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1000v for the 1N4007, with the small 1N4007GP being the most popular for general purpose
mains voltage rectification.
Half Wave Rectification
A rectifier is a circuit which converts the Alternating Current (AC) input power into a Direct
Current (DC) output power. The input power supply may be either a single-phase or a multi-
phase supply with the simplest of all the rectifier circuits being that of the Half Wave Rectifier.
The power diode in a half wave rectifier circuit passes just one half of each complete sine wave
of the AC supply in order to convert it into a DC supply. Then this type of circuit is called a
"half-wave" rectifier because it passes only half of the incoming AC power supply as shown
below.
Half Wave Rectifier Circuit


During each "positive" half cycle of the AC sine wave, the diode is forward biased as the anode
is positive with respect to the cathode resulting in current flowing through the diode. Since the
DC load is resistive (resistor, R), the current flowing in the load resistor is therefore proportional
to the voltage (Ohms Law), and the voltage across the load resistor will therefore be the same
as the supply voltage, Vs (minus Vf), that is the "DC" voltage across the load is sinusoidal for
the first half cycle only so Vout = Vs.
During each "negative" half cycle of the AC sine wave, the diode is reverse biased as the anode
is negative with respect to the cathode therefore, No current flows through the diode or circuit.
Then in the negative half cycle of the supply, no current flows in the load resistor as no voltage
appears across it so Vout = 0.
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The current on the DC side of the circuit flows in one direction only making the circuit
Unidirectional and the value of the DC voltage V
DC
across the load resistor is calculated as
follows.


Where V
max
is the maximum voltage value of the AC supply, and V
S
is the r.m.s. value of the
supply.
Example No1.
Calculate the current, ( I
DC
) flowing through a 100 resistor connected to a 240v single phase
half-wave rectifier as shown above. Also calculate the power consumed by the load.


During the rectification process the resultant output DC voltage and current are therefore both
"ON" and "OFF" during every cycle. As the voltage across the load resistor is only present
during the positive half of the cycle (50% of the input waveform), this results in a low average
DC value being supplied to the load. The variation of the rectified output waveform between this
ON and OFF condition produces a waveform which has large amounts of "ripple" which is an
undesirable feature. The resultant DC ripple has a frequency that is equal to that of the AC
supply frequency.
Very often when rectifying an alternating voltage we wish to produce a "steady" and continuous
DC voltage free from any voltage variations or ripple. One way of doing this is to connect a large
value Capacitor across the output voltage terminals in parallel with the load resistor as shown
below. This type of capacitor is known commonly as a "Reservoir" or Smoothing Capacitor.
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Half-wave Rectifier with Smoothing Capacitor

When rectification is used to provide a direct voltage power supply from an alternating source,
the amount of ripple can be further reduced by using larger value capacitors but there are limits
both on cost and size. For a given capacitor value, a greater load current (smaller load resistor)
will discharge the capacitor more quickly (RC Time Constant) and so increases the ripple
obtained. Then for single phase, half-wave rectifier circuits it is not very practical to try and
reduce the ripple voltage by capacitor smoothing alone, it is more practical to use "Full-wave
Rectification" instead.
In practice, the half-wave rectifier is used most often in low-power applications because of their
major disadvantages being. The output amplitude is less than the input amplitude, there is no
output during the negative half cycle so half the power is wasted and the output is pulsed DC
resulting in excessive ripple. To overcome these disadvantages a number of Power Diodes are
connected together to produce a Full Wave Rectifier as discussed in the next tutorial.

Full Wave Rectifier

In the previous Power Diodes tutorial we discussed ways of reducing the ripple or voltage
variations on a direct DC voltage by connecting capacitors across the load resistance. While this
method may be suitable for low power applications it is unsuitable to applications which need a
"steady and smooth" DC supply voltage. One method to improve on this is to use every half-
cycle of the input voltage instead of every other half-cycle. The circuit which allows us to do this
is called a Full Wave Rectifier.
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Like the half wave circuit, a full wave rectifier circuit produces an output voltage or current
which is purely DC or has some specified DC component. Full wave rectifiers have some
fundamental advantages over their half wave rectifier counterparts. The average (DC) output
voltage is higher than for half wave, the output of the full wave rectifier has much less ripple
than that of the half wave rectifier producing a smoother output waveform.
In a Full Wave Rectifier circuit two diodes are now used, one for each half of the cycle. A
transformer is used whose secondary winding is split equally into two halves with a common
centre tapped connection, (C). This configuration results in each diode conducting in turn when
its anode terminal is positive with respect to the transformer centre point C producing an output
during both half-cycles, twice that for the half wave rectifier so it is 100% efficient as shown
below.
Full Wave Rectifier Circuit


The full wave rectifier circuit consists of two power diodes connected to a single load resistance
(R
L
) with each diode taking it in turn to supply current to the load. When point A of the
transformer is positive with respect to point B, diode D
1
conducts in the forward direction as
indicated by the arrows. When point B is positive (in the negative half of the cycle) with respect
to point A, diode D
2
conducts in the forward direction and the current flowing through resistor R
is in the same direction for both circuits. As the output voltage across the resistor R is the phasor
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sum of the two waveforms combined, this type of full wave rectifier circuit is also known as a
"bi-phase" circuit.
As the spaces between each half-wave developed by each diode is now being filled in by the
other diode the average DC output voltage across the load resistor is now double that of the
single half-wave rectifier circuit and is about 0.637V
max
of the peak voltage, assuming no
losses. However in reality, during each half cycle the current flows through two diodes instead of
just one so the amplitude of the output voltage is two voltage drops ( 2 x 0.7 = 1.4V ) less than
the input V
MAX
amplitude.


The peak voltage of the output waveform is the same as before for the half-wave rectifier
provided each half of the transformer windings have the same rms voltage value. To obtain a
different DC voltage output different transformer ratios can be used. The main disadvantage of
this type of full wave rectifier circuit is that a larger transformer for a given power output is
required with two separate but identical secondary windings making this type of full wave
rectifying circuit costly compared to the "Full Wave Bridge Rectifier" circuit equivalent.
The Full Wave Bridge Rectifier
Another type of circuit that produces the same output waveform as the full wave rectifier circuit
above, is that of the Full Wave Bridge Rectifier. This type of single phase rectifier uses four
individual rectifying diodes connected in a closed loop "bridge" configuration to produce the
desired output. The main advantage of this bridge circuit is that it does not require a special
centre tapped transformer, thereby reducing its size and cost. The single secondary winding is
connected to one side of the diode bridge network and the load to the other side as shown below.
The Diode Bridge Rectifier

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The four diodes labelled D
1
to D
4
are arranged in "series pairs" with only two diodes conducting
current during each half cycle. During the positive half cycle of the supply, diodes D1 and D2
conduct in series while diodes D3 and D4 are reverse biased and the current flows through the
load as shown below.
The Positive Half-cycle


During the negative half cycle of the supply, diodes D3 and D4 conduct in series, but diodes D1
and D2 switch of as they are now reverse biased. The current flowing through the load is the
same direction as before.





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The Negative Half-cycle


As the current flowing through the load is unidirectional, so the voltage developed across the
load is also unidirectional the same as for the previous two diode full-wave rectifier, therefore
the average DC voltage across the load is 0.637V
max
and the ripple frequency is now twice the
supply frequency (e.g. 100Hz for a 50Hz supply).

Typical Bridge Rectifier
Although we can use four individual power diodes to make a full wave bridge rectifier, pre-made
bridge rectifier components are available "off-the-shelf" in a range of different voltage and
current sizes that can be soldered directly into a PCB circuit board or be connected by spade
connectors. The image to the right shows a typical single phase bridge rectifier with one corner
cut off. This cut-off corner indicates that the terminal nearest to the corner is the positive or +ve
output terminal or lead with the opposite (diagonal) lead being the negative or -ve output lead.
The other two connecting leads are for the input alternating voltage from a transformer
secondary winding.


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The Smoothing Capacitor
We saw in the previous section that the single phase half-wave rectifier produces an output wave
every half cycle and that it was not practical to use this type of circuit to produce a steady DC
supply. The full-wave bridge rectifier however, gives us a greater mean DC value (0.637 Vmax)
with less superimposed ripple while the output waveform is twice that of the frequency of the
input supply frequency. We can therefore increase its average DC output level even higher by
connecting a suitable smoothing capacitor across the output of the bridge circuit as shown below.
Full-wave Rectifier with Smoothing Capacitor


The smoothing capacitor converts the full-wave rippled output of the rectifier into a smooth DC
output voltage. Generally for DC power supply circuits the smoothing capacitor is an Aluminium
Electrolytic type that has a capacitance value of 100uF or more with repeated DC voltage pulses
from the rectifier charging up the capacitor to peak voltage. However, their are two important
parameters to consider when choosing a suitable smoothing capacitor and these are its Working
Voltage, which must be higher than the no-load output value of the rectifier and its Capacitance
Value, which determines the amount of ripple that will appear superimposed on top of the DC
voltage. Too low a value and the capacitor has little effect but if the smoothing capacitor is large
enough (parallel capacitors can be used) and the load current is not too large, the output voltage
will be almost as smooth as pure DC. As a general rule of thumb, we are looking to have a ripple
voltage of less than 100mV peak to peak.
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The maximum ripple voltage present for a Full Wave Rectifier circuit is not only determined by
the value of the smoothing capacitor but by the frequency and load current, and is calculated as:
Bridge Rectifier Ripple Voltage

Where: I is the DC load current in amps, is the frequency of the ripple or twice the input
frequency in Hertz, and C is the capacitance in Farads.
The main advantages of a full-wave bridge rectifier is that it has a smaller AC ripple value for a
given load and a smaller reservoir or smoothing capacitor than an equivalent half-wave rectifier.
Therefore, the fundamental frequency of the ripple voltage is twice that of the AC supply
frequency (100Hz) where for the half-wave rectifier it is exactly equal to the supply frequency
(50Hz).
The amount of ripple voltage that is superimposed on top of the DC supply voltage by the diodes
can be virtually eliminated by adding a much improved -filter (pi-filter) to the output terminals
of the bridge rectifier. This type of low-pass filter consists of two smoothing capacitors, usually
of the same value and a choke or inductance across them to introduce a high impedance path to
the alternating ripple component. Another more practical and cheaper alternative is to use a 3-
terminal voltage regulator IC, such as a LM78xx for a positive output voltage or the LM79xx for
a negative output voltage which can reduce the ripple by more than 70dB (Datasheet) while
delivering a constant output current of over 1 amp.
In the next tutorial about diodes, we will look at the Zener Diode which takes advantage of its
reverse breakdown voltage characteristic to produce a constant and fixed output voltage across
itself.
Zener Diodes

The Zener Diode
In the previous Signal Diode tutorial, we saw that a "reverse biased" diode blocks current in the
reverse direction, but will suffer from premature breakdown or damage if the reverse voltage
applied across it is too high. However, the Zener Diode or "Breakdown Diode" as they are
sometimes called, are basically the same as the standard PN junction diode but are specially
designed to have a low pre-determined Reverse Breakdown Voltage that takes advantage of
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this high reverse voltage. The point at which a zener diode breaks down or conducts is called the
"Zener Voltage" (Vz).
The Zener diode is like a general-purpose signal diode consisting of a silicon PN junction.
When biased in the forward direction it behaves just like a normal signal diode passing the rated
current, but when a reverse voltage is applied to it the reverse saturation current remains fairly
constant over a wide range of voltages. The reverse voltage increases until the diodes breakdown
voltage V
B
is reached at which point a process called Avalanche Breakdown occurs in the
depletion layer and the current flowing through the zener diode increases dramatically to the
maximum circuit value (which is usually limited by a series resistor). This breakdown voltage
point is called the "zener voltage" for zener diodes.
The point at which current flows can be very accurately controlled (to less than 1% tolerance) in
the doping stage of the diodes construction giving the diode a specific zener breakdown voltage,
(Vz) ranging from a few volts up to a few hundred volts. This zener breakdown voltage on the I-
V curve is almost a vertical straight line.
Zener Diode I-V Characteristics


The Zener Diode is used in its "reverse bias" or reverse breakdown mode, i.e. the diodes anode
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connects to the negative supply. From the I-V characteristics curve above, we can see that the
zener diode has a region in its reverse bias characteristics of almost a constant negative voltage
regardless of the value of the current flowing through the diode and remains nearly constant even
with large changes in current as long as the zener diodes current remains between the breakdown
current I
Z(min)
and the maximum current rating I
Z(max)
.
This ability to control itself can be used to great effect to regulate or stabilise a voltage source
against supply or load variations. The fact that the voltage across the diode in the breakdown
region is almost constant turns out to be an important application of the zener diode as a voltage
regulator. The function of a regulator is to provide a constant output voltage to a load connected
in parallel with it in spite of the ripples in the supply voltage or the variation in the load current
and the zener diode will continue to regulate the voltage until the diodes current falls below the
minimum I
Z(min)
value in the reverse breakdown region.
The Zener Diode Regulator
Zener Diodes can be used to produce a stabilised voltage output with low ripple under varying
load current conditions. By passing a small current through the diode from a voltage source, via a
suitable current limiting resistor (R
S
), the zener diode will conduct sufficient current to maintain
a voltage drop of Vout. We remember from the previous tutorials that the DC output voltage
from the half or full-wave rectifiers contains ripple superimposed onto the DC voltage and that
as the load value changes so to does the average output voltage. By connecting a simple zener
stabiliser circuit as shown below across the output of the rectifier, a more stable output voltage
can be produced.
Zener Diode Regulator

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The resistor, R
S
is connected in series with the zener diode to limit the current flow through the
diode with the voltage source, V
S
being connected across the combination. The stabilised output
voltage V
out
is taken from across the zener diode. The zener diode is connected with its cathode
terminal connected to the positive rail of the DC supply so it is reverse biased and will be
operating in its breakdown condition. Resistor R
S
is selected so to limit the maximum current
flowing in the circuit.
With no load connected to the circuit, the load current will be zero, ( I
L
= 0 ), and all the circuit
current passes through the zener diode which inturn dissipates its maximum power. Also a small
value of the series resistor R
S
will result in a greater diode current when the load resistance R
L
is
connected and large as this will increase the power dissipation requirement of the diode so care
must be taken when selecting the appropriate value of series resistance so that the zeners
maximum power rating is not exceeded under this no-load or high-impedance condition.
The load is connected in parallel with the zener diode, so the voltage across R
L
is always the
same as the zener voltage, ( V
R
= V
Z
). There is a minimum zener current for which the
stabilization of the voltage is effective and the zener current must stay above this value operating
under load within its breakdown region at all times. The upper limit of current is of course
dependant upon the power rating of the device. The supply voltage V
S
must be greater than V
Z
.
One small problem with zener diode stabiliser circuits is that the diode can sometimes generate
electrical noise on top of the DC supply as it tries to stabilise the voltage. Normally this is not a
problem for most applications but the addition of a large value decoupling capacitor across the
zeners output may be required to give additional smoothing.
Then to summarise a little. A zener diode is always operated in its reverse biased condition. A
voltage regulator circuit can be designed using a zener diode to maintain a constant DC output
voltage across the load in spite of variations in the input voltage or changes in the load current.
The zener voltage regulator consists of a current limiting resistor R
S
connected in series with the
input voltage V
S
with the zener diode connected in parallel with the load R
L
in this reverse
biased condition. The stabilized output voltage is always selected to be the same as the
breakdown voltage V
Z
of the diode.
Example No1
A 5.0V stabilised power supply is required to be produced from a 12V DC power supply
input source. The maximum power rating P
Z
of the zener diode is 2W. Using the zener
regulator circuit above calculate:
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a) The maximum current flowing through the zener diode.



b) The value of the series resistor, R
S




c) The load current I
L
if a load resistor of 1k is connected across the Zener diode.



d) The total supply current I
S



Zener Diode Voltages
As well as producing a single stabilised voltage output, zener diodes can also be connected
together in series along with normal silicon signal diodes to produce a variety of different
reference voltage output values as shown below.





Zener Diodes Connected in Series
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The values of the individual Zener diodes can be chosen to suit the application while the silicon
diode will always drop about 0.6 - 0.7V in the forward bias condition. The supply voltage, Vin
must of course be higher than the largest output reference voltage and in our example above this
is 19v.
A typical zener diode for general electronic circuits is the 500mW, BZX55 series or the larger
1.3W, BZX85 series were the zener voltage is given as, for example, C7V5 for a 7.5V diode
giving a diode reference number of BZX55C7V5. The 500mW series of zener diodes are
available from about 2.4 up to about 100 volts and typically have the same sequence of values as
used for the 5% (E24) resistor series with the individual voltage ratings for these small but very
useful diodes are given in the table below.
Zener Diode Standard Voltages
BZX55 Zener Diode Power Rating 500mW
2.4V 2.7V 3.0V 3.3V 3.6V 3.9V 4.3V 4.7V
5.1V 5.6V 6.2V 6.8V 7.5V 8.2V 9.1V 10V
11V 12V 13V 15V 16V 18V 20V 22V
24V 27V 30V 33V 36V 39V 43V 47V
BZX85 Zener Diode Power Rating 1.3W
3.3V 3.6V 3.9V 4.3V 4.7V 5.1V 5.6 6.2V
6.8V 7.5V 8.2V 9.1V 10V 11V 12V 13V
15V 16V 18V 20V 22V 24V 27V 30V
33V 36V 39V 43V 47V 51V 56V 62V
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Zener Diode Clipping Circuits
Thus far we have looked at how a zener diode can be used to regulate a constant DC source but
what if the input signal was not steady state DC but an alternating AC waveform how would the
zener diode react to a constantly changing signal.
Diode clipping and clamping circuits are circuits that are used to shape or modify an input AC
waveform (or any sinusoid) producing a differently shape output waveform depending on the
circuit arrangement. Diode clipper circuits are also called limiters because they limit or clip-off
the positive (or negative) part of an input AC signal. As zener clipper circuits limit or cut-off part
of the waveform across them, they are mainly used for circuit protection or in waveform shaping
circuits. For example, if we wanted to clip an output waveform at +7.5V, we would use a 7.5V
zener diode. If the output waveform tries to exceed the 7.5V limit, the zener diode will "clip-off"
the excess voltage from the input producing a waveform with a flat top still keeping the output
constant at +7.5V. Note that in the forward bias condition a zener diode is still a diode and when
the AC waveform output goes negative below -0.7V, the zener diode turns "ON" like any normal
silicon diode would and clips the output at -0.7V as shown below.
Square Wave Signal

The back to back connected zener diodes can be used as an AC regulator producing what is
jokingly called a "poor man's square wave generator". Using this arrangement we can clip the
waveform between a positive value of +7.5V and a negative value of -7.5V. If we wanted to clip
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an output waveform between different minimum and maximum values for example, +8V and -
6V, use would simply use two differently rated zener diodes.
Note that the output will actually clip the AC waveform between +8.7V and -6.7V due to the
addition of the forward biasing diode voltage, which adds another 0.7V voltage drop to it. This
type of clipper configuration is fairly common for protecting an electronic circuit from over
voltage. The two zeners are generally placed across the power supply input terminals and during
normal operation, one of the zener diodes is "OFF" and the diodes have little or no affect.
However, if the input voltage waveform exceeds its limit, then the zeners turn "ON" and clip the
input to protect the circuit.
In the next tutorial about diodes, we will look at using the forward biased PN junction of a diode
to produce light. We know from the previous tutorials that when charge carriers move across the
junction, electrons combine with holes and energy is lost in the form of heat, but also some of
this energy is dissipated as photons but we can not see them. If we place a translucent lens
around the junction, visible light will be produced and the diode becomes a light source. This
effect produces another type of diode known commonly as the Light Emitting Diode which
takes advantage of this light producing characteristic to emit light (photons) in a variety of
colours and wavelengths.



Light Emitting Diodes
Light Emitting Diodes or LEDs, are among the most widely used of all the different types of
semiconductor diodes available today. They are the most visible type of diode, that emit a fairly
narrow bandwidth of either visible light at different coloured wavelengths, invisible infra-red
light for remote controls or laser type light when a forward current is passed through them. A
"Light Emitting Diode" or LED as it is more commonly called, is basically just a specialised
type of PN junction diode, made from a very thin layer of fairly heavily doped semiconductor
material. When the diode is forward biased, electrons from the semiconductors conduction band
recombine with holes from the valence band releasing sufficient energy to produce photons
which emit a monochromatic (single colour) of light. Because of this thin layer a reasonable
number of these photons can leave the junction and radiate away producing a coloured light
output. Then we can say that when operated in a forward biased direction Light Emitting
Diodes are semiconductor devices that convert electrical energy into light energy.
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The construction of a light emitting diode is very different from that of a normal signal diode.
The PN junction of an LED is surrounded by a transparent, hard plastic epoxy resign
hemispherical shaped shell or body which protects the LED from both vibration and shock.
Surprisingly, an LED junction does not actually emit that much light so the epoxy resin body is
constructed in such a way that the photons of light emitted by the junction are reflected away
from the surrounding substrate base to which the diode is attached and are focused upwards
through the domed top of the LED, which itself acts like a lens concentrating the amount of light.
This is why the emitted light appears to be brightest at the top of the LED.
However, not all LEDs are made with a hemispherical shaped dome for their epoxy shell. Some
LEDs have a rectangular or cylindrical shaped construction that has a flat surface on top. Also,
nearly all LEDs have their cathode, (K) terminal identified by either a notch or flat spot on the
body, or by one of the leads being shorter than the other, (the Anode, A).
Unlike normal incandescent lamps and bulbs which generate large amounts of heat when
illuminated, the light emitting diode produces a "cold" generation of light which leads to high
efficiencies than the normal "light bulb" because most of the generated energy radiates away
within the visible spectrum. Because LEDs are solid-state devices, they can be extremely small
and durable and provide much longer lamp life than normal light sources.
Light Emitting Diode Colours
So how does a light emitting diode get its colour. Unlike normal signal diodes which are made
for detection or power rectification, and which are made from either Germanium or Silicon
semiconductor materials, Light Emitting Diodes are made from exotic semiconductor
compounds such as Gallium Arsenide (GaAs), Gallium Phosphide (GaP), Gallium Arsenide
Phosphide (GaAsP), Silicon Carbide (SiC) or Gallium Indium Nitride (GaInN) all mixed
together at different ratios to produce a distinct wavelength of colour. Different LED compounds
emit light in specific regions of the visible light spectrum and therefore produce different
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intensity levels. The exact choice of the semiconductor material used will determine the overall
wavelength of the photon light emissions and therefore the resulting colour of the light emitted.
Thus, the actual colour of a light
emitting diode is determined by the
wavelength of the light emitted, which
inturn is determined by the actual
semiconductor compound used in
forming the PN junction during
manufacture and NOT by the colouring
of the LEDs plastic body although these
are slightly coloured to both enhance
the light and indicate its colour when its
not be used. Light emitting diodes are
available in a wide range of colours with the most common being RED, AMBER, YELLOW
and GREEN and are thus widely used as visual indicators and as moving light displays.
Recently developed blue and white coloured LEDs are also available but these tend to be much
more expensive than the normal standard colours due to the production costs of mixing together
two or more complementary colours at an exact ratio within the semiconductor compound and
also by injecting nitrogen atoms into the crystal structure during the doping process.
From the table above we can see that the main P-type dopant used in the manufacture of Light
Emitting Diodes is Gallium (Ga, atomic number 31) and that the main N-type dopant used is
Arsenic (As, atomic number 31) giving the resulting compound of Gallium Arsenide (GaAs)
crystal structure. The problem with using Gallium Arsenide on its own as the semiconductor
compound is that it radiates large amounts of low brightness infra-red radiation (850nm-940nm
approx.) from its junction when a forward current is flowing through it. This infra-red light is ok
for television remote controls but not very useful if we want to use the LED as an indicating
light. But by adding Phosphorus (P, atomic number 15), as a third dopant the overall wavelength
of the emitted radiation is reduced to below 680nm giving visible red light to the human eye.
Further refinements in the doping process of the PN junction have resulted in a range of colours
spanning the spectrum of visible light as we have seen above as well as infra-red and ultra-violet
wavelengths.
By mixing together a variety of semiconductor, metal and gas compounds the following list of
LEDs can be produced.
- Gallium Arsenide (GaAs) - infra-red
Typical LED Characteristics
Semiconductor
Material
Wavelength Colour V
F
@ 20mA
GaAs 850-940nm Infra-Red 1.2v
GaAsP 630-660nm Red 1.8v
GaAsP 605-620nm Amber 2.0v
GaAsP:N 585-595nm Yellow 2.2v
AlGaP 550-570nm Green 3.5v
SiC 430-505nm Blue 3.6v
GaInN 450nm White 4.0v
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- Gallium Arsenide Phosphide (GaAsP) - red to infra-red, orange
- Aluminium Gallium Arsenide Phosphide (AlGaAsP) - high-brightness red, orange-red, orange,
and yellow
- Gallium Phosphide (GaP) - red, yellow and green
- Aluminium Gallium Phosphide (AlGaP) - green
- Gallium Nitride (GaN) - green, emerald green
- Gallium Indium Nitride (GaInN) - near ultraviolet, bluish-green and blue
- Silicon Carbide (SiC) - blue as a substrate
- Zinc Selenide (ZnSe) - blue
- Aluminium Gallium Nitride (AlGaN) - ultraviolet
Like conventional PN junction diodes, LEDs are current-dependent devices with its forward
voltage drop V
F
, depending on the semiconductor compound (its light colour) and on the forward
biased LED current. The point where conduction begins and light is produced is about 1.2V for a
standard red LED to about 3.6V for a blue LED. The exact voltage drop will of course depend on
the manufacturer because of the different dopant materials and wavelengths used. The voltage
drop across the LED at a particular current value, for example 20mA, will also depend on the
initial conduction V
F
point. As an LED is effectively a diode, its forward current to voltage
characteristics curves can be plotted for each diode colour as shown below.
Light Emitting Diodes I-V Characteristics.
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Light Emitting Diode (LED) Schematic symbol and its I-V Characteristics Curves showing
the different colours available.
Before a light emitting diode can "emit" any form of light it needs a current to flow through it, as
it is a current dependant device with their light output intensity being direct ly proportional to the
forward current flowing through the LED. As the LED is to be connected in a forward bias
condition across a power supply it should be current limited using a series resistor to protect it
from excessive current flow. Never connect an LED directly to a battery or power supply as it
will be destroyed almost instantly because too much current will pass through and burn it out.
From the table above we can see that each LED has its own forward voltage drop across the PN
junction and this parameter which is determined by the semiconductor material used, is the
forward voltage drop for a specified amount of forward conduction current, typically for a
forward current of 20mA. In most cases LEDs are operated from a low voltage DC supply, with
a series resistor, R
S
used to limit the forward current to a safe value from say 5mA for a simple
LED indicator to 30mA or more where a high brightness light output is needed.
LED Series Resistance.
The series resistor value R
S
is calculated by simply using Ohms Law, by knowing the required
forward current I
F
of the LED, the supply voltage V
S
across the combination and the expected
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forward voltage drop of the LED, V
F
at the required current level, the current limiting resistor is
calculated as:
LED Series Resistor Circuit

Example No1
An amber coloured LED with a forward volt drop of 2 volts is to be connected to a 5.0v
stabilised DC power supply. Using the circuit above calculate the value of the series resistor
required to limit the forward current to less than 10mA. Also calculate the current flowing
through the diode if a 100 series resistor is used instead of the calculated first.
1). series resistor value at 10mA.

