ANALOG & DIGITAL ELECTRONICS (A30461) SYLLABUS

B. Tech. (CSE) III-Semester

Unit-I:P-N Junction Diode
P-N junction as a Diode, Diode Equation, Volt-Ampere Characteristics, Temperature dependence of V-I characteristics,
Ideal versus Practical- Resistance levels (Static and Dynamic). Transition and Diffusion Capacitances, Diode Equivalent
Circuits, Load Line Analysis, Breakdown Mechanisms in Semiconductor Diodes, Zener Diode Characteristics.

Diode Applications: Operation of Diode Rectifiers (Half Wave, Full Wave & Bridge) and Zener Voltage Regulator.

Unit –II: Bipolar Junction Transistor and UJT

The