2). with a 100 series resistor.

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We remember from the Resistors tutorials, that resistors come in standard preferred values. Our
first calculation above shows to limit the current flowing through the LED to 10mA exactly, we
would require a 300 resistor. In the E12 series of resistors there is no 300 resistor so we
would need to choose the next highest value, which is 330. A quick re-calculation shows the
new forward current value is now 9.1mA, and this is ok.
LED Driver Circuits
Now that we know what is an LED, we need some way of controlling it by switching it "ON"
and "OFF". The output stages of both TTL and CMOS logic gates can both source and sink
useful amounts of current therefore can be used to drive an LED. Normal integrated circuits
(ICs) have an output drive current of up to 50mA in the sink mode configuration, but have an
internally limited output current of about 30mA in the source mode configuration. Either way the
LED current must be limited to a safe value using a series resistor as we have already seen.
Below are some examples of driving light emitting diodes using inverting ICs but the idea is the
same for any type of integrated circuit output whether combinational or sequential.
IC Driver Circuit

If more than one LED requires driving at the same time, such as in large LED arrays, or the load
current is to high for the integrated circuit or we may just want to use discrete components
instead of ICs, then an alternative way of driving the LEDs using either bipolar NPN or PNP
transistors as switches is given below. Again as before, a series resistor, R
S
is required to limit
the LED current.
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Transistor Driver Circuit


The brightness of a light emitting diode cannot be controlled by simply varying the current
flowing through it. Allowing more current to flow through the LED will make it glow brighter
but will also cause it to dissipate more heat. LEDs are designed to produce a set amount of light
operating at a specific forward current ranging from about 10 to 20mA. In situations were power
savings are important, less current may be possible. However, reducing the current to below say
5mA may dim its light output too much or even turn the LED "OFF" completely. A much better
way to control the brightness of LEDs is to use a control process known as "Pulse Width
Modulation" or PWM, in which the LED is repeatedly turned "ON" and "OFF" at varying
frequencies depending upon the required light intensity.
LED Light Intensity using PWM
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When higher light outputs are required, a pulse width modulated current with a fairly short duty
cycle ("ON-OFF" Ratio) allows the diode current and therefore the output light intensity to be
increased significantly during the actual pulses, while still keeping the LEDs "average current
level" and power dissipation within safe limits. This "ON-OFF" flashing condition does not
affect what is seen as the human eyes fills in the gaps between the "ON" and "OFF" light pulses,
providing the pulse frequency is high enough, making it appear as a continuous light output. So
pulses at a frequency of 100Hz or more actually appear brighter to the eye than a continuous
light of the same average intensity.
Multi-coloured Light Emitting Diode
LEDs are available in a wide range of shapes, colours and various sizes with different light
output intensities available, with the most common (and cheapest to produce) being the standard
5mm Red Gallium Arsenide Phosphide (GaAsP) LED. LED's are also available in various
"packages" arranged to produce both letters and numbers with the most common being that of
the "seven segment display" arrangement. Nowadays, full colour flat screen LED displays are
available with a large number of dedicated ICs available for driving the displays directly. Most
light emitting diodes produce just a single output of coloured light however, multi-coloured
LEDs are now available that can produce a range of different colours from within a single
device. Most of these are actually two or three LEDs fabricated within a single package.
Bicolour Light Emitting Diodes
A bicolour light emitting diode has two LEDs chips connected together in "inverse parallel" (one
forwards, one backwards) combined in one single package. Bicolour LEDs can produce any one
of three colours for example, a red colour is emitted when the device is connected with current
flowing in one direction and a green colour is emitted when it is biased in the other direction.
This type of bi-directional arrangement is useful for giving polarity indication, for example, the
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correct connection of batteries or power supplies etc. Also, a bi-directional current produces both
colours mixed together as the two LEDs would take it in turn to illuminate if the device was
connected (via a suitable resistor) to a low voltage, low frequency AC supply.
A Bicolour LED

LED
Selected
Terminal A
AC
+ -
LED 1 ON OFF ON
LED 2 OFF ON ON
Colour Green Red Yellow

Tricolour Light Emitting Diodes
The most popular type of tricolour LED comprises of a single Red and a Green LED combined
in one package with their cathode terminals connected together producing a three terminal
device. They are called tricolour LEDs because they can give out a single red or a green colour
by turning "ON" only one LED at a time. They can also generate additional shades of colours
(the third colour) such as Orange or Yellow by turning "ON" the two LEDs in different ratios of
forward current as shown in the table thereby generating 4 different colours from just two diode
junctions.
A Multi or Tricolour LED

Output
Colour
Red Orange Yellow Green
LED 1
Current
0 5mA 9.5mA 15mA
LED 2
Current
10mA 6.5mA 3.5mA 0


LED Displays
As well as individual colour or multi-colour LEDs, several light emitting diodes can be
combined together within a single package to produce displays such as bargraphs, strips, arrays
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and seven segment displays. A seven segment LED display provides a very convenient way
when decoded properly of displaying information or digital data in the form of numbers, letters
or even alpha-numerical characters and as their name suggests, they consist of seven individual
LEDs (the segments), within one single display package.
In order to produce the required numbers or characters from 0 to 9 and A to F respectively, on
the display the correct combination of LED segments need to be illuminated. A standard seven
segment LED display generally has eight input connections, one for each LED segment and one
that acts as a common terminal or connection for all the internal segments.
- The Common Cathode Display (CCD) - In the common cathode display, all the cathode
connections of the LEDs are joined together and the individual segments are illuminated by
application of a HIGH, logic "1" signal.
-
- The Common Anode Display (CAD) - In the common anode display, all the anode connections
of the LEDs are joined together and the individual segments are illuminated by connecting the
terminals to a LOW, logic "0" signal.
A Typical Seven Segment LED Display

Opto-coupler
Finally, another useful application of light emitting diodes is Opto-coupling. An opto-coupler or
opto-isolator as it is also called, is a single electronic device that consists of a light emitting
diode combined with either a photo-diode, photo-transistor or photo-triac to provide an optical
signal path between an input connection and an output connection while maintaining electrical
isolation between two circuits. An opto-isolator consists of a light proof plastic body that has a
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typical breakdown voltages between the input (photo-diode) and the output (photo-transistor)
circuit of up to 5000 volts. This electrical isolation is especially useful where the signal from a
low voltage circuit such as a battery powered circuit, computer or microcontroller, is required to
operate or control another external circuit operating at a potentially dangerous mains voltage.
Photo-diode and Photo-transistor Opto-couplers

The two components used in an opto-isolator, an optical transmitter such as an infra-red emitting
Gallium Arsenide LED and an optical receiver such as a photo-transistor are closely optically
coupled and use light to send signals and/or information between its input and output. This
allows information to be transferred between circuits without an electrical connection or
common ground potential. Opto-isolators are digital or switching devices, so they transfer either
"ON-OFF" control signals or digital data. Analogue signals can be transferred by means of
frequency or pulse-width modulation.

Diode
From Wikipedia, the free encyclopedia
Jump to: navigation, search


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Figure 1: Closeup of a diode, showing the square shaped semiconductor crystal (black object on
left).


Figure 2: Various semiconductor diodes. Bottom: A bridge rectifier. In most diodes, a white or
black painted band identifies the cathode terminal, that is, the terminal which conventional
current flows out of when the diode is conducting.


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Figure 3: Structure of a vacuum tube diode. The filament may be bare, or more commonly (as
shown here), embedded within and insulated from an enclosing cathode
In electronics, a diode is a two-terminal electronic component that conducts electric current in
only one direction. The term usually refers to a semiconductor diode, the most common type
today. This is a crystalline piece of semiconductor material connected to two electrical
terminals.
[1]
A vacuum tube diode (now little used except in some high-power technologies) is a
vacuum tube with two electrodes; a plate and a cathode.
The most common function of a diode is to allow an electric current to pass in one direction
(called the diode's forward direction) while blocking current in the opposite direction (the
reverse direction). Thus, the diode can be thought of as an electronic version of a check valve.
This unidirectional behavior is called rectification, and is used to convert alternating current to
direct current, and to extract modulation from radio signals in radio receivers.
However, diodes can have more complicated behavior than this simple on-off action, due to their
complex non-linear electrical characteristics, which can be tailored by varying the construction
of their P-N junction. These are exploited in special purpose diodes that perform many different
functions. For example, specialized diodes are used to regulate voltage (Zener diodes), to
electronically tune radio and TV receivers (varactor diodes), to generate radio frequency
oscillations (tunnel diodes), and to produce light (light emitting diodes).
Diodes were the first semiconductor electronic devices. The discovery of crystals' rectifying
abilities was made by German physicist Ferdinand Braun in 1874. The first semiconductor
diodes, called cat's whisker diodes were made of crystals of minerals such as galena. Today most
diodes are made of silicon, but other semiconductors such as germanium are sometimes used.
[2]

o
Thermionic and gaseous state diodes


Figure 4: The symbol for an indirect heated vacuum tube diode. From top to bottom, the
components are the anode, the cathode, and the heater filament.
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Thermionic diodes are thermionic-valve devices (also known as vacuum tubes, tubes, or valves),
which are arrangements of electrodes surrounded by a vacuum within a glass envelope. Early
examples were fairly similar in appearance to incandescent light bulbs.
In thermionic valve diodes, a current through the heater filament indirectly heats the cathode,
another internal electrode treated with a mixture of barium and strontium oxides, which are
oxides of alkaline earth metals; these substances are chosen because they have a small work
function. (Some valves use direct heating, in which a tungsten filament acts as both heater and
cathode.) The heat causes thermionic emission of electrons into the vacuum. In forward
operation, a surrounding metal electrode called the anode is positively charged so that it
electrostatically attracts the emitted electrons. However, electrons are not easily released from
the unheated anode surface when the voltage polarity is reversed. Hence, any reverse flow is
negligible.
For much of the 20th century, thermionic valve diodes were used in analog signal applications,
and as rectifiers in many power supplies. Today, valve diodes are only used in niche applications
such as rectifiers in electric guitar and high-end audio amplifiers as well as specialized high-
voltage equipment.
Semiconductor diodes
A modern semiconductor diode is made of a crystal of semiconductor like silicon that has
impurities added to it to create a region on one side that contains negative charge carriers
(electrons), called n-type semiconductor, and a region on the other side that contains positive
charge carriers (holes), called p-type semiconductor. The diode's terminals are attached to each
of these regions. The boundary within the crystal between these two regions, called a PN
junction, is where the action of the diode takes place. The crystal conducts conventional current
in a direction from the p-type side (called the anode) to the n-type side (called the cathode), but
not in the opposite direction.
Another type of semiconductor diode, the Schottky diode, is formed from the contact between a
metal and a semiconductor rather than by a p-n junction.
Currentvoltage characteristic
A semiconductor diodes behavior in a circuit is given by its currentvoltage characteristic, or I
V graph (see graph below). The shape of the curve is determined by the transport of charge
carriers through the so-called depletion layer or depletion region that exists at the p-n junction
between differing semiconductors. When a p-n junction is first created, conduction band
(mobile) electrons from the N-doped region diffuse into the P-doped region where there is a
large population of holes (vacant places for electrons) with which the electrons recombine.
When a mobile electron recombines with a hole, both hole and electron vanish, leaving behind
an immobile positively charged donor (dopant) on the N-side and negatively charged acceptor
(dopant) on the P-side. The region around the p-n junction becomes depleted of charge carriers
and thus behaves as an insulator.
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However, the width of the depletion region (called the depletion width) cannot grow without
limit. For each electron-hole pair that recombines, a positively charged dopant ion is left behind
in the N-doped region, and a negatively charged dopant ion is left behind in the P-doped region.
As recombination proceeds more ions are created, an increasing electric field develops through
the depletion zone which acts to slow and then finally stop recombination. At this point, there is
a built-in potential across the depletion zone.
If an external voltage is placed across the diode with the same polarit y as the built-in potential,
the depletion zone continues to act as an insulator, preventing any significant electric current
flow (unless electron/hole pairs are actively being created in the junction by, for instance, light.
see photodiode). This is the reverse bias phenomenon. However, if the polarity of the external
voltage opposes the built-in potential, recombination can once again proceed, resulting in
substantial electric current through the p-n junction (i.e. substantial numbers of electrons and
holes recombine at the junction). For silicon diodes, the built-in potential is approximately 0.7 V
(0.3 V for Germanium and 0.2 V for Schottky). Thus, if an external current is passed through the
diode, about 0.7 V will be developed across the diode such that the P-doped region is positive
with respect to the N-doped region and the diode is said to be turned on as it has a forward
bias.

Figure 5: IV characteristics of a P-N junction diode (not to scale).
A diodes 'IV characteristic' can be approximated by four regions of operation (see the figure
at right).
At very large reverse bias, beyond the peak inverse voltage or PIV, a process called reverse
breakdown occurs which causes a large increase in current (i.e. a large number of electrons and
holes are created at, and move away from the pn junction) that usually damages the device
permanently. The avalanche diode is deliberately designed for use in the avalanche region. In the
zener diode, the concept of PIV is not applicable. A zener diode contains a heavily doped p-n
junction allowing electrons to tunnel from the valence band of the p-type material to the
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conduction band of the n-type material, such that the reverse voltage is clamped to a known
value (called the zener voltage), and avalanche does not occur. Both devices, however, do have a
limit to the maximum current and power in the clamped reverse voltage region. Also, following
the end of forward conduction in any diode, there is reverse current for a short time. The device
does not attain its full blocking capability until the reverse current ceases.
The second region, at reverse biases more positive than the PIV, has only a very small reverse
saturation current. In the reverse bias region for a normal P-N rectifier diode, the current through
the device is very low (in the A range). However, this is temperature dependent, and at
suffiently high temperatures, a substantial amount of reverse current can be observed (mA or
more).
The third region is forward but small bias, where only a small forward current is conducted.
As the potential difference is increased above an arbitrarily defined cut-in voltage or on-
voltage or diode forward voltage drop (V
d
), the diode current becomes appreciable (the level
of current considered appreciable and the value of cut-in voltage depends on the application),
and the diode presents a very low resistance. The currentvoltage curve is exponential. In a
normal silicon diode at rated currents, the arbitrary cut-in voltage is defined as 0.6 to 0.7 volts.
The value is different for other diode types Schottky diodes can be rated as low as 0.2 V,
Germanium diodes 0.25-0.3 V, and red or blue light-emitting diodes (LEDs) can have values of
1.4 V and 4.0 V respectively.
At higher currents the forward voltage drop of the diode increases. A drop of 1 V to 1.5 V is
typical at full rated current for power diodes.
Shockley diode equation
The Shockley ideal diode equation or the diode law (named after transistor co-inventor William
Bradford Shockley, not to be confused with tetrode inventor Walter H. Schottky) gives the IV
characteristic of an ideal diode in either forward or reverse bias (or no bias). The equation is:

where
I is the diode current,
I
S
is the reverse bias saturation current (or scale current),
V
D
is the voltage across the diode,
V
T
is the thermal voltage, and
n is the ideality factor, also known as the quality factor or sometimes emission
coefficient. The ideality factor n varies from 1 to 2 depending on the fabrication process
and semiconductor material and in many cases is assumed to be approximately equal to 1
(thus the notation n is omitted).
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The thermal voltage V
T
is approximately 25.85 mV at 300 K, a temperature close to room
temperature commonly used in device simulation software. At any temperature it is a known
constant defined by:

where k is the Boltzmann constant, T is the absolute temperature of the p-n junction, and q is the
magnitude of charge on an electron (the elementary charge).
The Shockley ideal diode equation or the diode law is derived with the assumption that the only
processes giving rise to the current in the diode are drift (due to electrical field), diffusion, and
thermal recombination-generation. It also assumes that the recombination-generation (R-G)
current in the depletion region is insignificant. This means that the Shockley equation doesnt
account for the processes involved in reverse breakdown and photon-assisted R-G. Additionally,
it doesnt describe the leveling off of the IV curve at high forward bias due to internal
resistance.
Under reverse bias voltages (see Figure 5) the exponential in the diode equation is negligible,
and the current is a constant (negative) reverse current value of I
S
. The reverse breakdown
region is not modeled by the Shockley diode equation.
For even rather small forward bias voltages (see Figure 5) the exponential is very large because
the thermal voltage is very small, so the subtracted 1 in the diode equation is negligible and the
forward diode current is often approximated as

The use of the diode equation in circuit problems is illustrated in the article on diode modeling.
Small-signal behaviour
For circuit design, a small-signal model of the diode behavior often proves useful. A specific
example of diode modeling is discussed in the article on small-signal circuits.
Types of semiconductor diode

Diode
Zener
diode
Schottky
diode
Tunnel
diode
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Light-emitting
diode
Photodiode Varicap Silicon controlled rectifier
Figure 6: Some diode symbols.


Figure 7: Typical diode packages in same alignment as diode symbol. Thin bar depicts the
cathode.


Figure 8: Several types of diodes. The scale is centimeters.
There are several types of junction diodes, which either emphasize a different physical aspect of
a diode often by geometric scaling, doping level, choosing the right electrodes, are just an
application of a diode in a special circuit, or are really different devices like the Gunn and laser
diode and the MOSFET:
Normal (p-n) diodes, which operate as described above, are usually made of doped silicon or,
more rarely, germanium. Before the development of modern silicon power rectifier diodes,
cuprous oxide and later selenium was used; its low efficiency gave it a much higher forward
voltage drop (typically 1.41.7 V per cell, with multiple cells stacked to increase the peak
inverse voltage rating in high voltage rectifiers), and required a large heat sink (often an
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extension of the diodes metal substrate), much larger than a silicon diode of the same current
ratings would require. The vast majority of all diodes are the p-n diodes found in CMOS
integrated circuits, which include two diodes per pin and many other internal diodes.
Avalanche diodes
Diodes that conduct in the reverse direction when the reverse bias voltage exceeds the
breakdown voltage. These are electrically very similar to Zener diodes, and are often
mistakenly called Zener diodes, but break down by a different mechanism, the avalanche
effect. This occurs when the reverse electric field across the p-n junction causes a wave of
ionization, reminiscent of an avalanche, leading to a large current. Avalanche diodes are
designed to break down at a well-defined reverse voltage without being destroyed. The
difference between the avalanche diode (which has a reverse breakdown above about
6.2 V) and the Zener is that the channel length of the former exceeds the mean free
path of the electrons, so there are collisions between them on the way out. The only
practical difference is that the two types have temperature coefficients of opposite
polarities.
Cats whisker or crystal diodes
These are a type of point-contact diode. The cats whisker diode consists of a thin or
sharpened metal wire pressed against a semiconducting crystal, typically galena or a
piece of coal.
[9]
The wire forms the anode and the crystal forms the cathode. Cats
whisker diodes were also called crystal diodes and found application in crystal radio
receivers. Cats whisker diodes are generally obsolete, but may be available from a few
manufacturers.
[citation needed]

Constant current diodes
These are actually a JFET
[10]
with the gate shorted to the source, and function like a two-
terminal current-limiter analog to the Zener diode, which is limiting voltage. They allow
a current through them to rise to a certain value, and then level off at a specific value.
Also called CLDs, constant-current diodes, diode-connected transistors, or current-
regulating diodes.
Esaki or tunnel diodes
These have a region of operation showing negative resistance caused by quantum
tunneling, thus allowing amplification of signals and very simple bistable circuits. These
diodes are also the type most resistant to nuclear radiation.
Gunn diodes
These are similar to tunnel diodes in that they are made of materials such as GaAs or InP
that exhibit a region of negative differential resistance. With appropriate biasing, dipole
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domains form and travel across the diode, allowing high frequency microwave oscillators
to be built.
Light-emitting diodes (LEDs)
In a diode formed from a direct band-gap semiconductor, such as gallium arsenide,
carriers that cross the junction emit photons when they recombine with the majority
carrier on the other side. Depending on the material, wavelengths (or colors)
[11]
from the
infrared to the near ultraviolet may be produced.
[12]
The forward potential of these diodes
depends on the wavelength of the emitted photons: 1.2 V corresponds to red, 2.4 V to
violet. The first LEDs were red and yellow, and higher-frequency diodes have been
developed over time. All LEDs produce incoherent, narrow-spectrum light; white
LEDs are actually combinations of three LEDs of a different color, or a blue LED with a
yellow scintillator coating. LEDs can also be used as low-efficiency photodiodes in
signal applications. An LED may be paired with a photodiode or phototransistor in the
same package, to form an opto-isolator.
Laser diodes
When an LED-like structure is contained in a resonant cavity formed by polishing the
parallel end faces, a laser can be formed. Laser diodes are commonly used in optical
storage devices and for high speed optical communication.
Peltier diodes
These diodes are used as sensors, heat engines for thermoelectric cooling. Charge carriers
absorb and emit their band gap energies as heat.
Photodiodes
All semiconductors are subject to optical charge carrier generation. This is typically an
undesired effect, so most semiconductors are packaged in light blocking material.
Photodiodes are intended to sense light(photodetector), so they are packaged in materials
that allow light to pass, and are usually PIN (the kind of diode most sensitive to light).
[13]

A photodiode can be used in solar cells, in photometry, or in optical communications.
Multiple photodiodes may be packaged in a single device, either as a linear array or as a
two-dimensional array. These arrays should not be confused with charge-coupled
devices.
Point-contact diodes
These work the same as the junction semiconductor diodes described above, but their
construction is simpler. A block of n-type semiconductor is built, and a conducting sharp-
point contact made with some group-3 metal is placed in contact with the semiconductor.
Some metal migrates into the semiconductor to make a small region of p-type
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semiconductor near the contact. The long-popular 1N34 germanium version is still used
in radio receivers as a detector and occasionally in specialized analog electronics.
PIN diodes
A PIN diode has a central un-doped, or intrinsic, layer, forming a p-type/intrinsic/n-type
structure.
[14]
They are used as radio frequency switches and attenuators. They are also
used as large volume ionizing radiation detectors and as photodetectors. PIN diodes are
also used in power electronics, as their central layer can withstand high voltages.
Furthermore, the PIN structure can be found in many power semiconductor devices, such
as IGBTs, power MOSFETs, and thyristors.
Schottky diodes
Schottky diodes are constructed from a metal to semiconductor contact. They have a
lower forward voltage drop than p-n junction diodes. Their forward voltage drop at
forward currents of about 1 mA is in the range 0.15 V to 0.45 V, which makes them
useful in voltage clamping applications and prevention of transistor saturation. They can
also be used as low loss rectifiers although their reverse leakage current is generally
higher than that of other diodes. Schottky diodes are majority carrier devices and so do
not suffer from minority carrier storage problems that slow down many other diodes
so they have a faster reverse recovery than p-n junction diodes. They also tend to have
much lower junction capacitance than p-n diodes which provides for high switching
speeds and their use in high-speed circuitry and RF devices such as switched-mode
power supply, mixers and detectors.
Super barrier diodes
Super barrier diodes are rectifier diodes that incorporate the low forward voltage drop of
the Schottky diode with the surge-handling capability and low reverse leakage current of
a normal p-n junction diode.
Gold-doped diodes
As a dopant, gold (or platinum) acts as recombination centers, which help a fast
recombination of minority carriers. This allows the diode to operate at signal frequencies,
at the expense of a higher forward voltage drop. Gold doped diodes are faster than other
p-n diodes (but not as fast as Schottky diodes). They also have less reverse-current
leakage than Schottky diodes (but not as good as other p-n diodes).
[15][16]
A typical
example is the 1N914.
Snap-off or Step recovery diodes
The term step recovery relates to the form of the reverse recovery characteristic of these
devices. After a forward current has been passing in an SRD and the current is interrupted
or reversed, the reverse conduction will cease very abruptly (as in a step waveform).
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SRDs can therefore provide very fast voltage transitions by the very sudden
disappearance of the charge carriers.
Transient voltage suppression diode (TVS)
These are avalanche diodes designed specifically to protect other semiconductor devices
from high-voltage transients.
[17]
Their p-n junctions have a much larger cross-sectional
area than those of a normal diode, allowing them to conduct large currents to ground
without sustaining damage.
Varicap or varactor diodes
These are used as voltage-controlled capacitors. These are important in PLL (phase-
locked loop) and FLL (frequency-locked loop) circuits, allowing tuning circuits, such as
those in television receivers, to lock quickly, replacing older designs that took a long time
to warm up and lock. A PLL is faster than an FLL, but prone to integer harmonic locking
(if one attempts to lock to a broadband signal). They also enabled tunable oscillators in
early discrete tuning of radios, where a cheap and stable, but fixed-frequency, crystal
oscillator provided the reference frequency for a voltage-controlled oscillator.
Zener diodes
Diodes that can be made to conduct backwards. This effect, called Zener breakdown,
occurs at a precisely defined voltage, allowing the diode to be used as a precision voltage
reference. In practical voltage reference circuits Zener and switching diodes are
connected in series and opposite directions to balance the temperature coefficient to near
zero. Some devices labeled as high-voltage Zener diodes are actually avalanche diodes
(see above). Two (equivalent) Zeners in series and in reverse order, in the same package,
constitute a transient absorber (or Transorb, a registered trademark). The Zener diode is
named for Dr. Clarence Melvin Zener of Southern Illinois University, inventor of the
device.
Other uses for semiconductor diodes include sensing temperature, and computing analog
logarithms (see Operational amplifier applications#Logarithmic).
Numbering and coding schemes
There are a number of common, standard and manufacturer-driven numbering and coding
schemes for diodes; the two most common being the EIA/JEDEC standard and the European Pro
Electron standard:
EIA/JEDEC
A standardized 1N-series numbering system was introduced in the US by EIA/JEDEC (Joint
Electron Device Engineering Council) about 1960. Among the most popular in this series were:
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1N34A/1N270 (Germanium signal), 1N914/1N4148 (Silicon signal), 1N4001-1N4007 (Silicon
1A power rectifier) and 1N54xx (Silicon 3A power rectifier)
[18][19][20]

Pro Electron
The European Pro Electron coding system for active components was introduced in 1966 and
comprises two letters followed by the part code. The first letter represents the semiconductor
material used for the component (A = Germanium and B = Silicon) and the second letter
represents the general function of the part (for diodes: A = low-power/signal, B = Variable
capacitance, X = Multiplier, Y = Rectifier and Z = Voltage reference), for example:
- AA-series germanium low-power/signal diodes (e.g.: AA119)
- BA-series silicon low-power/signal diodes (e.g.: BAT18 Silicon RF Switching Diode)
- BY-series silicon rectifier diodes (e.g.: BY127 1250V, 1A rectifier diode)
- BZ-series silicon zener diodes (e.g.: BZY88C4V7 4.7V zener diode)
Other common numbering / coding systems (generally manufacturer-driven) include:
- GD-series germanium diodes (ed: GD9) this is a very old coding system
- OA-series germanium diodes (e.g.: OA47) a coding sequence developed by Mullard, a
UK company
As well as these common codes, many manufacturers or organisations have their own systems
too for example:
- HP diode 1901-0044 = JEDEC 1N4148
- UK military diode CV448 = Mullard type OA81 = GEC type GEX23
Related devices
- Rectifier
- Transistor
- Thyristor or silicon controlled rectifier (SCR)
- TRIAC
- Diac
- Varistor
In optics, an equivalent device for the diode but with laser light would be the Optical isolator,
also known as an Optical Diode, that allows light to only pass in one direction. It uses a Faraday
rotator as the main component.
Applications
Radio demodulation
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The first use for the diode was the demodulation of amplitude modulated (AM) radio broadcasts.
The history of this discovery is treated in depth in the radio article. In summary, an AM signal
consists of alternating positive and negative peaks of voltage, whose amplitude or envelope is
proportional to the original audio signal. The diode (originally a crystal diode) rectifies the AM
radio frequency signal, leaving an audio signal which is the original audio signal, minus
atmospheric noise. The audio is extracted using a simple filter and fed into an audio amplifier or
transducer, which generates sound waves.
Power conversion
Rectifiers are constructed from diodes, where they are used to convert alternating current (AC)
electricity into direct current (DC). Automotive alternators are a common example, where the
diode, which rectifies the AC into DC, provides better performance than the commutator of
earlier dynamo. Similarly, diodes are also used in CockcroftWalton voltage multipliers to
convert AC into higher DC voltages.
Over-voltage protection
Diodes are frequently used to conduct damaging high voltages away from sensitive electronic
devices. They are usually reverse-biased (non-conducting) under normal circumstances. When
the voltage rises above the normal range, the diodes become forward-biased (conducting). For
example, diodes are used in (stepper motor and H-bridge) motor controller and relay circuits to
de-energize coils rapidly without the damaging voltage spikes that would otherwise occur. (Any
diode used in such an application is called a flyback diode). Many integrated circuits also
incorporate diodes on the connection pins to prevent external voltages from damaging their
sensitive transistors. Specialized diodes are used to protect from over-voltages at higher power
(see Diode types above).
Logic gates
Diodes can be combined with other components to construct AND and OR logic gates. This is
referred to as diode logic.
Ionizing radiation detectors
In addition to light, mentioned above, semiconductor diodes are sensitive to more energetic
radiation. In electronics, cosmic rays and other sources of ionizing radiation cause noise pulses
and single and multiple bit errors. This effect is sometimes exploited by particle detectors to
detect radiation. A single particle of radiation, with thousands or millions of electron volts of
energy, generates many charge carrier pairs, as its energy is deposited in the semiconductor
material. If the depletion layer is large enough to catch the whole shower or to stop a heavy
particle, a fairly accurate measurement of the particles energy can be made, simply by
measuring the charge conducted and without the complexity of a magnetic spectrometer or etc.
These semiconductor radiation detectors need efficient and uniform charge collection and low
leakage current. They are often cooled by liquid nitrogen. For longer range (about a centimetre)
particles they need a very large depletion depth and large area. For short range particles, they
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need any contact or un-depleted semiconductor on at least one surface to be very thin. The back-
bias voltages are near breakdown (around a thousand volts per centimetre). Germanium and
silicon are common materials. Some of these detectors sense position as well as energy. They
have a finite life, especially when detecting heavy particles, because of radiation damage. Silicon
and germanium are quite different in their ability to convert gamma rays to electron showers.
Semiconductor detectors for high energy particles are used in large numbers. Because of energy
loss fluctuations, accurate measurement of the energy deposited is of less use.
Temperature measurements
A diode can be used as a temperature measuring device, since the forward voltage drop across
the diode depends on temperature, as in a Silicon bandgap temperature sensor. From the
Shockley ideal diode equation given above, it appears the voltage has a positive temperature
coefficient (at a constant current) but depends on doping concentration and operating
temperature (Sze 2007). The temperature coefficient can be negative as in typical thermistors or
positive for temperature sense diodes down to about 20 kelvins. Typically, silicon diodes have
approximately 2 mV/C temperature coefficient at room temperature.
Current steering
Diodes will prevent currents in unintended directions. To supply power to an electrical circuit
during a power failure, the circuit can draw current from a battery. An Uninterruptible power
supply may use diodes in this way to ensure that current is only drawn from the battery when
necessary. Similarly, small boats typically have two circuits each with their own
battery/batteries: one used for engine starting; one used for domestics. Normally both are
charged from a single alternator, and a heavy duty split charge diode is used to prevent the higher
charge battery (typically the engine battery) from discharging through the lower charged battery
when the alternator is not running.
Diodes are also used in electronic musical keyboards. To reduce the amount of wiring needed in
electronic musical keyboards, these instruments often use keyboard matrix circuits. The
keyboard controller scans the rows and columns to determine which note the player has pressed.
The problem with matrix circuits is that when several notes are pressed at once, the current can
flow backwards through the circuit and trigger "phantom keys" that cause ghost notes to play.
To avoid triggering unwanted notes, most keyboard matrix circuits have diodes soldered with the
switch under each key of the musical keyboard. The same principle is also used for the switch
matrix in solid state pinball machines.

p-n Junction Diode
Diode:
A pure silicon crystal or germanium crystal is known as an intrinsic semiconductor. There are
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not enough free electrons and holes in an intrinsic semi-conductor to produce a usable current.
The electrical action of these can be modified by doping means adding impurity atoms to a
crystal to increase either the number of free holes or no of free electrons.
When a crystal has been doped, it is called a extrinsic semi-conductor. They are of two types
n-type semiconductor having free electrons as majority carriers
p-type semiconductor having free holes as majority carriers
By themselves, these doped materials are of little use. However, if a junction is made by joining
p-type semiconductor to n-type semiconductor a useful device is produced known as diode. It
will allow current to flow through it only in one direction. The unidirectional properties of a
diode allow current flow when forward biased and disallow current flow when reversed biased.
This is called rectification process and therefore it is also called rectifier.
How is it possible that by properly
joining two semiconductors each of
which, by itself, will freely conduct
the current in any direct refuses to
allow conduction in one direction.
Consider first the condition of p-
type and n-type germanium just
prior to joining fig. 1. The majority
and minority carriers are in constant
motion.
The minority carriers are thermally
produced and they exist only for
short time after which they
recombine and neutralize each
other. In the mean time, other
minority carriers have been
produced and this process goes on
and on.
The number of these electron hole
pair that exist at any one time
depends upon the temperature. The
number of majority carriers is
however, fixed depending on the
number of impurity atoms available.
While the electrons and holes are in
motion but the atoms are fixed in
place and do not move.

Fig.1
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As soon as, the junction is formed, the following processes are initiated fig. 2.

Fig.2
- Holes from the p-side diffuse into n-side where
they recombine with free electrons.
- Free electrons from n-side diffuse into p-side
where they recombine with free holes.
- The diffusion of electrons and holes is due to the
fact that large no of electrons are concentrated in
one area and large no of holes are concentrated in
another area.
- When these electrons and holes begin to diffuse
across the junction then they collide each other
and negative charge in the electrons cancels the
positive charge of the hole and both will lose their
charges.
- The diffusion of holes and electrons is an electric
current referred to as a recombination current.
The recombination process decay exponentially
with both time and distance from the junction.
Thus most of the recombination occurs just after
the junction is made and very near to junction.
- A measure of the rate of recombination is
the lifetime defined as the time required for the
density of carriers to decrease to 37% to the
original concentration
The impurity atoms are fixed in their individual places. The atoms itself is a part of the crystal
and so cannot move. When the electrons and hole meet, their individual charge is cancelled and
this leaves the originating impurity atoms with a net charge, the atom that produced the electron
now lack an electronic and so becomes charged positively, whereas the atoms that produced the
hole now lacks a positive charge and becomes negative.
The electrically charged atoms are called ions since they are no longer neutral. These ions
produce an electric field as shown in fig. 3. After several collisions occur, the electric field is
great enough to repel rest of the majority carriers away of the junction. For example, an electron
trying to diffuse from n to p side is repelled by the negative charge of the p-side. Thus diffusion
process does not continue indefinitely but continues as long as the field is developed.
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Fig.3
This region is produced immediately surrounding the junction that has no majority carriers. The
majority carriers have been repelled away from the junction and junction is depleted from
carriers. The junction is known as the barrier region or depletion region. The electric field
represents a potential difference across the junction also called space charge potential or barrier
potential . This potential is 0.7v for Si at 25
o
celcious and 0.3v for Ge.
The physical width of the depletion region depends on the doping level. If very heavy doping is
used, the depletion region is physically thin because diffusion charge need not travel far across
the junction before recombination takes place (short life time). If doping is light, then depletion
is more wide (long life time).

p-n Junction Diode
The symbol of diode is shown in fig. 4. The terminal connected to p-layer is called anode (A) and the terminal connected to
n-layer is called cathode (K)




Fig.4
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Reverse Bias:
If positive terminal of dc source is connected to cathode and negative terminal is connected to anode, the diode is called reverse
biased as shown in fig. 5.

Fig.5
When the diode is reverse biased then the depletion region width increases, majority carriers move away from the junction and
there is no flow of current due to majority carriers but there are thermally produced electron hole pair also. If these elect rons and
holes are generated in the vicinity of junction then there is a flow of current. The negative voltage applied to the diode will tend to
attract the holes thus generated and repel the electrons. At the same time, the positive voltage will attract the electrons towards the
battery and repel the holes. This will cause current to flow in the circuit. This current is usually very small (interms of micro amp
to nano amp). Since this current is due to minority carriers and these number of minority carriers are fixed at a given temperature
therefore, the current is almost constant known as reverse saturation current I
CO
.
In actual diode, the current is not almost constant but increases slightly with voltage. This is due to surface leakage current. The
surface of diode follows ohmic law (V=IR). The resistance under reverse bias condition is very high 100k to mega ohms. When
the reverse voltage is increased, then at certain voltage, then breakdown to diode takes place and it conducts heavily. This is due
to avalanche or zener breakdown. The characteristic of the diode is shown in fig. 6.
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Fig.6
Forward bias:
When the diode is forward bias, then majority carriers are pushed towards junction, when they collide and recombination takes
place. Number of majority carriers are fixed in semiconductor. Therefore as each electron is eliminated at the junction, a new
electron must be introduced, this comes from battery. At the same time, one hole must be created in p-layer. This is formed by
extracting one electron from p-layer. Therefore, there is a flow of carriers and thus flow of current.

Diode
Space charge capacitance C
T
of diode:
Reverse bias causes majority carriers to move away from the junction, thereby creating more
ions. Hence the thickness of depletion region increases. This region behaves as the dielectric
material used for making capacitors. The p-type and n-type conducting on each side of dielectric
act as the plate. The incremental capacitance C
T
is defined by

Since
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Therefore, (E-1)
where, dQ is the increase in charge caused by a change dV in voltage. C
T
is not constant, it
depends upon applied voltage, there fore it is defined as dQ / dV.
When p-n junction is forward biased, then also a capacitance is defined called diffusion
capacitance C
D
(rate of change of injected charge with voltage) to take into account the time
delay in moving the charges across the junction by the diffusion process. It is considered as a
fictitious element that allow us to predict time delay.
If the amount of charge to be moved across the junction is increased, the time delay is greater, it
follows that diffusion capacitance varies directly with the magnitude of forward current.
(E-2)
Relationship between Diode Current and Diode Voltage
An exponential relationship exists between the carrier density and applied potential of diode
junction as given in equation E-3. This exponential relationship of the current i
D
and the voltage
v
D
holds over a range of at least seven orders of magnitudes of current - that is a factor of 10
7
.
(E-3)
Where,
i
D
= Current through the diode (dependent variable in this expression)
v
D
= Potential difference across the diode terminals (independent variable in this expression)
I
O
= Reverse saturation current (of the order of 10
-15
A for small signal diodes, but I
O
is a strong
function of temperature)
q = Electron charge: 1.60 x 10
-19
joules/volt
k = Boltzmann's constant: 1.38 x l0
-23
joules / K
T = Absolute temperature in degrees Kelvin (K = 273 + temperature in C)
n = Empirical scaling constant between 0.5 and 2, sometimes referred to as the Exponential
Ideality Factor
The empirical constant, n, is a number that can vary according to the voltage and current levels.
It depends on electron drift, diffusion, and carrier recombination in the depletion region. Among
the quantities affecting the value of n are the diode manufacture, levels of doping and purity of
materials. If n=1, the value of k T/ q is 26 mV at 25C. When n=2, k T/ q becomes 52 mV.
For germanium diodes, n is usually considered to be close to 1. For silicon diodes, n is in the
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range of 1.3 to 1.6. n is assumed 1 for all junctions all throughout unless otherwise noted.
Equation (E-3) can be simplified by defining V
T
=k T/q, yielding
(E-4)
At room temperature (25C) with forward-bias voltage only the first term in the parentheses is
dominant and the current is approximately given by
(E-5)
The current-voltage (l-V) characteristic of the diode, as defined by (E-3) is illustrated in fig. 1.
The curve in the figure consists of two exponential curves. However, the exponent values are
such that for voltages and currents experienced in practical circuits, the curve sections are close
to being straight lines. For voltages less than V
ON
, the curve is approximated by a straight line of
slope close to zero. Since the slope is the conductance (i.e., i / v), the conductance is very small
in this region, and the equivalent resistance is very high. For voltages above V
ON
, the curve is
approximated by a straight line with a very large slope. The conductance is therefore very large,
and the diode has a very small equivalent resistance.

Fig.1 - Diode Voltage relationship
The slope of the curves of fig.1 changes as the current and voltage change since the l-V
characteristic follows the exponential relationship of relationship of equation (E-4). Differentiate
the equation (E-4) to find the slope at any arbitrary value of v
D
or i
D
,
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(E-6)
This slope is the equivalent conductance of the diode at the specified values of v
D
or i
D
.
We can approximate the slope as a linear function of the diode current. To eliminate the
exponential function, we substitute equation (E-4) into the exponential of equation (E-7) to
obtain
(E-7)
A realistic assumption is that I
O
<< i
D
equation (E-7) then yields,
(E-8)
The approximation applies if the diode is forward biased. The dynamic resistance is the
reciprocal of this expression.
(E-9)
Although r
d
is a function of i
d
, we can approximate it as a constant if the variation of i
D
is small.
This corresponds to approximating the exponential function as a straight line within a specific
operating range.
Normally, the term R
f
to denote diode forward resistance. R
f
is composed of r
d
and the contact
resistance. The contact resistance is a relatively small resistance composed of the resistance of
the actual connection to the diode and the resistance of the semiconductor prior to the junction.
The reverse-bias resistance is extremely large and is often approximated as infinity.

Diode
Temperature Effects:
Temperature plays an important role in determining the characteristic of diodes. As temperature
increases, the turn-on voltage, v
ON
, decreases. Alternatively, a decrease in temperature results in
an increase in v
ON
. This is illustrated in fig. 2, where V
ON
varies linearly with temperature which
is evidenced by the evenly spaced curves for increasing temperature in 25 C increments.
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The temperature relationship is described by equation
V
ON
(T
New
) V
ON
(T
room
) = k
T
(T
New
T
room
) (E-10)

Fig. 2 - Dependence of iD on temperature versus vD for real diode (kT = -2.0 mV /C)
where,
T
room
= room temperature, or 25C.
T
New
= new temperature of diode in C.
V
ON
(T
room
) = diode voltage at room temperature.
V
ON
(T
New
) = diode voltage at new temperature.
k
T
= temperature coefficient in V/C.
Although k
T
varies with changing operating parameters, standard engineering practice permits
approximation as a constant. Values of k
T
for the various types of diodes at room temperature are
given as follows:
k
T
= -2.5 mV/C for germanium diodes
k
T
= -2.0 mV/C for silicon diodes
The reverse saturation current, I
O
also depends on temperature. At room temperature, it increases
approximately 16% per C for silicon and 10% per C for germanium diodes. In other words, I
O

approximately doubles for every 5 C increase in temperature for silicon, and for every 7 C for
germanium. The expression for the reverse saturation current as a function of temperature can be
approximated as
(E-11)
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where K
i
= 0.15/C ( for silicon) and T1 and T2 are two arbitrary temperatures.

Applications of Diode
Applications of diode:
Half wave Rectifier:
The single phase half wave rectifier is shown in fig. 8.


Fig. 8 Fig. 9
In positive half cycle, D is forward biased and conducts. Thus the output voltage is same as the
input voltage. In the negative half cycle, D is reverse biased, and therefore output voltage is zero.
The output voltage waveform is shown in fig. 9.
The average output voltage of the rectifier is given by

The average output current is given by

When the diode is reverse biased, entire transformer voltage appears across the diode. The
maximum voltage across the diode is V
m
. The diode must be capable to withstand this voltage.
Therefore PIV half wave rating of diode should be equal to V
m
in case of single-phase rectifiers.
The average current rating must be greater than I
avg

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Full Wave Rectifier:
A single phase full wave rectifier using center tap transformer is shown in fig. 10. It supplies
current in both half cycles of the input voltage.


Fig. 10 Fig. 11
In the first half cycle D
1
is forward biased and conducts. But D
2
is reverse biased and does not
conduct. In the second half cycle D
2
is forward biased, and conducts and D
1
is reverse biased. It
is also called 2 pulse midpoint converter because it supplies current in both the half cycles. The
output voltage waveform is shown in fig. 11.
The average output voltage is given by

and the average load current is given by

When D
1
conducts, then full secondary voltage appears across D
2
, therefore PIV rating of the
diode should be 2 V
m
.

Bridge Rectifier:
The single phase full wave bridge rectifier is shown in fig. 1. It is the most widely used
rectifier. It also provides currents in both the half cycle of input supply.
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Fig. 1 Fig. 2
In the positive half cycle, D
1
& D
4
are forward biased and D
2
& D
3
are reverse biased. In the
negative half cycle, D
2
& D
3
are forward biased, and D
1
& D
4
are reverse biased. The output
voltage waveform is shown in fig. 2 and it is same as full wave rectifier but the advantage is that
PIV rating of diodes are V m and only single secondary transformer is required.
The main disadvantage is that it requires four diodes. When low dc voltage is required then
secondary voltage is low and diodes drop (1.4V) becomes significant. For low dc output, 2-pulse
center tap rectifier is used because only one diode drop is there.
The ripple factor is the measure of the purity of dc output of a rectifier and is defined as

Therefore,


Zener Diode:
The diodes designed to work in breakdown region are called zener diode. If the reverse voltage
exceeds the breakdown voltage, the zener diode will normally not be destroyed as long as the
current does not exceed maximum value and the device closes not over load.
When a thermally generated carrier (part of the reverse saturation current) falls down the
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junction and acquires energy of the applied potential, the carrier collides with crystal ions and
imparts sufficient energy to disrupt a covalent bond. In addition to the original carrier, a new
electron-hole pair is generated. This pair may pick up sufficient energy from the applied field to
collide with another crystal ion and create still another electron-hole pair. This action continues
and thereby disrupts the covalent bonds. The process is referred to as impact ionization,
avalanche multiplication or avalanche breakdown.
There is a second mechanism that disrupts the covalent bonds. The use of a sufficiently strong
electric field at the junction can cause a direct rupture of the bond. If the electric field exerts a
strong force on a bound electron, the electron can be torn from the covalent bond thus causing
the number of electron-hole pair combinations to multiply. This mechanism is called high field
emission or Zener breakdown. The value of reverse voltage at which this occurs is controlled by
the amount ot doping of the diode. A heavily doped diode has a low Zener breakdown voltage,
while a lightly doped diode has a high Zener breakdown voltage.
At voltages above approximately 8V, the predominant mechanism is the avalanche breakdown.
Since the Zener effect (avalanche) occurs at a predictable point, the diode can be used as a
voltage reference. The reverse voltage at which the avalanche occurs is called the breakdown or
Zener voltage.
A typical Zener diode characteristic is shown in fig. 1. The circuit symbol for the Zener diode is
different from that of a regular diode, and is illustrated in the figure. The maximum reverse
current, I
Z(max)
, which the Zener diode can withstand is dependent on the design and construction
of the diode. A design guideline that the minimum Zener current, where the characteristic curve
remains at V
Z
(near the knee of the curve), is 0.1/ I
Z(max)
.

Fig. 1 - Zener diode characteristic
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The power handling capacity of these diodes is better. The power dissipation of a zener diode
equals the product of its voltage and current.
P
Z
= V
Z
I
Z

The amount of power which the zener diode can withstand ( V
Z
.I
Z(max)
) is a limiting factor in
power supply design.
Zener Regulator:
When zener diode is forward biased it works as a diode and drop across it is 0.7 V. When it
works in breakdown region the voltage across it is constant (V
Z
) and the current through diode is
decided by the external resistance. Thus, zener diode can be used as a voltage regulator in the
configuration shown in fig. 2 for regulating the dc voltage. It maintains the output voltage
constant even through the current through it changes.


Fig. 2 Fig. 3
The load line of the circuit is given by V
s
= I
s
R
s
+ V
z
. The load line is plotted along with zener
characteristic in fig. 3. The intersection point of the load line and the zener characteristic gives
the output voltage and zener current.
To operate the zener in breakdown region V
s
should always be greater then V
z
. R
s
is used to limit
the current. If the V
s
voltage changes, operating point also changes simultaneously but voltage
across zener is almost constant. The first approximation of zener diode is a voltage source of V
z

magnitude and second approximation includes the resistance also. The two approximate
equivalent circuits are shown in fig. 4.
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If second approximation of zener diode is considered, the output voltage varies slightly as shown
in fig. 5. The zener ON state resistance produces more I * R drop as the current increases. As the
voltage varies form V
1
to V
2
the operating point shifts from Q
1
to Q
2
.
The voltage at Q
1
is
V
1
= I
1
R
Z
+V
Z

and at Q
2

V
2
= I
2
R
Z
+V
Z

Thus, change in voltage is
V
2
V
1
= ( I
2
I
1
) R
Z

V
Z
= I
Z
R
Z


Zener Diode
Design of Zener regulator circuit:
A zenere regulator circuit is shown in fig. 6. The varying load current is represented by a
variable load resistance R
L
.
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The zener will work in the breakdown
region only if the Thevenin voltage across
zener is more than V
Z
.

If zener is operating in breakdown region,
the current through R
S
is given by


Fig. 6
and load current
I
s
= I
z
+ I
L

The circuit is designed such that the diode always operates in the breakdown region and the
voltage V
Z
across it remains fairly constant even though the current I
Z
through it vary
considerably.
If the load I
L
should increase, the current I
Z
should decrease by the same percentage in order to
maintain load current constant I
s
. This keeps the voltage drop across R
s
constant and hence the
output voltage.
If the input voltage should increase, the zener diode passes a larger current, that extra voltage is
dropped across the resistance R
s
. If input voltage falls, the current I
Z
falls such that V
Z
is
constant.
In the practical application the source voltage, v
s
, varies and the load current also varies. The
design challenge is to choose a value of R
s
which permits the diode to maintain a relatively
constant output voltage, even when the input source voltage varies and the load current also
varies.
We now analyze the circuit to determine the proper choice of R
s
. For the circuit shown in figure,
(E-1)
(E-2)
The variable quantities in Equation (E-2) are v
Z
and i
L
. In order to assure that the diode remains
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in the constant voltage (breakdown) region, we examine the two extremes of input/output
conditions, as follows:
- The current through the diode, i
Z
, is a minimum (I
Z min
) when the load current, i
L
is
maximum (I
L max
) and the source voltage, v
s
is minimum (V
s min
).
- The current through the diode, i
Z
, is a maximum (I
Z max
) when the load current, i
L
, is
minmum (i
L min
) and the source voltage v
s
is minimum(V
s max
).
When these characteristics of the two extremes are inserted into Equation (E-1),
we find (E-3)
(E-4)
In a practical problem, we know the range of input voltages, the range of output load currents,
and the desired Zener voltage. Equation (E-4) thus represents one equation in two unknowns, the
maximum and minimum Zener current. A second equation is found from the characteristic of
zener. To avoid the non-constant portion of the characteristic curve, we use an accepted rule of
thumb that the minimum Zener current should be 0.1 times the maximum (i.e., 10%), that is,
(E-5)
Solving the equations E-4 and E-5, we get,
(E-6)
Now that we can solve for the maximum Zener current, the value of R
s
, is calculated from
Equation (E-3).
Zener diodes are manufactured with breakdown voltages V
Z
in the range of a few volts to a few
hundred volts. The manufacturer specifies the maximum power the diode can dissipate. For
example, a 1W, 10 V zener can operate safely at currents up to 100mA.


Special Purpose Diodes
Light Emitting Diode :
In a forward biased diode free electrons cross the junction and enter into p-layer where they
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recombine with holes. Each recombination radiates energy as electron falls from higher energy
level to a lower energy level. I n ordinary diodes this energy is in the form of heat. In light
emitting diode, this energy is in the form of light.
The symbol of LED is shown in fig. 2. Ordinary diodes are made of Ge or Si. This material
blocks the passage of light. LEDs are made of different materials such as gallium, arsenic and
phosphorus. LEDs can radiate red, green, yellow, blue, orange or infrared (invisible). The LED's
forward voltage drop is more approximately 1.5V. Typical LED current is between 10 mA to 50
mA.



Fig. 2 Fig. 3
Seven Segment Display :
Seven segment displays are used to display digits and few alphabets. It contains seven
rectangular LEDs. Each LED is called a Segment. External resistors are used to limit the currents
to safe Values. It can display any letters a, b, c, d, e, f, g.as shown in fig. 3.
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Fig. 4
The LEDs of seven-segment display are connected in either in common anode configuration or
in common cathode configuration as shown in fig. 4.
Photo diode :
When a diode is reversed biased as shown in fig. 5, a reverse current flows due to minority
carriers. These carriers exist because thermal energy keeps on producing free electrons and
holes. The lifetime of the minority carriers is short, but while they exist they can contribute to the
reverse current. When light energy bombards a p-n junction, it too can produce free electrons.
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Fig. 5
In other words, the amount of light striking the junction can control the reverse current in a
diode. A photo diode is made on the same principle. It is sensitive to the light. In this diode,
through a window light falls to the junction. The stronger the light, the greater the minority
carriers and larger the reverse current.


Voltage regulator
A voltage regulator is an electrical regulator designed to automatically maintain a constant
voltage level.
It may use an electromechanical mechanism, or passive or active electronic components.
Depending on the design, it may be used to regulate one or more AC or DC voltages.
With the exception of passive shunt regulators, all modern electronic voltage regulators operate
by comparing the actual output voltage to some internal fixed reference voltage. Any difference
is amplified and used to control the regulation element in such a way as to reduce the voltage
error. This forms a negative feedback control loop; increasing the open-loop gain tends to
increase regulation accuracy but reduce stability (avoidance of oscillation, or ringing during step
changes). There will also be a trade-off between stability and the speed of the response to
changes. If the output voltage is too low (perhaps due to input voltage reducing or load current
increasing), the regulation element is commanded, up to a point, to produce a higher output
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voltage - by dropping less of the input voltage (for linear series regulators and buck switching
regulators), or to draw input current for longer periods (boost-type switching regulators); if the
output voltage is too high, the regulation element will normally be commanded to produce a
lower voltage. However, many regulators have over-current protection, so that they will entirely
stop sourcing current (or limit the current in some way) if the output current is too high, and
some regulators may also shut down if the input voltage is outside a given range.
Measures of regulator quality
The output voltage can only be held roughly constant; the regulation is specified by two
measurements:
- load regulation is the change in output voltage for a given change in load current (for
example: "typically 15mV, maximum 100mV for load currents between 5mA and 1.4A,
at some specified temperature and input voltage").
- line regulation or input regulation is the degree to which output voltage changes with
input (supply) voltage changes - as a ratio of output to input change (for example
"typically 13mV/V"), or the output voltage change over the entire specified input voltage
range (for example "plus or minus 2% for input voltages between 90V and 260V, 50-
60Hz").
Other important parameters are:
- Temperature coefficient of the output voltage is the change in output voltage with
temperature (perhaps averaged over a given temperature range), while...
- Initial accuracy of a voltage regulator (or simply "the voltage accuracy") reflects the
error in output voltage for a fixed regulator without taking into account temperature or
aging effects on output accuracy.
- Dropout voltage - the minimum difference between input voltage and output voltage for
which the regulator can still supply the specified current. A Low Drop-Out (LDO)
regulator is designed to work well even with an input supply only a Volt or so above the
output voltage.
- Absolute Maximum Ratings are defined for regulator components, specifying the
continuous and peak output currents that may be used (sometimes internally limited), the
maximum input voltage, maximum power dissipation at a given temperature, etc.
- Output noise (thermal white noise) and output dynamic impedance may be specified as
graphs versus frequency, while output ripple noise (mains "hum" or switch-mode "hash"
noise) may be given as peak-to-peak or RMS voltages, or in terms of their spectra.
- Quiescent current in a regulator circuit is the current drawn internally, not available to
the load, normally measured as the input current while no load is connected (and hence a
source of inefficiency; some linear regulators are, surprisingly, more efficient at very low
current loads than switch-mode designs because of this).


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Electromechanical regulators


Circuit design for a simple electromechanical voltage regulator.


Interior of an old electromechanical voltage regulator.


Graph of voltage output on a time scale.
In older electromechanical regulators, voltage regulation is easily accomplished by coiling the
sensing wire to make an electromagnet. The magnetic field produced by the current attracts a
moving ferrous core held back under spring tension or gravitational pull. As voltage increases, so
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does the current, strengthening the magnetic field produced by the coil and pulling the core
towards the field. The magnet is physically connected to a mechanical power switch, which
opens as the magnet moves into the field. As voltage decreases, so does the current, releasing
spring tension or the weight of the core and causing it to retract. This closes the switch and
allows the power to flow once more.
If the mechanical regulator design is sensitive to small voltage fluctuations, the motion of the
solenoid core can be used to move a selector switch across a range of resistances or transformer
windings to gradually step the output voltage up or down, or to rotate the position of a moving-
coil AC regulator.
Early automobile generators and alternators had a mechanical voltage regulator using one, two,
or three relays and various resistors to stabilize the generator's output at slightly more than 6 or
12 V, independent of the engine's rpm or the varying load on the vehicle's electrical system.
Essentially, the relay(s) employed pulse width modulation to regulate the output of the generator,
controlling the field current reaching the generator (or alternator) and in this way controlling the
output voltage produced.
The regulators used for generators (but not alternators) also disconnect the generator when it was
not producing electricity, thereby preventing the battery from discharging back into the generator
and attempting to run it as a motor. The rectifier diodes in an alternator automatically perform
this function so that a specific relay is not required; this appreciably simplified the regulator
design.
More modern designs now use solid state technology (transistors) to perform the same function
that the relays perform in electromechanical regulators.
Electromechanical regulators are used for mains voltage stabilisationsee Voltage
regulator#AC voltage stabilizers below.
Coil-rotation AC voltage regulator


Basic design principle and circuit diagram for the rotating-coil AC voltage regulator.
This is an older type of regulator used in the 1920s that uses the principle of a fixed-position
field coil and a second field coil that can be rotated on an axis in parallel with the fixed coil.
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When the movable coil is positioned perpendicular to the fixed coil, the magnetic forces acting
on the movable coil balance each other out and voltage output is unchanged. Rotating the coil in
one direction or the other away from the center position will increase or decrease voltage in the
secondary movable coil.
This type of regulator can be automated via a servo control mechanism to advance the movable
coil position in order to provide voltage increase or decrease. A braking mechanism or high ratio
gearing is used to hold the rotating coil in place against the powerful magnetic forces acting on
the moving coil.
AC voltage stabilizers


Magnetic mains regulator
Electromechanical
Electromechanical regulators, usually called voltage stabilizers, have also been used to regulate
the voltage on AC power distribution lines. These regulators operate by using a servomechanism
to select the appropriate tap on an autotransformer with multiple taps, or by moving the wiper on
a continuously variable autotransfomer. If the output voltage is not in the acceptable range, the
servomechanism switches connections or moves the wiper to adjust the voltage into the
acceptable region. The controls provide a deadband wherein the controller will not act,
preventing the controller from constantly adjusting the voltage ("hunting") as it varies by an
acceptably small amount.
Constant-voltage transformer
An alternative method is the use of a type of saturating transformer called a ferro resonant
transformer or constant-voltage transformer. These transformers use a tank circuit composed
of a high-voltage resonant winding and a capacitor to produce a nearly constant average output
with a varying input. The ferroresonant approach is attractive due to its lack of active
components, relying on the square loop saturation characteristics of the tank circuit to absorb
variations in average input voltage. Older designs of ferroresonant transformers had an output
with high harmonic content, leading to a distorted output waveform. Modern devices are used to
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construct a perfect sinewave. The ferroresonant action is a flux limiter rather than a voltage
regulator, but with a fixed supply frequency it can maintain an almost constant average output
voltage even as the input voltage varies widely.
The ferroresonant transformers, which are also known as Constant Voltage Transformers (CVTs)
or ferros, are also good surge suppressors, as they provide high isolation and inherent short-
circuit protection.
A ferroresonant transformer can operate with an input voltage range 40% or more of the
nominal voltage.
Output power factor remains in the range of 0.96 or higher from half to full load.
Because it regenerates an output voltage waveform, output distortion, which is typically less than
4%, is independent of any input voltage distortion, including notching.
Efficiency at full load is typically in the range of 89% to 93%. However, at low loads, efficiency
can drop below 60% and no load losses can be as high as 20%. The current-limiting capability
also becomes a handicap when a CVT is used in an application with moderate to high inrush
current like motors, transformers or magnets. In this case, the CVT has to be sized to
accommodate the peak current, thus forcing it to run at low loads and poor efficiency.
Minimum maintenance is required. Transformers and capacitors can be very reliable. Some units
have included redundant capacitors to allow several capacitors to fail between inspections
without any noticeable effect on the device's performance.
Output voltage varies about 1.2% for every 1% change in supply frequency. For example, a 2-Hz
change in generator frequency, which is very large, results in an output voltage change of only
4%, which has little effect for most loads.
It accepts 100% single-phase switch-mode power supply loading without any requirement for
derating, including all neutral components.
Input current distortion remains less than 8% THD even when supplying nonlinear loads with
more than 100% current THD.
Drawbacks of CVTs (constant voltage transformers) are their larger size, audible humming
sound, and high heat generation.




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DC voltage stabilizers


KPEH8A stabilizers
Many simple DC power supplies regulate the voltage using a shunt regulator such as a zener
diode, avalanche breakdown diode, or voltage regulator tube. Each of these devices begins
conducting at a specified voltage and will conduct as much current as required to hold its
terminal voltage to that specified voltage. The power supply is designed to only supply a
maximum amount of current that is within the safe operating capability of the shunt regulating
device (commonly, by using a series resistor). In shunt regulators, the voltage reference is also
the regulating device.
If the stabilizer must provide more power, the shunt regulator output is only used to provide the
standard voltage reference for the electronic device, known as the voltage stabilizer. The voltage
stabilizer is the electronic device, able to deliver much larger currents on demand.
Active regulators
Active regulators employ at least one active (amplifying) component such as a transistor or
operational amplifier. Shunt regulators are often (but not always) passive and simple, but always
inefficient because they (essentially) dump the excess current not needed by the load. When
more power must be supplied, more sophisticated circuits are used. In general, these active
regulators can be divided into several classes:
- Linear series regulators
- Switching regulators
- SCR regulators
Linear regulators
Linear regulators are based on devices that operate in their linear region (in contrast, a switching
regulator is based on a device forced to act as an on/off switch). In the past, one or more vacuum
tubes were commonly used as the variable resistance. Modern designs use one or more
transistors instead, perhaps within an Integrated Circuit. Linear designs have the advantage of
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very "clean" output with little noise introduced into their DC output, but are most often much
less efficient and unable to step-up or invert the input voltage like switched supplies. All linear
regulators require a higher input than the output. All linear regulators are subject to the parameter
of dropout voltage.
Entire linear regulators are available as integrated circuits. These chips come in either fixed or
adjustable voltage types.
Switching regulators
Switching regulators rapidly switch a series device on and off. The duty cycle of the switch sets
how much charge is transferred to the load. This is controlled by a similar feedback mechanism
as in a linear regulator. Because the series element is either fully conducting, or switched off, it
dissipates almost no power; this is what gives the switching design its efficiency. Switching
regulators are also able to generate output voltages which are higher than the input, or of
opposite polarity something not possible with a linear design.
Like linear regulators, nearly-complete switching regulators are also available as integrated
circuits. Unlike linear regulators, these usually require one external component: an inductor that
acts as the energy storage element. (Large-valued inductors tend to be physically large relative to
almost all other kinds of componentry, so they are rarely fabricated within integrated circuits and
IC regulators with some exceptions.
[1]
)
Comparing linear vs. switching regulators
The two types of regulators have their different advantages:
- Linear regulators are best when low output noise (and low RFI radiated noise) is required
- Linear regulators are best when a fast response to input and output disturbances is
required.
- At low levels of power, linear regulators are cheaper and occupy less printed circuit
board space.
- Switching regulators are best when power efficiency is critical (such as in portable
computers), except linear regulators are more efficient in a small number of cases (such
as a 5V microprocessor often in "sleep" mode fed from a 6V battery, if the complexity of
the switching circuit and the junction capacitance charging current means a high
quiescent current in the switching regulator).
- Switching regulators are required when the only power supply is a DC voltage, and a
higher output voltage is required.
- At high levels of power (above a few watts), switching regulators are cheaper (for
example, the cost of removing heat generated is less).
SCR regulators
Regulators powered from AC power circuits can use silicon controlled rectifiers (SCRs) as the
series device. Whenever the output voltage is below the desired value, the SCR is triggered,
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allowing electricity to flow into the load until the AC mains voltage passes through zero (ending
the half cycle). SCR regulators have the advantages of being both very efficient and very simple,
but because they can not terminate an on-going half cycle of conduction, they are not capable of
very accurate voltage regulation in response to rapidly-changing loads.
Combination (hybrid) regulators
Many power supplies use more than one regulating method in series. For example, the output
from a switching regulator can be further regulated by a linear regulator. The switching regulator
accepts a wide range of input voltages and efficiently generates a (somewhat noisy) voltage
slightly above the ultimately desired output. That is followed by a linear regulator that generates
exactly the desired voltage and eliminates nearly all the noise generated by the switching
regulator. Other designs may use an SCR regulator as the "pre-regulator", followed by another
type of regulator. An efficient way of creating a variable-voltage, accurate output power supply
is to combine a multi-tapped transformer with an adjustable linear post-regulator.
Voltage stabilizer
A voltage stabilizer is an electronic device able to deliver relatively constant output voltage
while input voltage and load current changes over time.
The voltage stabilizer is the shunt regulator such as a Zener diode or avalanche diode. Each of
these devices begins conducting at a specified voltage and will conduct as much current as
required to hold its terminal voltage to that specified voltage. Hence the shunt regulator can be
viewed as the limited power parallel stabilizer. The shunt regulator output is used as a voltage
reference.
The Zener diode and avalanche diode have opposite threshold voltage dependence on
temperature. By connecting these two devices sequentially, it is possible to construct a voltage
reference with improved thermal stability. Sometimes (mostly for the voltages around 5.6 V)
both effects are combined in the same diode.
Simple voltage stabilizer
In the simplest case emitter follower is used, the base of the regulating transistor is directly
connected to the voltage reference:
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The stabilizer uses the power source, having voltage U
in
that may vary over time. It delivers the
relatively constant voltage U
out
. The output load R
L
can also vary over time. For such a device to
work properly, the input voltage must be larger than the output voltage and Voltage drop must
not exceed the limits of the transistor used.
The output voltage of the stabilizer is equal to U
Z
- U
BE
where U
BE
is about 0.7v and depends on
the load current. If the output voltage drops below that limit, this increases the voltage difference
between the base and emitter (U
be
), opening the transistor and delivering more current.
Delivering more current through the same output resistor R
L
increases the voltage again.
Voltage stabilizer with an operational amplifier
The stability of the output voltage can be significantly increased by using the operational
amplifier:

In this case, the operational amplifier opens the transistor more if the voltage at its inverting
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input drops significantly below the output of the voltage reference at the non-inverting input.
Using the voltage divider (R1, R2 and R3) allows choice of the arbitrary output voltage between
U
z
and U
in
.
Zener diode


Current-voltage characteristic of a Zener diode with a breakdown voltage of 17 volt. Notice the
change of voltage scale between the forward biased (positive) direction and the reverse biased
(negative) direction.
A Zener diode is a type of diode that permits current not only in the forward direction like a
normal diode, but also in the reverse direction if the voltage is larger than the breakdown voltage
known as "Zener knee voltage" or "Zener voltage". The device was named after Clarence Zener,
who discovered this electrical property.
A conventional solid-state diode will not allow significant current if it is reverse-biased below its
reverse breakdown voltage. When the reverse bias breakdown voltage is exceeded, a
conventional diode is subject to high current due to avalanche breakdown. Unless this current is
limited by circuitry, the diode will be permanently damaged. In case of large forward bias
(current in the direction of the arrow), the diode exhibits a voltage drop due to its junction built-
in voltage and internal resistance. The amount of the voltage drop depends on the semiconductor
material and the doping concentrations.
A Zener diode exhibits almost the same properties, except the device is specially designed so as
to have a greatly reduced breakdown voltage, the so-called Zener voltage. By contrast with the
conventional device, a reverse-biased Zener diode will exhibit a controlled breakdown and allow
the current to keep the voltage across the Zener diode at the Zener voltage. For example, a diode
with a Zener breakdown voltage of 3.2 V will exhibit a voltage drop of 3.2 V if reverse bias
voltage applied across it is more than its Zener voltage. The Zener diode is therefore ideal for
applications such as the generation of a reference voltage (e.g. for an amplifier stage), or as a
voltage stabilizer for low-current applications.
The Zener diode's operation depends on the heavy doping of its p-n junction allowing electrons
to tunnel from the valence band of the p-type material to the conduction band of the n-type
material. In the atomic scale, this tunneling corresponds to the transport of valence band
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electrons into the empty conduction band states; as a result of the reduced barrier between these
bands and high electric fields that are induced due to the relatively high levels of dopings on both
sides.
[1]
The breakdown voltage can be controlled quite accurately in the doping process. While
tolerances within 0.05% are available, the most widely used tolerances are 5% and 10%.
Breakdown voltage for commonly available zener diodes can vary widely from 1.2 volts to 200
volts.
Another mechanism that produces a similar effect is the avalanche effect as in the avalanche
diode. The two types of diode are in fact constructed the same way and both effects are present
in diodes of this type. In silicon diodes up to about 5.6 volts, the Zener effect is the predominant
effect and shows a marked negative temperature coefficient. Above 5.6 volts, the avalanche
effect becomes predominant and exhibits a positive temperature coefficient
[1]
. In a 5.6 V diode,
the two effects occur together and their temperature coefficients neatly cancel each other out,
thus the 5.6 V diode is the component of choice in temperature-critical applications. Modern
manufacturing techniques have produced devices with voltages lower than 5.6 V with negligible
temperature coefficients, but as higher voltage devices are encountered, the temperature
coefficient rises dramatically. A 75 V diode has 10 times the coefficient of a 12 V diode.
All such diodes, regardless of breakdown voltage, are usually marketed under the umbrella term
of "Zener diode".
-
Uses


Zener diode shown with typical packages. Reverse current i
Z
is shown.
Zener diodes are widely used as voltage references and as shunt regulators to regulate the voltage
across small circuits. When connected in parallel with a variable voltage source so that it is
reverse biased, a Zener diode conducts when the voltage reaches the diode's reverse breakdown
voltage. From that point on, the relatively low impedance of the diode keeps the voltage across
the diode at that value.
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In this circuit, a typical voltage reference or regulator, an input voltage, U
IN
, is regulated down to
a stable output voltage U
OUT
. The intrinsic voltage drop of diode D is stable over a wide current
range and holds U
OUT
relatively constant even though the input voltage may fluctuate over a
fairly wide range. Because of the low impedance of the diode when operated like this, Resistor R
is used to limit current through the circuit.
In the case of this simple reference, the current flowing in the diode is determined using Ohms
law and the known voltage drop across the resistor R. I
Diode
= (U
IN
- U
OUT
) / R

The value of R must satisfy two conditions:
1. R must be small enough that the current through D keeps D in reverse breakdown. The
value of this current is given in the data sheet for D. For example, the common
BZX79C5V6
[2]
device, a 5.6 V 0.5 W Zener diode, has a recommended reverse current of
5 mA. If insufficient current exists through D, then U
OUT
will be unregulated, and less
than the nominal breakdown voltage (this differs to voltage regulator tubes where the
output voltage will be higher than nominal and could rise as high as U
IN
). When
calculating R, allowance must be made for any current through the external load, not
shown in this diagram, connected across U
OUT
.
2. R must be large enough that the current through D does not destroy the device. If the
current through D is I
D
, its breakdown voltage V
B
and its maximum power dissipation
P
MAX
, then I
D
V
B
< P
MAX
.
A load may be placed across the diode in this reference circuit, and as long as the zener stays in
reverse breakdown, the diode will provide a stable voltage source to the load.
A Zener diode used in this way is known as a shunt voltage regulator (shunt, in this context,
meaning connected in parallel, and voltage regulator being a class of circuit that produces a
stable voltage across any load). In a sense, a portion of the current through the resistor is shunted
through the Zener diode, and the rest is through the load. Thus the voltage that the load sees is
controlled by causing some fraction of the current from the power source to bypass ithence the
name, by analogy with locomotive switching points.
Shunt regulators are simple, but the requirements that the ballast resistor be small enough to
avoid excessive voltage drop during worst-case operation (low input voltage concurrent with
high load current) tends to leave a lot of current flowing in the diode much of the time, making
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for a fairly wasteful regulator with high quiescent power dissipation, only suitable for smaller
loads.
Zener diodes in this configuration are often used as stable references for more advanced voltage
regulator circuits.
These devices are also encountered, typically in series with a base-emitter junction, in transistor
stages where selective choice of a device centered around the avalanche/Zener point can be used
to introduce compensating temperature co-efficient balancing of the transistor PN junction. An
example of this kind of use would be a DC error amplifier used in a stabilized power supply
circuit feedback loop system.
Zener diodes are also used in surge protectors to limit transient voltage spikes.
Another notable application of the zener diode is the use of noise caused by its avalanche
breakdown in a random number generator that never repeats.
Voltage Regulators:
An ideal power supply maintains a constant voltage at its output terminals under all operating
conditions. The output voltage of a practical power supply changes with load generally dropping
as load current increases as shown in fig. 1.

Fig. 1
The terminal voltage when full load current is drawn is called full load voltage (V
FL
). The no
load voltage is the terminal voltage when zero current is drawn from the supply, that is, the open
circuit terminal voltage.
Power supply performance is measured in terms of percent voltage regulation, which indicates its
ability to maintain a constant voltage. It is defined as

The Thevenin's equivalent of a power supply is shown in fig. 2. The Thevenin voltage is the no-
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load voltage V
NL
and the Thevenin resistance is called the output resistance R
o
. Let the full load
current be I
FL
. Therefore, the full load resistance R
FL
is given by


Fig. 2
From the equivalent circuit, we have

and the voltage regulation is given by

It is clear that the ideal power supply has zero outut resistance.
Example-1
A power supply having output resistance 1.5 supplies a full load current of 500mA to a 50
load. Determine:
1. percent voltage regulation of the supply
2. no load output voltage.
Solution:
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(a). Full load output voltage V
FL
= (500mA ) (50) = 25V.
Therefore,
(b). The no load voltage
Voltage Regulators:
An unregulated power supply consists of a transformer (step down), a rectifier and a filter. These
power supplies are not good for some applications where constant voltage is required
irrespective of external disturbances. The main disturbances are:
1. As the load current varies, the output voltage also varies because of its poor regulation.
2. The dc output voltage varies directly with ac input supply. The input voltage may vary
over a wide range thus dc voltage also changes.
3. The dc output voltage varies with the temperature if semiconductor devices are used.
An electronic voltage regulator is essentially a controller used along with unregulated power
supply to stabilize the output dc voltage against three major disturbances
a. Load current (I
L
)
b. Supply voltage (V
i
)
c. Temperature (T)
Fig. 3, shows the basic block diagram of voltage regulator. where
V
i
= unregulated dc voltage.
V
o
= regulated dc voltage.

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Fig. 3
Since the output dc voltage V
Lo
depends on the input unregulated dc voltage V
i
, load current I
L

and the temperature t, then the change V
o
in output voltage of a power supply can be expressed
as follows
V
O
= V
O
(V
i
, I
L
, T)
Take partial derivative of V
O
, we get,

S
V
gives variation in output voltage only due to unregulated dc voltage. R
O
gives the output
voltage variation only due to load current. S
T
gives the variation in output voltage only due to
temperature.
The smaller the value of the three coefficients, the better the regulations of power supply. The
input voltage variation is either due to input supply fluctuations or presence of ripples due to
inadequate filtering.
Voltage Regulator:
A voltage regulator is a device designed to maintain the output voltage of power supply nearly
constant. It can be regarded as a closed loop system because it monitors the output voltage and
generates the control signal to increase or decrease the supply voltage as necessary to
compensate for any change in the output voltage. Thus the purpose of voltage regulator is to
eliminate any output voltage variation that might occur because of changes in load, changes in
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supply voltage or changes in temperature.
Zener Voltage Regulator:
The regulated power supply may use zener diode as the voltage controlling device as shown in
fig. 4. The output voltage is determined by the reverse breakdown voltage of the zener diode.
This is nearly constant for a wide range of currents. The load voltage can be maintained constant
by controlling the current through zener.

Fig. 4
The zener diode regulator has limitations of range. The load current range for which regulation is
maintained, is the difference between maximum allowable zener current and minimum current
required for the zener to operate in breakdown region. For example, if zener diode requires a
minimum current of 10 mA and is limited to a maximum of 1A (to prevent excessive
dissipation), the range is 1 - 0.01 = 0.99A. If the load current variation exceeds 0.99A, regulation
may be lost.
Emitter Follower Regulator:
To obtain better voltage regulation in shunt regulator, the zener diode can be connected to the
base circuit of a power transistor as shown in fig. 5. This amplifies the zener current range. It is
also known as emitter follower regulation.

Fig. 5
This configuration reduces the current flow in the diode. The power transistor used in this
configuration is known as pass transistor. The purpose of C
L
is to ensure that the variations in
one of the regulated power supply loads will not be fed to other loads. That is, the capacitor
effectively shorts out high-frequency variations.
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Because of the current amplifying property of the transistor, the current in the zenor dioide is
small. Hence there is little voltage drop across the diode resistance, and the zener approximates
an ideal constant voltage source.
Operation of the circuit:
The current through resistor R is the sum of zener current I
Z
and the transistor base current I
B
( =
I
L
/ ).
I
L
= I
Z
+ I
B

The output voltage across R
L
resistance is given by
V
O
= V
Z
V
BE

Where V
BE
0.7 V
Therefore, V
O
= constant.
The emitter current is same as load current. The current I
R
is assumed to be constant for a given
supply voltage. Therefore, if I
L
increases, it needs more base currents, to increase base current I
z

decreases. The difference in this regulator with zener regulator is that in later case the zener
current decreases (increase) by same amount by which the load current increases (decreases).
Thus the current range is less, while in the shunt regulators, if I
L
increases by I
L
then I
B
should
increase by I
L
/ or I
Z
should decrease by I
L
/ . Therefore the current range control is more
for the same rating zener.
The simplified circuit of the shunt regulator is shown in fig. 6.

Fig. 6
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In a power supply the power regulation is basically, because of its high internal impedance. In
the circuit discussed, the unregulated supply has resistance R
S
of the order of 100 ohm. The use
of emitter follower is to reduce the output resistance and it becomes approximately.
R
O
= ( R
z
+ h
ie
) / (1 + h
fe
)
Where R
Z
represents the dynamic zener resistance. The voltage stabilization ratio S
V
is
approximately
S
V
= V
o
/ V
I
= R
z
/ (R
z
+ R)
S
V
can be improved by increasing R. This increases V
CE
and power dissipated in the transistor.
Other disadvantages of the circuit are.
1. No provision for varying the output voltage since it is almost equal to the zener voltage.
2. Change in V
BE
and V
z
due to temperature variations appear at the output since the
transistor is connected in series with load, it is called series regulator and transistor is
allow series pass transistor.
Design of Series Voltage Regulator:
Fig. 1 shows the basic circuit of a series voltage regulator. The operation of this regulator has
been discussed in previous lecture. It consists of series (pass) transistor to control the output
voltage.

Fig. 1
The circuit can be designed taking two extreme operating conditions,
1. V
S max
, I
Z max
, I
load min
/
2. V
S min
, I
Z min
, I
load max
/
We calculate R
s
for both conditions and since R
si
is constant, we equate these two expressions
as in Equation E-1.
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(E-1)
A design guideline that set I
Z min
= 0.1 I
Z max
. Then we equate the expressions for Equation (E-1)
to obtain,
(E-2)
Solving for I
Z max
, we obtain,
(E-3)
We estimate the load resistance by taking the ratio of the minimum source voltage to the
maximum load current. Since R
load
is large and in parallel, it can be ignored. This is the worst
case since it represents the smallest load and therefore the maximum load current.
(E-4)
The output filter capacitor size can be estimated according to the permissible output voltage
variation and ripple voltage frequency and is given by
(E-5)
Since the voltage gain of an EF amplifier is unity, the output voltage of teh regulated power
supply is,
V
load
=V
Z
- V
BE
(E-6)
The percent regulation of the power supply is given by
(E-7)
Voltage Regulator
The maximum power dissipated in this type of series regulator is the power dissipated in the
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internal pass transistor, which is approximately (V
S max
- V
out
) I
L max
. Hence, as the load current
increases, the power dissipated in the internal pass transistor increases. If I
Load
exceeds 0.75 A,
the IC package should be secured to a heat sink. When this is done, I
Load
can increase to about
1.5 A.
We now focus our attention on the 78XX series of regulators. The last two digits of the IC part
number denote the output voltage of the device. Thus, for example, a 7808 IC package produces
an 8V regulated output. These packages, although internally complex, are inexpensive and easy
to use.
There are a number of different voltages that can be obtained from the 78XX series 1C; they are
5, 6, 8, 8.5, 10, 12, 15, 18, and 24 V. In order to design a regulator around one of these ICs, we
need only select a transformer, diodes, and filter. The physical configuration is shown in fig.
3(a). The ground lead and the metal tab are connected together. This permits direct attachment to
a heat sink for cooling purposes. A typical circuit application is shown in fig. 3(c).

(a)

(b)

(c)
Fig. 3
The specification sheet for this IC indicates that there must be a common ground between the
input and output, and the minimum voltage at the IC input must be above the regulated output. In
order to assure this last condition, it is necessary to filter the output from the rectifier. The C
F
in
fig. 3(b) performs this filtering when combined with the input resistance to the IC. We use an n:1
step down transformer, with the secondary winding center-tapped, to drive a full-wave rectifier.


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The minimum and maximum input voltages for the 78XX family of regulators are shown in
Table-1.
Type Min Max
7805 7 25
7806 8 25
7808 10.5 25
7885 10.5 25
7810 12.5 28
7812 14.5 30
7815 17.5 30
7818 21 33
7824 27 38
Table - 1
We use Table -1 to select the turns ratio, n, for a 78XX regulator. As a design guide, we will
take the average of V
max
and V
min
of the particular IC regulator to calculate n. For example, using
a 7805 regulator, we obtain

The center tap provides division by 2 so the peak voltage out of the rectifier is 115 2 / 2n = 16.
Therefore, n = 5. This is a conservative method of selecting the transformer ratio.
The filter capacitor, C
F
, is chosen to maintain the voltage input range to the regulator as specified
in Table 8.1.
The output capacitor, C
Load
, aids in isolating the effect of the transients that may appear on the
regulated supply line. C
Load
should be a high quality tantalum capacitor with a capacitance of 1.0
F. It should be connected close to the 78XX regulator using short leads in order to improve the
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stability performance.
This family of regulators can also be used for battery powered systems. Fig. 3(c) shows a battery
powered application. The value of C
F
is chosen in the same manner as for the standard filter.
The 79XX series regulator is identical to the 78XX series except that it provides negative
regulated voltages instead of positive.

UNIVERSITY QUESTIONS
1. What is mean by depletion region?
2. Define the transition capacitance of a diode.
3. Differentiate a PN junction diode and zener diode.
4. Give the diode current equation.
5. Define cutin voltage.
6. What assumptions are made while analyzing the motion of an electron in an electric field?
7. How do you increase the conductivity of intrinsic semiconductor?
8. What is Hall effect?
9. How do the transition region width and contact potential across a pn junction vary with the
applied bias voltage?
10. Mention the two mechanisms of breakdown in a pn junction.
11. State law of mass action.
12. Why is the mobility of electrons greater than the holes?
13. The reverse saturation current of a silicon PN junction diode is 10 A.Calculate the diode
current for the forward bias voltage of 0.6 V at 25 C.
14. Give the equation for diode current under reverse bias.
15. Compare LED and LCD.
16. Define the potential-energy barrier.
17. What is mean by Diffusion capacitance?
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18. Name any two material used to manufacture LEDs.
19. What is precision rectifier?
20. Find the ripple factor for a FWR with capacitor filter with the output waveform as shown in
the figure.Assume RL=100 with capacitor C=1000F.
21. Derive the ripple factor for a HWR and FWR .
22. Why a series resistor is necessary when a diode is forward biased?
23. List any four applications of light emitting diode.

BIG QUESTIONS
1. With the volt-ampere characteristics,explain the working principle of the diode and also
explain the static ,dynamic resistance of the diodes? (16)
2. Write a detailed note on:
i) diode switching times (6)
ii)applications of diodes. (4)
iii)capacitance of diodes.(6)
3. Zener diode can be used as a voltage regulator-Justify it.(8)
4. Explain the working of a PN junction diode under various biasing conditions using the
relevant circuit sketch. (16)
5. Explain how a PN junction is formed? (8).
6. Write a note on diode capacitance.(8)
7. Write a detailed note on: LED (8)
8. Describe the conduction of current in an intrinsic semiconductor.(8)
9. Find the conductivity and resistivity of an intrinsic semiconductor at temperature of 300k.It is
given that
Ni=2.5*1013/cm3,n=3800cm2/Sv;
p=1800 cm2/Sv,q=1.6*10-19 C (8)
10. Derive the continuity equation for a semiconductor. (10)
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11.In an N-type semiconductor ,the Fermi-level lies 0.3 Ev below the conduction band at 27 C .If
the temperature is increased to 55 C ,find the new position of the Fermi level.(6)
12. Give the theory of current components of pn junction diode.(10)
13. Determine the ac resistance for a semiconductor diode having a forward bias of 200 mV and
reverse saturation current of 1A at room temperature.(6)
14. Derive an expression for the diffusion capacitance of a pn junction diode. (8)
15. What is zener effect ?Explain the function of a zener diode and draw its characteristics. (10)
16. Write a note on temperature dependence of breakdown voltages. (6)
17. Derive an expression for total current in a semiconductor due to drift and diffusion
phenomena.
18. What is meant by carrier life time? Discuss. (6)
19. Describe the action of the PN junction diode under forward and reverse biased conditions and
hence draw the volt amp characteristics of the diode . (10)
20. Distinguish between avalanche breakdown and zener breakdown. (8)
21.Explain the switching characteristics of PN junction diode. (8)
22. Derive an expression for the current under forward bias and reverse bias. (10)
23. The diode current is 0.6 Ma,when the applied voltage is 400 Mv 20Ma when the applied
voltage is 500Mv.Determine .Assume kt/q=25Mv.
24. Explain the effect of temperature on PN junction diodes. (5)
25. Explain the use of zener diode as voltage regulator . (6)
26.With neat diagram explain the operation of LCD.
27. Discuss the V-I characteristics of P-N junction diode and zener diode.
28. Explain the operation of full wave rectifier. Also derive the expression for its average output
voltage.
29. Derive the ripple factor for FWR with capacitor filter. (10)
30.Explain in detailabout the operation of the following type of filters and derive the ripple
factor for all i)C-filter ii) L-filter iii)LC-filter iv)CLC-filter (16)
31. Explain in detail the operation of the electronic voltage regulators.
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UNIT 2
Bipolar Junction Transistor And Its Applications

Introduction
A bipolar (junction) transistor (BJT) is a three-terminal electronic device constructed of doped
semiconductor material and may be used in amplifying or switching applications. Bipolar
transistors are so named because their operation involves both electrons and holes. Charge flow
in a BJT is due to bidirectional diffusion of charge carriers across a junction between two regions
of different charge concentrations. This mode of operation is contrasted with unipolar
transistors, such as field-effect transistors, in which only one carrier type is involved in charge
flow due to drift. By design, most of the BJT collector current is due to the flow of charges
injected from a high-concentration emitter into the base where they are minority carriers that
diffuse toward the collector, and so BJTs are classified as minority-carrier devices.

PNP

NPN
Schematic symbols for
PNP- and NPN-type
BJTs.
-
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NPN BJT with forward-biased EB junction and reverse-biased BC junction
An NPN transistor can be considered as two diodes with a shared anode. In typical operation, the
base-emitter junction is forward biased and the basecollector junction is reverse biased. In an
NPN transistor, for example, when a positive voltage is applied to the baseemitter junction, the
equilibrium between thermally generated carriers and the repelling electric field of the depletion
region becomes unbalanced, allowing thermally excited electrons to inject into the base region.
These electrons wander (or "diffuse") through the base from the region of high concentration
near the emitter towards the region of low concentration near the collector. The electrons in the
base are called minority carriers because the base is doped p-type which would make holes the
majority carrier in the base.
To minimize the percentage of carriers that recombine before reaching the collectorbase
junction, the transistor's base region must be thin enough that carriers can diffuse across it in
much less time than the semiconductor's minority carrier lifetime. In particular, the thickness of
the base must be much less than the diffusion length of the electrons. The collectorbase junction
is reverse-biased, and so little electron injection occurs from the collector to the base, but
electrons that diffuse through the base towards the collector are swept into the collector by the
electric field in the depletion region of the collectorbase junction. The thin shared base and
asymmetric collectoremitter doping is what differentiates a bipolar transistor from two separate
and oppositely biased diodes connected in series.
Voltage, current, and charge control
The collectoremitter current can be viewed as being controlled by the baseemitter current
(current control), or by the baseemitter voltage (voltage control). These views are related by the
currentvoltage relation of the baseemitter junction, which is just the usual exponential current
voltage curve of a p-n junction (diode).
[1]

The physical explanation for collector current is the amount of minority-carrier charge in the
base region.
[1][2][3]
Detailed models of transistor action, such as the GummelPoon model,
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account for the distribution of this charge explicitly to explain transistor behavior more exactly.
[4]

The charge-control view easily handles phototransistors, where minority carriers in the base
region are created by the absorption of photons, and handles the dynamics of turn-off, or
recovery time, which depends on charge in the base region recombining. However, because base
charge is not a signal that is visible at the terminals, the current- and voltage-control views are
generally used in circuit design and analysis.
In analog circuit design, the current-control view is sometimes used because it is approximately
linear. That is, the collector current is approximately
F
times the base current. Some basic
circuits can be designed by assuming that the emitterbase voltage is approximately constant,
and that collector current is beta times the base current. However, to accurately and reliably
design production BJT circuits, the voltage-control (for example, EbersMoll) model is
required
[1]
. The voltage-control model requires an exponential function to be taken into account,
but when it is linearized such that the transistor can be modelled as a transconductance, as in the
EbersMoll model, design for circuits such as differential amplifiers again becomes a mostly
linear problem, so the voltage-control view is often preferred. For translinear circuits, in which
the exponential IV curve is key to the operation, the transistors are usually modelled as voltage
controlled with transconductance proportional to collector current. In general, transistor level
circuit design is performed using SPICE or a comparable analogue circuit simulator, so model
complexity is usually not of much concern to the designer.
Turn-on, turn-off, and storage delay
The Bipolar transistor exhibits a few delay characteristics when turning on and off. Most
transistors, and especially power transistors, exhibit long base storage time that limits maximum
frequency of operation in switching applications. One method for reducing this storage time is by
using a Baker clamp.
Transistor 'alpha' and 'beta'
The proportion of electrons able to cross the base and reach the collector is a measure of the BJT
efficiency. The heavy doping of the emitter region and light doping of the base region cause
many more electrons to be injected from the emitter into the base than holes to be injected from
the base into the emitter. The common-emitter current gain is represented by
F
or h
fe
; it is
approximately the ratio of the DC collector current to the DC base current in forward-active
region. It is typically greater than 100 for small-signal transistors but can be smaller in transistors
designed for high-power applications. Another important parameter is the common-base current
gain,
F
. The common-base current gain is approximately the gain of current from emitter to
collector in the forward-active region. This ratio usually has a value close to unity; between 0.98
and 0.998. Alpha and beta are more precisely related by the following identities (NPN
transistor):

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Structure

Simplified cross section of a planar NPN bipolar junction transistor


Die of a KSY34 high-frequency NPN transistor, base and emitter connected via bonded wires
A BJT consists of three differently doped semiconductor regions, the emitter region, the base
region and the collector region. These regions are, respectively, p type, n type and p type in a
PNP, and n type, p type and n type in a NPN transistor. Each semiconductor region is connected
to a terminal, appropriately labeled: emitter (E), base (B) and collector (C).
The base is physically located between the emitter and the collector and is made from lightly
doped, high resistivity material. The collector surrounds the emitter region, making it almost
impossible for the electrons injected into the base region to escape being collected, thus making
the resulting value of very close to unity, and so, giving the transistor a large . A cross section
view of a BJT indicates that the collectorbase junction has a much larger area than the emitter
base junction.
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The bipolar junction transistor, unlike other transistors, is usually not a symmetrical device. This
means that interchanging the collector and the emitter makes the transistor leave the forward
active mode and start to operate in reverse mode. Because the transistor's internal structure is
usually optimized for forward-mode operation, interchanging the collector and the emitter makes
the values of and in reverse operation much smaller than those in forward operation; often
the of the reverse mode is lower than 0.5. The lack of symmetry is primarily due to the doping
ratios of the emitter and the collector. The emitter is heavily doped, while the collector is lightly
doped, allowing a large reverse bias voltage to be applied before the collectorbase junction
breaks down. The collectorbase junction is reverse biased in normal operation. The reason the
emitter is heavily doped is to increase the emitter injection efficiency: the ratio of carriers
injected by the emitter to those injected by the base. For high current gain, most of the carriers
injected into the emitterbase junction must come from the emitter.
The low-performance "lateral" bipolar transistors sometimes used in CMOS processes are
sometimes designed symmetrically, that is, with no difference between forward and backward
operation.
Small changes in the voltage applied across the baseemitter terminals causes the current that
flows between the emitter and the collector to change significantly. This effect can be used to
amplify the input voltage or current. BJTs can be thought of as voltage-controlled current
sources, but are more simply characterized as current-controlled current sources, or current
amplifiers, due to the low impedance at the base.
Early transistors were made from germanium but most modern BJTs are made from silicon. A
significant minority are also now made from gallium arsenide, especially for very high speed
applications (see HBT, below).
NPN

The symbol of an NPN bipolar junction transistor
NPN is one of the two types of bipolar transistors, consisting of a layer of P-doped
semiconductor (the "base") between two N-doped layers. A small current entering the base is
amplified in the collector output. That is, an NPN transistor is "on" when its base is pulled high
relative to the emitter.
Most of the NPN current is carried by electrons, moving from emitter to collector as minority
carriers in the P-type base region. Most bipolar transistors used today are NPN, because electron
mobility is higher than hole mobility in semiconductors, allowing greater currents and faster
operation.
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PNP


The symbol of a PNP Bipolar Junction Transistor.
The other type of BJT is the PNP, consisting of a layer of N-doped semiconductor between two
layers of P-doped material. A small current leaving the base is amplified in the collector output.
That is, a PNP transistor is "on" when its base is pulled low relative to the emitter.
The arrows in the NPN and PNP transistor symbols are on the emitter legs and point in the
direction of the conventional current flow when the device is in forward active mode.
A mnemonic device for the NPN / PNP distinction, based on the arrows in their symbols and the
letters in their names, is not pointing in for NPN and pointing in for PNP.
[5]

Heterojunction bipolar transistor


Bands in graded heterojunction NPN bipolar transistor. Barriers indicated for electrons to move
from emitter to base, and for holes to be injected backward from base to emitter; Also, grading of
bandgap in base assists electron transport in base region; Light colors indicate depleted regions
The heterojunction bipolar transistor (HBT) is an improvement of the BJT that can handle
signals of very high frequencies up to several hundred GHz. It is common in modern ultrafast
circuits, mostly RF systems.
[6][7]
Heterojunction transistors have different semiconductors for the
elements of the transistor. Usually the emitter is composed of a larger bandgap material than the
base. The figure shows that this difference in bandgap allows the barrier for holes to inject
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backward into the base, denoted in figure as
p
, to be made large, while the barrier for electrons
to inject into the base
n
is made low. This barrier arrangement helps reduce minority carrier
injection from the base when the emitter-base junction is under forward bias, and thus reduces
base current and increases emitter injection efficiency.
The improved injection of carriers into the base allows the base to have a higher doping level,
resulting in lower resistance to access the base electrode. In the more traditional BJT, also
referred to as homojunction BJT, the efficiency of carrier injection from the emitter to the base is
primarily determined by the doping ratio between the emitter and base, which means the base
must be lightly doped to obtain high injection efficiency, making its resistance relatively high. In
addition, higher doping in the base can improve figures of merit like the Early voltage by
lessening base narrowing.
The grading of composition in the base, for example, by progressively increasing the amount of
germanium in a SiGe transistor, causes a gradient in bandgap in the neutral base, denoted in the
figure by
G
, providing a "built-in" field that assists electron transport across the base. That
drift component of transport aids the normal diffusive transport, increasing the frequency
response of the transistor by shortening the transit time across the base.
Two commonly used HBTs are silicongermanium and aluminum gallium arsenide, though a
wide variety of semiconductors may be used for the HBT structure. HBT structures are usually
grown by epitaxy techniques like MOCVD and MBE.
Regions of operation
Applied voltages
B-E Junction
Bias (NPN)
B-C Junction
Bias (NPN)
Mode (NPN)
E < B < C Forward Reverse Forward active
E < B > C Forward Forward Saturation
E > B < C Reverse Reverse Cut-off
E > B > C Reverse Forward Reverse-active
Bipolar transistors have five distinct regions of operation, defined by BJT junction biases.
The modes of operation can be described in terms of the applied voltages (this description
applies to NPN transistors; polarities are reversed for PNP transistors):
Forward active: base higher than emitter, collector higher than base (in this mode the collector
current is proportional to base current by
F
).
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- Saturation: base higher than emitter, but collector is not higher than base.
- Cut-Off: base lower than emitter, but collector is higher than base. It means the transistor
is not letting conventional current to go through collector to emitter.
- Reverse-action: base lower than emitter, collector lower than base: reverse conventional
current goes through transistor.
Applied voltages
B-E Junction
Bias (PNP)
B-C Junction
Bias (PNP)
Mode (PNP)
E < B < C Reverse Forward Reverse-active
E < B > C Reverse Reverse Cut-off
E > B < C Forward Forward Saturation
E > B > C Forward Reverse Forward active
In terms of junction biasing: ('reverse biased basecollector junction' means Vbc < 0 for NPN,
opposite for PNP)
- Forward-active (or simply, active): The baseemitter junction is forward biased and the base
collector junction is reverse biased. Most bipolar transistors are designed to afford the greatest
common-emitter current gain,
F
, in forward-active mode. If this is the case, the collector
emitter current is approximately proportional to the base current, but many times larger, for
small base current variations.
- Reverse-active (or inverse-active or inverted): By reversing the biasing conditions of the
forward-active region, a bipolar transistor goes into reverse-active mode. In this mode, the
emitter and collector regions switch roles. Because most BJTs are designed to maximize current
gain in forward-active mode, the
F
in inverted mode is several (23 for the ordinary
germanium transistor) times smaller. This transistor mode is seldom used, usually being
considered only for failsafe conditions and some types of bipolar logic. The reverse bias
breakdown voltage to the base may be an order of magnitude lower in this region.
- Saturation: With both junctions forward-biased, a BJT is in saturation mode and facilitates high
current conduction from the emitter to the collector. This mode corresponds to a logical "on",
or a closed switch.
- Cut-off: In cut-off, biasing conditions opposite of saturation (both junctions reverse biased) are
present. There is very little current, which corresponds to a logical "off", or an open switch.
- Avalanche breakdown region
Although these regions are well defined for sufficiently large applied voltage, they overlap
somewhat for small (less than a few hundred millivolts) biases. For example, in the typical
grounded-emitter configuration of an NPN BJT used as a pulldown switch in digital logic, the
"off" state never involves a reverse-biased junction because the base voltage never goes below
ground; nevertheless the forward bias is close enough to zero that essentially no current flows, so
this end of the forward active region can be regarded as the cutoff region.
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Active-mode NPN transistors in circuits

Structure and use of NPN transistor. Arrow according to schematic.
The diagram opposite is a schematic representation of an NPN transistor connected to two
voltage sources. To make the transistor conduct appreciable current (on the order of 1 mA) from
C to E, V
BE
must be above a minimum value sometimes referred to as the cut-in voltage. The
cut-in voltage is usually about 600 mV for silicon BJTs at room temperature but can be different
depending on the type of transistor and its biasing. This applied voltage causes the lower P-N
junction to 'turn-on' allowing a flow of electrons from the emitter into the base. In active mode,
the electric field existing between base and collector (caused by V
CE
) will cause the majority of
these electrons to cross the upper P-N junction into the collector to form the collector current I
C
.
The remainder of the electrons recombine with holes, the majority carriers in the base, making a
current through the base connection to form the base current, I
B
. As shown in the diagram, the
emitter current, I
E
, is the total transistor current, which is the sum of the other terminal currents
(i.e., ).
In the diagram, the arrows representing current point in the direction of conventional current
the flow of electrons is in the opposite direction of the arrows because electrons carry negative
electric charge. In active mode, the ratio of the collector current to the base current is called the
DC current gain. This gain is usually 100 or more, but robust circuit designs do not depend on
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the exact value (for example see op-amp). The value of this gain for DC signals is referred to as
h
FE
, and the value of this gain for AC signals is referred to as h
fe
. However, when there is no
particular frequency range of interest, the symbol is used
[citation needed]
.
It should also be noted that the emitter current is related to V
BE
exponentially. At room
temperature, an increase in V
BE
by approximately 60 mV increases the emitter current by a
factor of 10. Because the base current is approximately proportional to the collector and emitter
currents, they vary in the same way.
Active-mode PNP transistors in circuits

Structure and use of PNP transistor.
The diagram opposite is a schematic representation of a PNP transistor connected to two voltage
sources. To make the transistor conduct appreciable current (on the order of 1 mA) from E to C,
V
EB
must be above a minimum value sometimes referred to as the cut-in voltage. The cut-in
voltage is usually about 600 mV for silicon BJTs at room temperature but can be different
depending on the type of transistor and its biasing. This applied voltage causes the upper P-N
junction to 'turn-on' allowing a flow of holes from the emitter into the base. In active mode, the
electric field existing between the emitter and the collector (caused by V
CE
) causes the majority
of these holes to cross the lower P-N junction into the collector to form the collector current I
C
.
The remainder of the holes recombine with electrons, the majority carriers in the base, making a
current through the base connection to form the base current, I
B
. As shown in the diagram, the
emitter current, I
E
, is the total transistor current, which is the sum of the other terminal currents
(i.e., ).
In the diagram, the arrows representing current point in the direction of conventional current
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the flow of holes is in the same direction of the arrows because holes carry positive electric
charge. In active mode, the ratio of the collector current to the base current is called the DC
current gain. This gain is usually 100 or more, but robust circuit designs do not depend on the
exact value. The value of this gain for DC signals is referred to as h
FE
, and the value of this gain
for AC signals is referred to as h
fe
. However, when there is no particular frequency range of
interest, the symbol is used
[citation needed]
.
It should also be noted that the emitter current is related to V
EB
exponentially. At room
temperature, an increase in V
EB
by approximately 60 mV increases the emitter current by a
factor of 10. Because the base current is approximately proportional to the collector and emitter
currents, they vary in the same way.
History
The bipolar point-contact transistor was invented in December 1947 at the Bell Telephone
Laboratories by John Bardeen and Walter Brattain under the direction of William Shockley. The
junction version known as the bipolar junction transistor, invented by Shockley in 1948, enjoyed
three decades as the device of choice in the design of discrete and integrated circuits. Nowadays,
the use of the BJT has declined in favor of CMOS technology in the design of digital integrated
circuits.
Germanium transistors
The germanium transistor was more common in the 1950s and 1960s, and while it exhibits a
lower "cut off" voltage, typically around 0.2 V, making it more suitable for some applications, it
also has a greater tendency to exhibit thermal runaway.
Early manufacturing techniques
Various methods of manufacturing bipolar junction transistors were developed
[8]
.
- Point-contact transistor first type to demonstrate transistor action, limited commercial
use due to high cost and noise.
- Grown junction transistor first type of bipolar junction transistor made
[9]
. Invented by
William Shockley at Bell Labs. Invented on June 23, 1948
[10]
. Patent filed on June 26,
1948.
- Alloy junction transistor emitter and collector alloy beads fused to base. Developed at
General Electric and RCA
[11]
in 1951.
o Micro alloy transistor high speed type of alloy junction transistor. Developed at
Philco
[12]
.
o Micro alloy diffused transistor high speed type of alloy junction transistor.
Developed at Philco.
o Post alloy diffused transistor high speed type of alloy junction transistor.
Developed at Philips.
- Tetrode transistor high speed variant of grown junction transistor
[13]
or alloy junction
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transistor
[14]
with two connections to base.
- Surface barrier transistor high speed metal barrier junction transistor. Developed at
Philco
[15]
in 1953
[16]
.
- Drift-field transistor high speed bipolar junction transistor. Invented by Herbert
Kroemer
[17][18]
at the Central Bureau of Telecommunications Technology of the German
Postal Service, in 1953.
- Diffusion transistor modern type bipolar junction transistor. Prototypes
[19]
developed at
Bell Labs in 1954.
o Diffused base transistor first implementation of diffusion transistor.
o Mesa transistor Developed at Texas Instruments in 1957.
o Planar transistor the bipolar junction transistor that made mass produced
monolithic integrated circuits possible. Developed by Dr. Jean Hoerni
[20]
at
Fairchild in 1959.
- Epitaxial transistor a bipolar junction transistor made using vapor phase deposition. See
epitaxy. Allows very precise control of doping levels and gradients.
Theory and modeling
In the discussion below, focus is on the NPN bipolar transistor. In the NPN transistor in what is
called active mode the base-emitter voltage V
BE
and collector-base voltage V
CB
are positive,
forward biasing the emitter-base junction and reverse-biasing the collector-base junction. In
active mode of operation, electrons are injected from the forward biased n-type emitter region
into the p-type base where they diffuse to the reverse biased n-type collector and are swept away
by the electric field in the reverse biased collector-base junction. For a figure describing forward
and reverse bias, see the end of the article semiconductor diodes.
Large-signal models
EbersMoll model

EbersMoll Model for an NPN transistor.
[21]

- I
B
, I
C
, I
E
: base, collector and emitter currents
- I
CD
, I
ED
: collector and emitter diode currents
-
F
,
R
: forward and reverse common-base current gains
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EbersMoll Model for a PNP transistor.

The DC emitter and collector currents in active mode are well modeled by an approximation to the
EbersMoll model:


The base internal current is mainly by diffusion (see Fick's law) and

where
- V
T
is the thermal voltage kT / q (approximately 26 mV at 300 K room temperature).
- I
E
is the emitter current
- I
C
is the collector current
-
T
is the common base forward short circuit current gain (0.98 to 0.998)
- I
ES
is the reverse saturation current of the baseemitter diode (on the order of 10
15
to 10
12

amperes)
- V
BE
is the baseemitter voltage
- D
n
is the diffusion constant for electrons in the p-type base
- W is the base width
The and forward parameters are as described previously. A reverse is sometimes included
in the model.
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The unapproximated EbersMoll equations used to describe the three currents in any operating
region are given below. These equations are based on the transport model for a bipolar junction
transistor.
[22]




where
- i
C
is the collector current
- i
B
is the base current
- i
E
is the emitter current
-
F
is the forward common emitter current gain (20 to 500)
-
R
is the reverse common emitter current gain (0 to 20)
- I
S
is the reverse saturation current (on the order of 10
15
to 10
12
amperes)
- V
T
is the thermal voltage (approximately 26 mV at 300 K room temperature).
- V
BE
is the baseemitter voltage
- V
BC
is the basecollector voltage
Base-width modulation
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Top: NPN base width for low collector-base reverse bias; Bottom: narrower NPN base width for large
collector-base reverse bias. Hashed regions are depleted regions.
Main article: Early Effect
As the applied collectorbase voltage (V
CB
= V
CE
V
BE
) varies, the collectorbase depletion
region varies in size. An increase in the collectorbase voltage, for example, causes a greater
reverse bias across the collectorbase junction, increasing the collectorbase depletion region
width, and decreasing the width of the base. This variation in base width often is called the
"Early effect" after its discoverer James M. Early.
Narrowing of the base width has two consequences:
- There is a lesser chance for recombination within the "smaller" base region.
- The charge gradient is increased across the base, and consequently, the current of minority
carriers injected across the emitter junction increases.
Both factors increase the collector or "output" current of the transistor in response to an increase
in the collectorbase voltage.
In the forward-active region, the Early effect modifies the collector current (i
C
) and the forward
common emitter current gain (
F
) as given by:
[citation needed]


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where:
- V
CB
is the collectorbase voltage
- V
A
is the Early voltage (15 V to 150 V)
-
F0
is forward common-emitter current gain when V
CB
= 0 V
- r
o
is the output impedance
- I
C
is the collector current
Currentvoltage characteristics
The following assumptions are involved when deriving ideal current-voltage characteristics of
the BJT
- Low level injection
- Uniform doping in each region with abrupt junctions
- One-dimensional current
- Negligible recombination-generation in space charge regions
- Negligible electric fields outside of space charge regions.
It is important to characterize the minority diffusion currents induced by injection of carriers.
With regard to pn-junction diode, a key relation is the diffusion equation.

A solution of this equation is below, and two boundary conditions are used to solve and find C
1

and C
2
.

The following equations apply to the emitter and collector region, respectively, and the origins 0,
0', and 0'' apply to the base, collector, and emitter.

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A boundary condition of the emitter is below:

The values of the constants A
1
and B
1
are zero due to the following conditions of the emitter and
collector regions as and .


Because A
1
= B
1
= 0, the values of n
E
(0'') and n
c
(0') are A
2
and B
2
, respectively.


Expressions of I
En
and I
Cn
can be evaluated.


Because insignificant recombination occurs, the second derivative of p
B
(x) is zero. There is
therefore a linear relationship between excess hole density and x.

The following are boundary conditions of p
B
.


Substitute into the above linear relation.
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.
With this result, derive value of I
Ep
.


Use the expressions of I
Ep
, I
En
, p
B
(0), and p
B
(W) to develop an expression of the emitter
current.



Similarly, an expression of the collector current is derived.



An expression of the base current is found with the previous results.


Punchthrough
When the basecollector voltage reaches a certain (device specific) value, the basecollector
depletion region boundary meets the baseemitter depletion region boundary. When in this state
the transistor effectively has no base.The device thus loses all gain when in this state.
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GummelPoon charge-control model
The GummelPoon model is a detailed charge-controlled model of BJT dynamics, which has
been adopted and elaborated by others to explain transistor dynamics in greater detail than the
terminal-based models typically do. This model also includes the dependence of transistor -
values upon the direct current levels in the transistor, which are assumed current-independent in
the EbersMoll model.
Small-signal models
hybrid-pi model
h- parameter model

Generalized h-parameter model of an NPN BJT.
Replace x with e, b or c for CE, CB and CC topologies respectively.
Another model commonly used to analyze BJT circuits is the "h-parameter" model, closely
related to the hybrid-pi model and the y-parameter two-port, but using input current and output
voltage as independent variables, rather than input and output voltages. This two-port network is
particularly suited to BJTs as it lends itself easily to the analysis of circuit behaviour, and may be
used to develop further accurate models. As shown, the term "x" in the model represents a
different BJT lead depending on the topology used. For common-emitter mode the various
symbols take on the specific values as:
- x = 'e' because it is a common-emitter topology
- Terminal 1 = Base
- Terminal 2 = Collector
- Terminal 3 = Emitter
- i
i
= Base current (i
b
)
- i
o
= Collector current (i
c
)
- V
in
= Base-to-emitter voltage (V
BE
)
- V
o
= Collector-to-emitter voltage (V
CE
)
and the h-parameters are given by:
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- h
ix
= h
ie
The input impedance of the transistor (corresponding to the emitter resistance
r
e
).
- h
rx
= h
re
Represents the dependence of the transistor's I
B
V
BE
curve on the value of V
CE
.
It is usually very small and is often neglected (assumed to be zero).
- h
fx
= h
fe
The current-gain of the transistor. This parameter is often specified as h
FE
or
the DC current-gain (
DC
) in datasheets.
- h
ox
= h
oe
The output impedance of transistor. This term is usually specified as an
admittance and has to be inverted to convert it to an impedance.
As shown, the h-parameters have lower-case subscripts and hence signify AC conditions or
analyses. For DC conditions they are specified in upper-case. For the CE topology, an
approximate h-parameter model is commonly used which further simplifies the circuit analysis.
For this the h
oe
and h
re
parameters are neglected (that is, they are set to infinity and zero,
respectively). It should also be noted that the h-parameter model as shown is suited to low-
frequency, small-signal analysis. For high-frequency analyses the inter-electrode capacitances
that are important at high frequencies must be added.
Applications
The BJT remains a device that excels in some applications, such as discrete circuit design, due to
the very wide selection of BJT types available, and because of its high transconductance and
output resistance compared to MOSFETs. The BJT is also the choice for demanding analog
circuits, especially for very-high-frequency applications, such as radio-frequency circuits for
wireless systems. Bipolar transistors can be combined with MOSFETs in an integrated circuit by
using a BiCMOS process of wafer fabrication to create circuits that take advantage of the
application strengths of both types of transistor.
Temperature sensors
Main article: Silicon bandgap temperature sensor
Because of the known temperature and current dependence of the forward-biased baseemitter
junction voltage, the BJT can be used to measure temperature by subtracting two voltages at two
different bias currents in a known ratio.
Logarithmic converters
Because baseemitter voltage varies as the log of the baseemitter and collectoremitter
currents, a BJT can also be used to compute logarithms and anti-logarithms. A diode can also
perform these nonlinear functions, but the transistor provides more circuit flexibility.
Vulnerabilities
Exposure of the transistor to ionizing radiation causes radiation damage. Radiation causes a
buildup of 'defects' in the base region that act as recombination centers. The resulting reduction
in minority carrier lifetime causes gradual loss of gain of the transistor.
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Power BJTs are subject to a failure mode called secondary breakdown, in which excessive
current and normal imperfections in the silicon die cause portions of the silicon inside the device
to become disproportionately hotter than the others. The doped silicon has a negative
temperature coefficient, meaning that it conducts more current at higher temperatures. Thus, the
hottest part of the die conducts the most current, causing its conductivity to increase, which then
causes it to become progressively hotter again, until the device fails internally. The thermal
runaway process associated with secondary breakdown, once triggered, occurs almost instantly
and may catastrophically damage the transistor package.
If the emitter-base junction is reverse biased into avalanche (zener) mode and current flows for a
short period of time, the current gain of the BJT will be permanently degraded.
Why transistor(BJT) is called a current controlled device and why it is called a nonlinear
device?
A bipolar junction transistor (BJT) is a type of transistor. It is a three-terminal device constructed
of doped semiconductor material and may be used in amplifying or switching applications.
Bipolar transistors are so named because their operation involves both electrons and holes.

Although a small part of the transistor current is due to the flow of majority carriers, most of the
transistor current is due to the flow of minority carriers and so BJTs are classified as 'minority-
carrier' devices.

An NPN transistor can be considered as two diodes with a shared anode region. In typical
operation, the emitterbase junction is forward biased and the basecollector junction is reverse
biased. In an NPN transistor, for example, when a positive voltage is applied to the baseemitter
junction, the equilibrium between thermally generated carriers and the repelling electric field of
the depletion region becomes unbalanced, allowing thermally excited electrons to inject into the
base region. These electrons wander (or "diffuse") through the base from the region of high
concentration near the emitter towards the region of low concentration near the collector. The
electrons in the base are called minority carriers because the base is doped p-type which would
make holes the majority carrier in the base.

The base region of the transistor must be made thin, so that carriers can diffuse across it in much
less time than the semiconductor's minority carrier lifetime, to minimize the percentage of
carriers that recombine before reaching the collectorbase junction. The thickness of the base
should be less than the diffusion length of the electrons. The collectorbase junction is reverse-
biased, so little electron injection occurs from the collector to the base, but electrons that diffuse
through the base towards the collector are swept into the collector by the electric field in the
depletion region of the collectorbase junction.

Voltage, current, and charge control
The collectoremitter current can be viewed as being controlled by the baseemitter current
(current control), or by the baseemitter voltage (voltage control). These views are related by the
currentvoltage relation of the baseemitter junction, which is just the usual exponential current
voltage curve of a p-n junction (diode).
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The physical explanation for collector current is the amount of minority-carrier charge in the
base region. Detailed models of transistor action, such as the GummelPoon model, account for
the distribution of this charge explicitly to explain transistor behavior more exactly. The charge-
control view easily handles photo-transistors, where minority carriers in the base region are
created by the absorption of photons, and handles the dynamics of turn-off, or recovery time,
which depends on charge in the base region recombining. However, since base charge is not a
signal that is visible at the terminals, the current- and voltage-control views are usually used in
circuit design and analysis.

In analog circuit design, the current-control view is sometimes used since it is approximately
linear. That is, the collector current is approximately F times the base current. Some basic
circuits can be designed by assuming that the emitterbase voltage is approximately constant,
and that collector current is beta times the base current. However, to accurately and reliably
design production bjt circuits, the voltage-control (for example, EbersMoll) model is required
The voltage-control model requires an exponential function to be taken into account, but when it
is linearized such that the transistor can be modelled as a transconductance, as in the EbersMoll
model, design for circuits such as differential amplifiers again becomes a mostly linear problem,
so the voltage-control view is often preferred. For translinear circuits, in which the exponential
IV curve is key to the operation, the transistors are usually modelled as voltage controlled with
transconductance proportional to collector current. In general, transistor level circuit design is
performed using SPICE or a comparable analogue circuit simulator, so model complexity is
usually not of much concern to the designer.

Biploar transistor:
A transistor is basically a Si on Ge
crystal containing three separate
regions. It can be either NPN or
PNP type fig. 1. The middle region
is called the base and the outer two
regions are called emitter and the
collector. The outer layers although
they are of same type but their
functions cannot be changed. They
have different physical and
electrical properties.
In most transistors, emitter is
heavily doped. Its job is to emit or
inject electrons into the base. These
bases are lightly doped and very
thin, it passes most of the emitter-
injected electrons on to the
collector. The doping level of



fig. 1

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collector is intermediate between
the heavy doping of emitter and the
light doping of the base.
The collector is so named because
it collects electrons from base. The
collector is the largest of the three
regions; it must dissipate more heat
than the emitter or base. The
transistor has two junctions. One
between emitter and the base and
other between the base and the
collector. Because of this the
transistor is similar to two diodes,
one emitter diode and other
collector base diode.

The depletion layers do not have the same width, because different regions have different doping
levels. The more heavily doped a region is, the greater the concentration of ions near the
junction. This means the depletion layer penetrates more deeply into the base and slightly into
emitter. Similarly, it penetration more into collector. The thickness of collector depletion layer is
large while the base depletion layer is small as shown in fig. 2.
When transistor is made, the diffusion of free electrons across the junction produces two
depletion layers. For each of these depletion layers, the barrier potential is 0.7 V for Si transistor
and 0.3 V for Ge transistor.


fig. 2
If both the junctions are forward biased using two d.c sources, as shown in fig. 3a. free electrons
(majority carriers) enter the emitter and collector of the transistor, joins at the base and come out
of the base. Because both the diodes are forward biased, the emitter and collector currents are
large.
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Fig. 3a

Fig. 3b
If both the junction are reverse biased as shown in fig. 3b, then small currents flows through both
junctions only due to thermally produced minority carriers and surface leakage. Thermally
produced carriers are temperature dependent it approximately doubles for every 10 degree
celsius rise in ambient temperature. The surface leakage current increases with voltage.
When the emitter diode is forward biased and collector diode is reverse biased as shown in fig. 4
then one expect large emitter current and small collector current but collector current is almost as
large as emitter current.

Fig. 4
When emitter diodes forward biased and the applied voltage is more than 0.7 V (barrier
potential) then larger number of majority carriers (electrons in n-type) diffuse across the
junction.
Once the electrons are injected by the emitter enter into the base, they become minority carriers.
These electrons do not have separate identities from those, which are thermally generated, in the
base region itself. The base is made very thin and is very lightly doped. Because of this only few
electrons traveling from the emitter to base region recombine with holes. This gives rise to
recombination current. The rest of the electrons exist for more time. Since the collector diode is
reverse biased, (n is connected to positive supply) therefore most of the electrons are pushed into
collector layer. These collector elections can then flow into the external collector lead.
Thus, there is a steady stream of electrons leaving the negative source terminal and entering the
emitter region. The V
EB
forward bias forces these emitter electrons to enter the base region. The
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thin and lightly doped base gives almost all those electrons enough lifetime to diffuse into the
depletion layer. The depletion layer field pushes a steady stream of electron into the collector
region. These electrons leave the collector and flow into the positive terminal of the voltage
source. In most transistor, more than 95% of the emitter injected electrons flow to the collector,
less than 5% fall into base holes and flow out the external base lead. But the collector current is
less than emitter current.
Relation between different currents in a transistor:
The total current flowing into the transistor must be equal to the total current flowing out of it.
Hence, the emitter current I
E
is equal to the sum of the collector (I
C
) and base current (I
B
). That
is,
I
E
= I
C
+ I
B

The currents directions are positive directions. The total collector current I
C
is made up of two
components.
1. The fraction of emitter (electron) current which reaches the collector ( a
dc
I
E
)
2. The normal reverse leakage current I
CO


o
dc
is known as large signal current gain or dc alpha. It is always positive. Since collector current
is almost equal to the I
E
therefore dc I
E
varies from 0.9 to 0.98. Usually, the reverse leakage
current is very small compared to the total collector current.

NOTE: The forward bias on the emitter diode controls the number of free electrons infected into
the base. The larger (V
BE
) forward voltage, the greater the number of injected electrons. The
reverse bias on the collector diode has little influence on the number of electrons that enter the
collector. Increasing V
CB
does not change the number of free electrons arriving at the collector
junction layer.
The symbol of npn and pnp transistors are shown in fig. 5.
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Fig. 5
Breakdown Voltages:
Since the two halves of a transistor are diodes, two much reverse voltage on either diode can
cause breakdown. The breakdown voltage depends on the width of the depletion layer and the
doping levels. Because of the heavy doping level, the emitter diode has a low breakdown voltage
approximately 5 to 30 V. The collector diode is less heavily doped so its breakdown voltage is
higher around 20 to 300 V.
Common Base Configuration

If the base is common to the input and output circuits, it is know as common base configuration
as shown in fig. 1.

Fig. 1
For a pnp transistor the largest current components are due to holes. Holes flow from emitter to
collector and few holes flow down towards ground out of the base terminal. The current
directions are shown in fig. 1.
(I
E
= I
C
+ I
B
).
For a forward biased junction, V
EB
is positive and for a reverse biased junction V
CB
is negative.
The complete transistor can be described by the following two relations, which give the input
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voltage V
EB
and output current I
C
in terms of the output voltage (V
CB
) and input current I
E
.
V
EB
= f
1
(V
CB
, I
E
)
I
C
= f
2
(V
CB
, I
E
)
The output characteristic:
The collector current I
C
is completely determined by the input current I
E
and the V
CB
voltage.
The relationship is given in fig. 2. It is a plot of I
C
versus V
CB
, with emitter current I
E
as
parameter. The curves are known as the output or collector or static characteristics. The transistor
consists of two diodes placed in series back to back (with two cathodes connected together). The
complete characteristic can be divided in three regions.

Figure 7.2
(1). Active region:
In this region the collector diode is reverse biased and the emitter diode is forward biased.
Consider first that the emitter current is zero. Then the collector current is small and equals the
reverse saturation current I
CO
of the collector junction considered as a diode.
If the forward current I
B
is increased, then a fraction of I
E
ie. o
dc
I
E
will reach the collector. In the
active region, the collector current is essentially independent of collector voltage and depends
only upon the emitter current. Because o
dc
is, less than one but almost equal to unity, the
magnitude of the collector current is slightly less that of emitter current. The collector current is
almost constant and work as a current source.
The collector current slightly increases with voltage. This is due to early effect. At higher voltage
collector gathers in a few more electrons. This reduces the base current. The difference is so
small, that it is usually neglected. If the collector voltage is increased, then space charge width
increases; this decreased the effective base width. Then there is less chance for recombination
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within the base region.
(2). Saturation region:
The region to the left of the ordinate V
CB
= 0, and above the I
E
= 0, characteristic in which both
emitter and collector junction are forward biased, is called saturation region.
When collector diode is forward biased, there is large change in collector current with small
changes in collector voltage. A forward bias means, that p is made positive with respect to n,
there is a flow of holes from p to n. This changes the collector current direction. If diode is
sufficiently forward biased the current changes rapidly. It does not depend upon emitter current.
(3). Cut off region:
The region below I
E
= 0 and to the right of V
CB
for which emitter and collector junctions are both
reversed biased is referred to cutoff region. The characteristics I
E
= 0, is similar to other
characteristics but not coincident with horizontal axis. The collector current is same as I
CO
. I
CBO

is frequently used for I
CO
. It means collector to base current with emitter open. This is also
temperature dependent.


Common Base Amplifier
The common base amplifier circuit is
shown in Fig. 1. The V
EE
source
forward biases the emitter diode and
V
CC
source reverse biased collector
diode. The ac source v
in
is connected
to emitter through a coupling
capacitor so that it blocks dc. This ac
voltage produces small fluctuation in
currents and voltages. The load
resistance R
L
is also connected to
collector through coupling capacitor
so the fluctuation in collector base
voltage will be observed across R
L
.
The dc equivalent circuit is obtained
by reducing all ac sources to zero
and opening all capacitors. The dc
collector current is same as I
E
and

Fig. 1
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V
CB
is given by
V
CB
= V
CC
- I
C
R
C
.
These current and voltage fix the Q point. The ac equivalent circuit is obtained by reducing all dc
sources to zero and shorting all coupling capacitors. r'
e
represents the ac resistance of the diode
as shown in Fig. 2.

Fig. 2
Fig. 3, shows the diode curve relating I
E
and V
BE
. In the absence of ac signal, the transistor
operates at Q point (point of intersection of load line and input characteristic). When the ac
signal is applied, the emitter current and voltage also change. If the signal is small, the operating
point swings sinusoidally about Q point (A to B).
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Fig .3
If the ac signal is small, the points A and B are close to Q, and arc A B can be approximated by a
straight line and diode appears to be a resistance given by

If the input signal is small, input voltage and current will be sinusoidal but if the input voltage is
large then current will no longer be sinusoidal because of the non linearity of diode curve. The
emitter current is elongated on the positive half cycle and compressed on negative half cycle.
Therefore the output will also be distorted.
r'
e
is the ratio of V
BE
and I
E
and its value depends upon the location of Q. Higher up the Q
point small will be the value of r' e because the same change in V
BE
produces large change in I
E
.
The slope of the curve at Q determines the value of r'
e
. From calculation it can be proved that.
r'e = 25mV / IE

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Common Base Amplifier
Proof:
In general, the current through a diode is given by

Where q is he charge on electron, V is the drop across diode, T is the temperature and K is a
constant.
On differentiating w.r.t V, we get,

The value of (q / KT) at 25C is approximately 40.
Therefore,
or,

To a close approximation the small changes in collector current equal the small changes in
emitter current. In the ac equivalent circuit, the current i
C
' is shown upward because if i
e
'
increases, then i
C
' also increases in the same direction.
Voltage gain:
Since the ac input voltage source is connected across r'
e
. Therefore, the ac emitter current is
given by
i
e
= V
in
/ r'
e

or, V
in
= ie r'
e

The output voltage is given by V
out
= i
c
(R
C
|| R
L
)
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Under open circuit condition v
out
= i
c
R
c



Common Base Amplifier
Example-1
Find the voltage gain and output of the amplifier shown in fig. 4, if input voltage is 1.5mV.

Fig. 4
Solution:
The emitter dc current I E is given by
Therefore, emitter ac resistance =
or, A
V
= 56.6
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and, V
out
= 1.5 x 56.6 = 84.9 mV
Example-2
Repeat example-1 if ac source has resistance R s = 100 W .
Solution:
The ac equivalent circuit with ac source resistance is shown in fig. 5.

Fig. 5
The emitter ac current is given by
or,
Therefore, voltage gain of the amplifier =

and, V
out
= 1.5 x 8.71 =13.1 mV


Common Emitter Configuration
Common Emitter Curves:
The common emitter configuration of BJT is shown in fig. 1.
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Fig. 1
In C.E. configuration the emitter is made common to the input and output. It is also referred to as
grounded emitter configuration. It is most commonly used configuration. In this, base current
and output voltages are taken as impendent parameters and input voltage and output current as
dependent parameters
V
BE
= f
1
( I
B
, V
CE
)
I
C
= f
2
( I
B
, V
CE
)
Input Characteristic:
The curve between I
B
and V
BE
for different values of V
CE
are shown in fig. 2. Since the base
emitter junction of a transistor is a diode, therefore the characteristic is similar to diode one. With
higher values of V
CE
collector gathers slightly more electrons and therefore base current reduces.
Normally this effect is neglected. (Early effect). When collector is shorted with emitter then the
input characteristic is the characteristic of a forward biased diode when V
BE
is zero and I
B
is also
zero.
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Fig. 2


Common Emitter Configuration
Output Characteristic:
The output characteristic is the curve between V
CE
and I
C
for various values of I
B
. For fixed
value of I
B
and is shown in fig. 3. For fixed value of I
B
, I
C
is not varying much dependent on V
CE

but slopes are greater than CE characteristic. The output characteristics can again be divided into
three parts.

Fig. 3
(1) Active Region:
In this region collector junction is reverse biased and emitter junction is forward biased. It is the
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area to the right of V
CE
= 0.5 V and above I
B
= 0. In this region transistor current responds most
sensitively to I
B
. If transistor is to be used as an amplifier, it must operate in this region.

If o
dc
is truly constant then I
C
would be independent of V
CE
. But because of early effect, o
dc

increases by 0.1% (0.001) e.g. from 0.995 to 0.996 as V
CE
increases from a few volts to 10V.
Then |
dc
increases from 0.995 / (1-0.995) = 200 to 0.996 / (1-0.996) = 250 or about 25%. This
shows that small change in o reflects large change in |. Therefore the curves are subjected to
large variations for the same type of transistors.
(2) Cut Off:
Cut off in a transistor is given by I
B
= 0, I
C
= I
CO
. A transistor is not at cut off if the base current is
simply reduced to zero (open circuited) under this condition,
I
C
= I
E
= I
CO
/ ( 1-
dc
) = I
CEO

The actual collector current with base open is designated as I
CEO
. Since even in the neighborhood
of cut off, o
dc
may be as large as 0.9 for Ge, then I
C
=10 I
CO
(approximately), at zero base current.
Accordingly in order to cut off transistor it is not enough to reduce I
B
to zero, but it is necessary
to reverse bias the emitter junction slightly. It is found that reverse voltage of 0.1 V is sufficient
for cut off a transistor. In Si, the o
dc
is very nearly equal to zero, therefore, I
C
= I
CO
. Hence even
with I
B
= 0, I
C
= I
E
= I
CO
so that transistor is very close to cut off.
In summary, cut off means I
E
= 0, I
C
= I
CO
, I
B
= -I
C
= -I
CO
, and V
BE
is a reverse voltage whose
magnitude is of the order of 0.1 V for Ge and 0 V for Si.
Reverse Collector Saturation Current I
CBO
:
When in a physical transistor emitter current is reduced to zero, then the collector current is
known as I
CBO
(approximately equal to I
CO
). Reverse collector saturation current I
CBO
also varies
with temperature, avalanche multiplication and variability from sample to sample. Consider the
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circuit shown in fig. 4. V
BB
is the reverse voltage applied to reduce the emitter current to zero.
I
E
= 0, I
B
= -I
CBO

If we require, V
BE
= - 0.1 V
Then - V
BB
+ I
CBO
R
B
< - 0.1 V

Fig. 4
If R
B
= 100 K, I
CBO
= 100 m A, Then V
BB
must be 10.1 Volts. Hence transistor must be capable
to withstand this reverse voltage before breakdown voltage exceeds.
(3).Saturation Region:
In this region both the diodes are forward biased by at least cut in voltage. Since the voltage V
BE

and V
BC
across a forward is approximately 0.7 V therefore, V
CE
= V
CB
+ V
BE
= - V
BC
+ V
BE
is
also few tenths of volts. Hence saturation region is very close to zero voltage axis, where all the
current rapidly reduces to zero. In this region the transistor collector current is approximately
given by V
CC
/ R
C
and independent of base current. Normal transistor action is last and it acts
like a small ohmic resistance.


Common Emitter Configuration
Large Signal Current Gain
dc
:-
The ratio I
c
/ I
B
is defined as transfer ratio or large signal current gain b
dc

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Where I
C
is the collector current and I
B
is the base current. The b
dc
is an indication if how well
the transistor works. The typical value of b
dc
varies from 50 to 300.
In terms of h parameters, b
dc
is known as dc current gain and in designated h
fE
( b
dc
= h
fE
).
Knowing the maximum collector current and b
dc
the minimum base current can be found which
will be needed to saturate the transistor.

This expression of b
dc
is defined neglecting reverse leakage current (I
CO
).
Taking reverse leakage current (I
CO
) into account, the expression for the b
dc
can be obtained as
follows:
b
dc
in terms of a
dc
is given by

Since, I
CO
= I
CBO


Cut off of a transistor means I
E
= 0, then I
C
= I
CBO
and I
B
= - I
CBO
. Therefore, the above
expression b
dc
gives the collector current increment to the base current change form cut off to I
B

and hence it represents the large signal current gain of all common emitter transistor.



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Biasing Techniques for CE Amplifiers
Biasing Circuit Techniques or Locating the Q - Point:
Fixed Bias or Base Bias:
In order for a transistor to amplify, it has to be properly biased. This means forward biasing the
base emitter junction and reverse biasing collector base junction. For linear amplification, the
transistor should operate in active region ( If I
E
increases, I
C
increases, V
CE
decreases
proportionally).
The source V
BB
, through a current limit resistor R
B
forward biases the emitter diode and V
CC

through resistor R
C
(load resistance) reverse biases the collector junction as shown in fig. 1.

Fig. 1
The dc base current through R
B
is given by
I
B
= (V
BB
- V
BE
) / R
B

or V
BE
= V
BB
- I
B
R
B

Normally V
BE
is taken 0.7V or 0.3V. If exact voltage is required, then the input characteristic ( I
B

vs V
BE
) of the transistor should be used to solve the above equation. The load line for the input
circuit is drawn on input characteristic. The two points of the load line can be obtained as given
below
For I
B
= 0, V
BE
= V
BB
.
and For V
BE
= 0, I
B
= V
BB
/ R
B
.
The intersection of this line with input characteristic gives the operating point Q as shown in
fig. 2. If an ac signal is connected to the base of the transistor, then variation in V
BE
is about
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Q - point. This gives variation in I
B
and hence I
C
.

Fig. 2


Biasing Techniques for CE Amplifiers
In the output circuit, the load equation can be written as
V
CE
= V
CC
- I
C
R
C

This equation involves two unknown V
CE
and I
C
and therefore can not be solved. To solve this
equation output characteristic ( I
C
vs V
CE
) is used.
The load equation is the equation of a straight line and given by two points:
I
C
= 0, V
CE
= V
CC

& V
CE
= 0, I
C
= V
CC
/ R
C

The intersection of this line which is also called dc load line and the characteristic gives the
operating point Q as shown in fig. 3.
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Fig. 3
The point at which the load line intersects with I
B
= 0 characteristic is known as cut off point. At
this point base current is zero and collector current is almost negligibly small. At cut off the
emitter diode comes out of forward bias and normal transistor action is lost. To a close
approximation.
V
CE
( cut off) ~ V
CC
(approximately).
The intersection of the load line and I
B
= I
B(max)
characteristic is known as saturation point . At
this point I
B
= I
B(max)
, I
C
= I
C(sat)
. At this point collector diodes comes out of reverse bias and again
transistor action is lost. To a close approximation,
I
C(sat)
~ V
CC
/ R
C
(approximately ).
The I
B(sat)
is the minimum current required to operate the transistor in saturation region. If the I
B

is less than I
B (sat)
, the transistor will operate in active region. If I
B
> I
B (sat)
it always operates in
saturation region.
If the transistor operates at saturation or cut off points and no where else then it is operating as a
switch is shown in fig. 4.
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Fig. 4
V
BB
= I
B
R
B
+ V
BE

I
B
= (V
BB
V
BE
) / R
B

If I
B
> I
B(sat)
, then it operates at saturation, If I
B
= 0, then it operates at cut off.
If a transistor is operating as an amplifier then Q point must be selected carefully. Although we
can select the operating point any where in the active region by choosing different values of R
B

& R
C
but the various transistor ratings such as maximum collector dissipation P
C(max)
maximum
collector voltage V
C(max)
and I
C(max)
& V
BE(max)
limit the operating range.
Once the Q point is established an ac input is connected. Due to this the ac source the base
current varies. As a result of this collector current and collector voltage also varies and the
amplified output is obtained.
If the Q-point is not selected properly then the output waveform will not be exactly the input
waveform. i.e. It may be clipped from one side or both sides or it may be distorted one.


Biasing Techniques for CE Amplifiers
Example-1
Find the transistor current in the circuit shown in fig. 5, if I
CO
= 20nA, =100.
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Solution:
For the base circuit, 5 = 200 x IB + 0.7
Therefore,
Since I
CO
<< I
B
, therefore, I
C
= I
B
= 2.15 mA
From the collector circuit, V
CE
= 10 - 3 x 2.15 = 3.55 V
Since, V
CE
= V
CB
+ V
BE

Thus, V
CB
= 3.55 - 0.7 = 2.55 V
Therefore, collector junction is reverse biased and
transistor is operating in its active region.

Fig. 5
Example - 2
If a resistor of 2K is connected in series with emitter in
the circuit as shown in fig. 6, find the currents. Given
I
CO
= 20 nA, =100.
Solution:
I
E
= I
B
+ I
C
= I
B
+ 100 I
B
= 101 I
B

For the base circuit, 5 = 200 x I
B
+ 0.7 + 2k x 101 I
B

Therefore,
Since I
CO
<< I
B
, therefore, I
C
= I
B
= 1.07 mA
From the collector circuit, V
CB
= 10 - 3 x 1.07 - 0.7 - 2 x
101 x 0.0107 = 3.93 V
Therefore collector junction is reverse biased and
transistor is operating in its active region.

Fig. 6


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Example - 3
Repeat the example-1 if R
B
is replaced by 50k.
Solution:
The circuit is shown in fig. 7.
Since the base resistance is reduced, the base current must
have increased and there is a possibility that the transistor
has entered into saturation region.
Assuming transistor is operating in its saturation region,
V
BE (sat)
= 0.8 V and V
CE (sat)
= 0.2V
Therefore,
and
The minimum base current required for operating the
transistor in saturation region is

Since I
B
> I
B(min)
, therefore, transistor is operating in its
saturation region.

Fig. 7
Example - 4
Repeat the example-2 if R
B
is replaced by 50k.
Solution:
The circuit is shown in fig. 8.
Since the base resistance is reduced, the base current must
have increased and there is a possibility that the transistor
has entered into saturation region.
Assuming transistor is operating in its saturation region,

Fig. 8
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Solving these equations, we get,
I
C
= 1.96mA and I
B
= 0.0035mA
The minimum base current required for operating the
transistor in saturation region is

Since I B < I B(min) , therefore, transistor is operating in
its active region and not in saturation. The base and the
collector currents can be recalculated assuming the
transistor to be in active region.
For the base circuit, 5 = 50 x I
B
+ 0.7 + 2k x 101 I
B

Therefore,
I
C
= 1.71mA
From the collector circuit, V
CB
= 10 - 3 x 1.71 - 0.7 - 2 x
101 x 0.0171 = 0.716 V


Stability of Operating Point
Let us consider three operating points of transistor operating in common emitter amplifier.
1. Near cut off
2. Near saturation
3. In the middle of active region
If the operating point is selected near the cutoff region, the output is clipped in negative half cycle as shown in fig. 1.
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Fig. 1
If the operating point is selected near saturation region, then the output is clipped in positive
cycle as shown in fig. 2.


Fig. 2 Fig. 3
If the operating point is selected in the middle of active region, then there is no clipping and the
output follows input faithfully as shown in fig. 3. If input is large then clipping at both sides will
take place. The first circuit for biasing the transistor is CE configuration is fixed bias.
In biasing circuit shown in fig. 4(a), two different power supplies are required. To avoid the use
of two supplies the base resistance R
B
is connected to V
CC
as shown in fig. 4(b).
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Fig. 4(a) Fig. 4(b)
Now V
CC
is still forward biasing emitter diode. In this circuit Q point is very unstable. The base
resistance R
B
is selected by noting the required base current I
B
for operating point Q.
I
B
= (V
CC
V
BE
) / R
B

Voltage across base emitter junction is approximately 0.7 V. Since V
CC
is usually very high
i.e. I
B
= V
CC
/ R
B

Since I
B
is constant therefore it is called fixed bias circuit.


Biasing
Stability of quiescent operating point:
Let us assume that the transistor is replaced by an other transistor of same type. The |
dc
of the
two transistors of same type may not be same. Therefore, if |
dc
increases then for same I
B
, output
characteristic shifts upward. If |
dc
decreases, the output characteristic shifts downward. Since I
B

is maintained constant, therefore the operating point shifts from Q to Q
1
as shown in fig. 5. The
new operating point may be completely unsatisfactory.
Therefore, to maintain operating point stable, I
B
should be allowed to change so as to maintain
V
CE
& I
C
constant as |
dc
changes.
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Fig. 5
A second cause for bias instability is a variation in temperature. The reverse saturation current
changes with temperature. Specifically, I
CO
doubles for every 10
o
C rise in temperature. The
collector current I
C
causes the collector junction temperature to rise, which in turn increases I
CO
.
As a result of this growth I
CO
, I
C
will increase ( |
dc
I
B
+ (1+ |
dc
) I
CO
) and so on. It may be
possible that this process goes on and the ratings of the transistors are exceeded. This increase in
I
C
changes the characteristic and hence the operating point.
Stability Factor:
The operating point can be made stable by keeping I
C
and V
CE
constant. There are two
techniques to make Q point stable.
1. stabilization techniques
2. compensation techniques
In first, resistor biasing circuits are used which allow I
B
to vary so as to keep I
C
relatively
constant with variations in |
dc
, I
CO
and V
BE
.
In second, temperature sensitive devices such as diodes, transistors are used which provide
compensating voltages and currents to maintain the operating point constant.
To compare different biasing circuits, stability factor S is defined as the rate of change of
collector current with respect to the I
CO
, keeping |
dc
and V
CE
constant
S = c I
C
/ c I
CO

If S is large, then circuit is thermally instable. S cannot be less than unity. The other stability
factors are, c I
C
/ c |
dc
and c I
C
/ c V
BE
. The bias circuit, which provide stability with I
CO
, also
show stability even if | and V
BE
changes.
I
C
= |
dc
I
B
+ (I + |
dc
) I
CO

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Differentiating with respect to I
C
,

In fixed bias circuit, I
B
& I
C
are independent. Therefore and S = 1 + |
dc
. If |
dc
=100, S =
101, which means I
C
increases 101 times as fast as I
CO
. Such a large change definitely operate the
transistor in saturation.


Biasing Techniques
Emitter Feedback Bias:
Fig. 1, shows the emitter feedback bias circuit. In this circuit, the voltage across resistor R
E
is
used to offset the changes in b
dc
. If b
dc
increases, the collector current increases. This increases
the emitter voltage which decrease the voltage across base resistor and reduces base current. The
reduced base current result in less collector current, which partially offsets the original increase
in b
dc
. The feedback term is used because output current ( I
C
) produces a change in input current
( I
B
). R
E
is common in input and output circuits.

Fig. 1
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In this case

Since I
E
= I
C
+ I
B


Therefore,

In this case, S is less compared to fixed bias circuit. Thus the stability of the Q point is better.
Further,

If I
C
is to be made insensitive to
dc
than

R
E
cannot be made large enough to swamp out the effects of
dc
without saturating the transistor.
Collector Feedback Bias:
In this case, the base resistor is returned back to collector as shown in fig. 2. If temperature
increases.
dc
increases. This produces more collectors current. As I
C
increases, collector emitter
voltage decreases. It means less voltage across R
B
and causes a decrease in base current this
decreasing I
C
, and compensating the effect of |
dc
.
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Fig. 2
In this circuit, the voltage equation is given by

Circuit is stiff sensitive to changes in
dc
. The advantage is only two resistors are used.
Then,

Therefore,

It is better as compared to fixed bias circuit.
Further,

Circuit is still sensitive to changes in
dc
. The advantage is only two resistors are used.



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Biasing Techniques
Voltage Divider Bias:
If the load resistance R
C
is very small, e.g. in a transformer coupled circuit, then there is no
improvement in stabilization in the collector to base bias circuit over fixed bias circuit. A circuit
which can be used even if there is no dc resistance in series with the collector, is the voltage
divider bias or self bias. fig. 3.
The current in the resistance R
E
in the emitter lead causes a voltage drop which is in the direction
to reverse bias the emitter junction. Since this junction must be forward biased, the base voltage
is obtained from the supply through R
1
, R
2
network. If R
b
= R
1
|| R
2
equivalent resistance is very
very small, then V
BE
voltage is independent of I
CO
and I
C
/ I
CO
0. For best stability R
1
&
R
2
must be kept small.

Fig. 3
If I
C
tends to increase, because of I
CO
, then the current in R
C
increases, hence base current is
decreased because of more reverse biasing and it reduces I
C
.
To analysis this circuit, the base circuit is replaced by its thevenin's equivalent as shown in fig. 4.
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Fig. 4
Thevenin's voltage is

R
b
is the effective resistance seen back from the base terminal.

If V
BE
is considered to be independent of I
C
, then


The smaller the value of R
b
, the better is the stabilization but S cannot be reduced be unity.
Hence I
C
always increases more than I
CO
. If R
b
is reduced, then current drawn from the supply
increases. Also if R
E
is increased then to operate at same Q-point, the magnitude of V
CC
must be
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increased. In both the cases the power loss increased and reduced h.
In order to avoid the loss of ac signal because of the feedback caused by R
E
, this resistance is
often by passed by a large capacitance (> 10 m F) so that its reactance at the frequency under
consideration is very small.


Biasing Techniques
Emitter Bias:
Fig. 5, shown the emitter bias circuit. The circuit gets this name because the negative supply V
EE

is used to forward bias the emitter junction through resistor R
E
. V
CC
still reverse biases collector
junction. This also gives the same stability as voltage divider circuit but it is used only if split
supply is available.

Fig. 5
In this circuit, the voltage equation is given by
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Biasing Techniques
Example-1
Determine the Q-point for the CE amplifier given in fig. 1, if R
1
= 1.5K O and R
s
= 7K O . A
2N3904 transistor is used with = 180, R
E
= 100O and R
C
= R
load
= 1K O . Also determine the
P
out
(ac) and the dc power delivered to the circuit by the source.

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Fig. 1
Solution:
We first obtain the Thevenin equivalent.

and

Note that this is not a desirable Q-point location since V
BB
is very close to V
BE
. Variation in V
BE

therefore significantly change I
C
.We find R
ac
= R
C
|| R
load
= 500 W and R
dc
= R
C
+ R
E
=1.1KO.
The value of V
CE
representing the quiescent value associated with I
CQ
is found as follows,

Then

Since the Q-point is on the lower half of the ac load line, the maximum possible symmetrical
output voltage swing is

The ac power output can be calculated as

The power drawn from the dc source is given by

The power loss in the transistor is given by

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The Q-point in this example is not in the middle of the load line so that output swing is not as
great as possible. However, if the input signal is small and maximum output is not required, a
small I
C
can be used to reduce the power dissipated in the circuit.

Biasing Techniques
Moving Ground Around:
Ground is a reference point that can be moved around. e.g. consider a collector feedback bias
circuit. The various stages of moving ground are shown in fig. 2.

Fig. 2
Biasing a pnp Transistor:
The biasing of pnp transistor is done similar to npn transistor except that supply is of opposite
polarity The various biasing circuits of pnp transistor are shown in fig. 3.
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Fig. 3
Example 2:
For the circuit shown in fig. 4, calculate I
C
and V
CE

Solution:


Fig. 4




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Small Signal CE Amplifiers
Small Signal CE Amplifiers:
CE amplifiers are very popular to amplify the small signal ac. After a transistor has been biased
with a Q point near the middle of a dc load line, ac source can be coupled to the base. This
produces fluctuations in the base current and hence in the collector current of the same shape and
frequency. The output will be enlarged sine wave of same frequency.
The amplifier is called linear if it does not change the wave shape of the signal. As long as the
input signal is small, the transistor will use only a small part of the load line and the operation
will be linear.
On the other hand, if the input signal is too large. The fluctuations along the load line will drive
the transistor into either saturation or cut off. This clips the peaks of the input and the amplifier is
no longer linear.
The CE amplifier configuration is shown in fig. 1.

Fig. 1
The coupling capacitor (C
C
) passes an ac signal from one point to another. At the same time it
does not allow the dc to pass through it. Hence it is also called blocking capacitor.
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Fig. 2
For example in fig. 2, the ac voltage at point A is transmitted to point B. For this series reactance
X
C
should be very small compared to series resistance R
S
. The circuit to the left of A may be a
source and a series resistor or may be the Thevenin equivalent of a complex circuit. Similarly R
L

may be the load resistance or equivalent resistance of a complex network. The current in the loop
is given by

As frequency increases, decreases, and current increases until it reaches to its
maximum value v
in
/ R. Therefore the capacitor couples the signal properly from A to B when
X
C
<< R. The size of the coupling capacitor depends upon the lowest frequency to be coupled.
Normally, for lowest frequency X
C
s 0.1R is taken as design rule.
The coupling capacitor acts like a switch, which is open to dc and shorted for ac.
The bypass capacitor C
b
is similar to a coupling capacitor, except that it couples an ungrounded
point to a grounded point. The C
b
capacitor looks like a short to an ac signal and therefore
emitter is said ac grounded. A bypass capacitor does not disturb the dc voltage at emitter because
it looks open to dc current. As a design rule X
Cb
s 0.1R
E
at lowest frequency.


Small Signal CE Amplifiers
Analysis of CE amplifier:
In a transistor amplifier, the dc source sets up quiescent current and voltages. The ac source then
produces fluctuations in these current and voltages. The simplest way to analyze this circuit is to
split the analysis in two parts: dc analysis and ac analysis. One can use superposition theorem for
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analysis .
AC & DC Equivalent Circuits:
For dc equivalent circuit, reduce all ac voltage sources to zero and open all ac current sources
and open all capacitors. With this reduced circuit shown in fig. 3 dc current and voltages can be
calculated.

Fig. 3
For ac equivalent circuits reduce dc voltage sources to zero and open current sources and short
all capacitors. This circuit is used to calculate ac currents and voltage as shown in fig. 4.

Fig. 4
The total current in any branch is the sum of dc and ac currents through that branch. The total
voltage across any branch is the sum of the dc voltage and ac voltage across that branch.
Phase Inversion:
Because of the fluctuation is base current; collector current and collector voltage also swings
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above and below the quiescent voltage. The ac output voltage is inverted with respect to the ac
input voltage, meaning it is 180
o
out of phase with input.
During the positive half cycle base current increase, causing the collector current to increase.
This produces a large voltage drop across the collector resistor; therefore, the voltage output
decreases and negative half cycle of output voltage is obtained. Conversely, on the negative half
cycle of input voltage less collector current flows and the voltage drop across the collector
resistor decreases, and hence collector voltage increases we get the positive half cycle of output
voltage as shown in fig. 5.

Fig. 5

Analysis of CE amplifier
AC Load line:
Consider the dc equivalent circuit fig. 1.
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Fig. 1
Assuming I
C
= I
C
(approx), the output circuit voltage equation can be written as

The slop of the d.c load line is .
When considering the ac equivalent circuit, the output impedance becomes R
C
|| R
L
which is less
than (R
C
+R
E
).
In the absence of ac signal, this load line passes through Q point. Therefore ac load line is a line
of slope (-1 / ( R
C
|| R
L
) ) passing through Q point. Therefore, the output voltage fluctuations will
now be corresponding to ac load line as shown in fig. 2. Under this condition, Q-point is not in
the middle of load line, therefore Q-point is selected slightly upward, means slightly shifted to
saturation side.
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Fig. 2


Analysis of CE amplifier
Voltage gain:
To find the voltage gain, consider an unloaded CE amplifier. The ac equival ent circuit is
shown in fig. 3. The transistor can be replaced by its collector equivalent model i.e. a
current source and emitter diode which offers ac resistance r'
e
.

Fig. 3
The input voltage appears directly across the emitter diode.
Therefore emitter current i
e
= V
in
/ r'
e
.
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Since, collector current approximately equals emitter current and i
C
= i
e
and v
out
= - i
e
R
C
(The
minus sign is used here to indicate phase inversion)
Further v
out
= - (V
in
R
C
) / r'
e

Therefore voltage gain A = v
out
/ v
in
= -R
C
/ r'
e

The ac source driving an amplifier has to supply alternating current to the amplifier. The
input impedance of an amplifier determines how much current the amplifier takes from
the ac source.
In a normal frequency range of an amplifier, where all capacitors look like ac shorts and
other reactance are negligible, the ac input impedance is defined as
z
in
= v
in
/ i
in

Where v
in
, i
in
are peak to peak values or rms values
The impedance looking directly into the base is symbolized z
in (base)
and is given by
Z
in(base)
= v
in
/ i
b
,
Since,v
in
= i
e
r'
e

b
i
b r'
e

z
in (base)
= b r'
e
.
From the ac equivalent circuit, the input impedance z
in
is the parallel combination of R
1
,
R
2
and b r'
e
.
Z
in
= R
1
|| R
2
|| b r'
e

The Thevenin voltage appearing at the output is
v
out
= A v
in

The Thevenin impedance is the parallel combination of R
C
and the internal impedance
of the current source. The collector current source is an ideal source, therefore it has an
infinite internal impedance.
z
out
= R
C
.
The simplified ac equivalent circuit is shown in fig. 4.
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Fig. 4

h-Parameters
Small signal low frequency transistor Models:
All the transistor amplifiers are two port networks having two voltages and two currents. The
positive directions of voltages and currents are shown in fig. 1.

Fig. 1
Out of four quantities two are independent and two are dependent. If the input current i
1
and
output voltage v
2
are taken independent then other two quantities i
2
and v
1
can be expressed in
terms of i
1
and V
2
.

The equations can be written as

where h
11
, h
12
, h
21
and h
22
are called h-parameters.

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= h
i
= input impedance with output short circuit to ac.

=h
r
= fraction of output voltage at input with input open circuited or reverse voltage gain with
input open circuited to ac (dimensions).

= h
f
= negative of current gain with output short circuited to ac.
The current entering the load is negative of I
2
. This is also known as forward short circuit current
gain.

= h
o
= output admittance with input open circuited to ac.
If these parameters are specified for a particular configuration, then suffixes e,b or c are also
included, e.g. h
fe
,h
ib
are h parameters of common emitter and common collector amplifiers
Using two equations the generalized model of the amplifier can be drawn as shown in fig. 2.

Fig. 2



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h-Parameters
The hybrid model for a transistor amplifier can be derived as follow:
Let us consider CE configuration as show in fig. 3. The variables, i
B
, i
C
,v
C
, and v
B
represent total
instantaneous currents and voltages i
B
and v
C
can be taken as independent variables and v
B
, I
C
as
dependent variables.

Fig. 3
v
B
= f
1
(i
B
,v
C
)
I
C
= f
2
( i
B
, v
C
).
Using Taylor 's series expression, and neglecting higher order terms we obtain.

The partial derivatives are taken keeping the collector voltage or base current constant. The v
B
,
v
C
, i
B
, i
C
represent the small signal (incremental) base and collector current and voltage
and can be represented as v
b
,i
b
,v
C
,i
C
.
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The model for CE configuration is shown in fig. 4.

Fig. 4

h-Parameters
Actcivotiov o| q tooctco:
To determine the four h-parameters of transister amplifier, input and output characteristic are
used. Input characteristic depicts the relationship between input voltage and input current with
output voltage as parameter. The output characteristic depicts the relationship between output
voltage and output current with input current as parameter. Fig. 5, shows the output
characterisitcs of CE amplifier.

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Fig. 5
The current increments are taken around the quiescent point Q which corresponds to i
B
= I
B
and
to the collector voltage V
CE
= V
C


The value of h
oe
at the quiescent operating point is given by the slope of the output characteristic
at the operating point (i.e. slope of tangent AB).

h
ie
is the slope of the appropriate input on fig. 6, at the operating point (slope of tangent EF at Q).

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Fig. 6
A vertical line on the input characteristic represents constant base current. The parameter hre can
be obtained from the ratio (V
B2
V
B1
) and (V
C2
V
C1
) for at Q.
Typical CE h-parametersof transistor 2N1573 are given below:
h
ie
= 1000 ohm.
h
re
= 2.5 * 10 4
h
fe
= 50
h
oe
= 25 m A / V

h-parameters
Analysis of a transistor amplifier using h-parameters:
To form a transistor amplifier it is only necessary to connect an external load and signal source
as indicated in fig. 1 and to bias the transistor properly.

Fig. 1
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Consider the two-port network of CE amplifier. R
S
is the source resistance and Z
L
is the load
impedence h-parameters are assumed to be constant over the operating range. The ac equivalent
circuit is shown in fig. 2. (Phasor notations are used assuming sinusoidal voltage input). The
quantities of interest are the current gain, input impedence, voltage gain, and output impedence.

Fig. 2
Current gain:
For the transistor amplifier stage, A
i
is defined as the ratio of output to input currents.

Input Impedence:
The impedence looking into the amplifier input terminals ( 1,1' ) is the input impedence Z
i

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h-parameters
Voltage gain:
The ratio of output voltage to input voltage gives the gain of the transistors.

Output Admittance:
It is defined as
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A
v
is the voltage gain for an ideal voltage source (R
v
= 0).
Consider input source to be a current source I
S
in parallel with a resistance R
S
as shown in fig. 3.

Fig. 3
In this case, overall current gain A
IS
is defined as
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h-parameters
To analyze multistage amplifier the h-parameters of the transistor used are obtained from
manufacture data sheet. The manufacture data sheet usually provides h-parameter in CE
configuration. These parameters may be converted into CC and CB values. For example fig. 4 h
rc

in terms of CE parameter can be obtained as follows.

Fig. 4
For CE transistor configuaration
V
be
= h
ie
I
b
+ h
re
V
ce

I
c
= h
fe
I
b
+ h
oe
V
ce

The circuit can be redrawn like CC transistor configuration as shown in fig. 5.
V
bc
= h
ie
I
b
+ h
rc
V
ec

I
c
= h
fe
I
b
+ h
oe
V
ec

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Fig. 5


h-parameters
Example - 1
For the circuits shown in fig. 1. (CECC configuration) various h-parameters are given
h
ie
= 2K, h
fe
= 50, h
re
= 6 * 10
-4
, h
oc
= 25 m A / V.
h
ic
= 2K, h
fe
= -51, h
re
= 1, h
oc
= 25 m A / V.

Fig. 1
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The small signal model of the transistor amplifier is shown in fig. 2.

Fig. 2
In the circuit, the collector resistance of first stage is shunted by the input impedence of last
stage. Therefore the analysis is started with last stage. It is convenient; to first compute current
gain, input impedence and voltage gain. Then output impedence is calculated starting from first
stage and moving towards end.
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The effective source resistance R'
S2
for the second stage is R
01
|| R
C1
. Thus R
S2
= R'
01
= 4.65K


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Frequency response

Overall current gain of the amplifier is A
i
and is
given by

The equivalent circuit of the amplifier is shown in
fig. 3. From the circuit it is clear that the current i
c1
is
divided into two parts.
Therefore,

and


Fig.3
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Overall voltage gain of the amplifier is given by



h-parameters
Simplified common emitter hybrid model:
In most practical cases it is appropriate to obtain approximate values of A
V
, A
i
etc rather than
calculating exact values. How the circuit can be modified without greatly reducing the accuracy.
Fig. 4 shows the CE amplifier equivalent circuit in terms of h-parameters Since 1 / h
oe
in parallel
with R
L
is approximately equal to R
L
if 1 / h
oe
>> R
L
then h
oe
may be neglected. Under these
conditions.
I
c
= h
fe
I
B
.
h
re
v
c
= h
re
I
c
R
L
= h
re
h
fe
I
b
R
L
.

Fig. 4
Since h
fe
.h
re
~ 0.01, this voltage may be neglected in comparison with h
ic
I
b
drop across h
ie

provided R
L
is not very large. If load resistance R
L
is small than hoe and h
re
can be neglected.
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Output impedence seems to be infinite. When V
s
= 0, and an external voltage is applied at the
output we fined I
b
= 0, I
C
= 0. True value depends upon R
S
and lies between 40 K and 80K.
On the same lines, the calculations for CC and CB can be done.
CE amplifier with an emitter resistor:
The voltage gain of a CE stage depends upon h
fe
. This transistor parameter depends upon
temperature, aging and the operating point. Moreover, h
fe
may vary widely from device to
device, even for same type of transistor. To stabilize voltage gain A
V
of each stage, it should be
independent of h
fe
. A simple and effective way is to connect an emitter resistor R
e
as shown in
fig. 5. The resistor provides negative feedback and provide stabilization.

Fig. 5
An approximate analysis of the circuit can be made using the simplified model.
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Subject to above approximation A
V
is completely stable. The output resistance is infinite for the
approximate model.
Opto Coupler:
It combines a LED and a photo diode in a single package as shown in fig. 6. LED radiates the light depending on the current
through LED. This light fails on photo diode and this sets up a reverse current. The advantage of an opto coupler is the electrical
isolation between the input and output circuits. The only contact between the input and output is a beam of light. Because of this,
it is possible to have an insulation resistance between the two circuits of the order of thousands of mega ohms. They can be used
to isolate two circuits of different voltage levels.

Fig. 6

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TRANSISTOR BIAS STABILITY
BJT: Bipolar Junction Transistors:
Bipolar transistor amplifiers must be properly biased to operate correctly. In circuits made with
individual devices (discrete circuits), biasing networks consisting of resistors are commonly
employed. Much more elaborate biasing arrangements are used in integrated circuits, for
example, bandgap voltage references and current mirrors.
Bias circuit requirements:
For analog circuit operation, the Q-point is placed so the transistor stays in active mode (does not
shift to operation in the saturation region or cut-off region) when input is applied. For digital
operation, the Q-point is placed so the transistor does the contrary - switches from "on" to "off"
state. Often, Q-point is established near the center of active region of transistor characteristic to
allow similar signal swings in positive and negative directions. Q-point should be stable. In
particular, it should be insensitive to variations in transistor parameters (for example, should not
shift if transistor is replaced by another of the same type), variations in temperature, variations in
power supply voltage and so forth. The circuit must be practical: easily implemented and cost-
effective.
Load line:
The concept of load line is very important in understanding the working of a transistor. It is
defined as the locus of operating points on the output characteristics of the transistor. It is the line
on which the operating point moves when ac signal is applied to the transistor.
The dc load line gives the value of I
c
and V
CE
corresponding to zero signal conditions. The ac
load line gives the value of I
C
and V
CE
when ac signal is applied. AC load line is steeper than the
dc load line but the two intersect at the Q-point determined by biasing dc voltages and
currents. AC load line takes into account the ac load resistance while dc load line considers only
the dc load resistance.
Q Point:
The operating point of a device, also known as bias point, quiescent point, or Q-point, is the
point on the output characteristics that shows the DC collectoremitter voltage (Vce) and the
collector current (Ic) with no input signal applied. The term is normally used in connection with
devices such as transistors.
For bipolar junction transistors the bias point is chosen to keep the transistor operating in the
active mode, using a variety of circuit techniques, establishing the Q-point DC voltage and
current. A small signal is then applied on top of the Q-point bias voltage, thereby either
modulating or switching the current, depending on the purpose of the circuit.
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The quiescent point of operation is typically near the middle of DC load line. The process of
obtaining certain DC collector current at a certain DC collector voltage by setting up operating
point is called biasing.
After establishing the operating point, when input signal is applied, the output signal should not
move the transistor either to saturation or to cut-off. However, this unwanted shift still might
occur, due to the following reasons:
1. Parameters of transistors depend on junction temperature. As junction temperature
increases, leakage current due to minority charge carriers (I
CBO
) increases. As I
CBO

increases, I
CEO
also increases, causing an increase in collector current I
C
. This produces
heat at the collector junction. This process repeats, and, finally, Q-point may shift into the
saturation region. Sometimes, the excess heat produced at the junction may even burn the
transistor. This is known as thermal runaway.
2. When a transistor is replaced by another of the same type, the Q-point may shift, due to
changes in parameters of the transistor, such as current gain () which varies slightly for
each unique transistor.
To avoid a shift of Q-point, bias-stabilization is necessary. Various biasing circuits can be used
for this purpose.
VARIOUS METHODS FOR TRANSISTOR BIASING
1. Fixed bias
2. Collector-to-base bias
3. Voltage divider bias
4. Emitter bias










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1)Fixed bias (base bias):



This form of biasing is also called base bias. In the example image on the right, the single power
source (for example, a battery) is used for both collector and base of transistor, although separate
batteries can also be used.
In the given circuit,
V
CC
= I
B
R
B
+ V
be

Therefore,
I
B
= (V
CC
- V
be
)/R
B

For a given transistor, V
be
does not vary significantly during use. As V
CC
is of fixed value, on
selection of R
B
, the base current I
B
is fixed. Therefore this type is called fixed bias type of circuit.
Also for given circuit,
V
CC
= I
C
R
C
+ V
ce

Therefore,
V
ce
= V
CC
- I
C
R
C

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The common-emitter current gain of a transistor is an important parameter in circuit design, and
is specified on the data sheet for a particular transistor. It is denoted as on this page.
Because I
C
= I
B

we can obtain I
C
as well. In this manner, operating point given as (V
CE
,I
C
) can be set for given
transistor.
Merits:
- It is simple to shift the operating point anywhere in the active region by merely changing
the base resistor (R
B
).
- A very small number of components are required.
Demerits:
- The collector current does not remain constant with variation in temperature or power
supply voltage. Therefore the operating point is unstable.
- Changes in V
be
will change I
B
and thus cause R
E
to change. This in turn will alter the gain
of the stage.
- When the transistor is replaced with another one, considerable change in the value of
can be expected. Due to this change the operating point will shift.
2)Collector-to-base bias:


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This configuration employs negative feedback to prevent thermal runaway and stabilize the
operating point. In this form of biasing, the base resistor RB is connected to the collector instead
of connecting it to the DC source V
CC
. So any thermal runaway will induce a voltage drop across
the R
C
resistor that will throttle the transistor's base current.
If V
be
is held constant and temperature increases, then the collector current I
c
increases. However,
a larger I
c
causes the voltage drop across resistor R
c
to increase, which in turn reduces the voltage
across the base resistor R
b
. A lower base-resistor voltage drop reduces the base current I
b
, which
results in less collector current I
c
. Because an increase in collector current with temperature is
opposed, the operating point is kept stable.
Merits:
- Circuit stabilizes the operating point against variations in temperature and (ie.
replacement of transistor)
Demerits:
- In this circuit, to keep I
c
independent of , the following condition must be met:
which is the case when
- As -value is fixed (and generally unknown) for a given transistor, this relation can be
satisfied either by keeping R
c
fairly large or making R
b
very low.
- If R
c
is large, a high V
cc
is necessary, which increases cost as well as precautions
necessary while handling.
- If R
b
is low, the reverse bias of the collectorbase region is small, which limits the
range of collector voltage swing that leaves the transistor in active mode.
- The resistor R
b
causes an AC feedback, reducing the voltage gain of the amplifier. This
undesirable effect is a trade-off for greater Q-point stability.







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3)Voltage divider bias

The voltage divider is formed using external resistors R
1
and R
2
. The voltage across R
2
forward
biases the emitter junction. By proper selection of resistors R
1
and R
2
, the operating point of the
transistor can be made independent of . In this circuit, the voltage divider holds the base voltage
fixed independent of base current provided the divider current is large compared to the base
current. However, even with a fixed base voltage, collector current varies with temperature (for
example) so an emitter resistor is added to stabilize the Q-point, similar to the above circuits with
emitter resistor.
Merits:
- Unlike above circuits, only one dc supply is necessary.
- Operating point is almost independent of variation.
- Operating point stabilized against shift in temperature.
Demerits:
- As -value is fixed for a given transistor, this relation can be satisfied either by keeping
R
E
fairly large, or making R
1
||R
2
very low.
- If R
E
is of large value, high V
CC
is necessary. This increases cost as well as
precautions necessary while handling.
- If R
1
|| R
2
is low, either R
1
is low, or R
2
is low, or both are low. A low R
1
raises
V
B
closer to V
C
, reducing the available swing in collector voltage, and limiting
how large R
C
can be made without driving the transistor out of active mode. A
low R
2
lowers V
be
, reducing the allowed collector current. Lowering both resistor
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values draws more current from the power supply and lowers the input resistance
of the amplifier as seen from the base.
- AC as well as DC feedback is caused by R
E
, which reduces the AC voltage gain of the
amplifier. A method to avoid AC feedback while retaining DC feedback is discussed
below.
4)Emitter bias:


When a split supply (dual power supply) is available, this biasing circuit is the most effective,
and provides zero bias voltage at the emitter or collector for load. The negative supply V
EE
is
used to forward-bias the emitter junction through R
E
. The positive supply V
CC
is used to reverse-
bias the collector junction. Only two resistors are necessary for the common collector stage and
four resistors for the common emitter or common base stage.
We know that,
V
B
- V
E
= V
be

If R
B
is small enough, base voltage will be approximately zero. Therefore emitter current is,
I
E
= (V
EE
- V
be
)/R
E

The operating point is independent of if R
E
>> R
B
/

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Merit:
Good stability of operating point similar to voltage divider bias.
Demerit:
This type can only be used when a split (dual) power supply is available.
Stability Factor:
The degree of success achieved in stabilizing I
C
in the face of variations in I
CO
is expressed in
terms of stability factor S and it is defined as the rate of change of I
C
w.r.t. I
CO
, keeping and
V
BE
constant.
i.e. S = dI
c
/dI
co
at constant and V
BE
(or I
B
)
From above expression it is obvious that smaller is the value of S, higher is the stability. It is
desirable to have as low stability factor as possible so as to achieve greater thermal stability. The
ideal value of S is unity (because I
C
includes I
CO
) but it is never possible to achieve it in practice.
Stability Factor in case of CB circuit,
S = dI
c
/dI
co
= d(I
E
+ I
co
)/dI
co
0+1 or S = 1
Stability factor in case of CE circuit
S = dI
c
/dI
co
= d[I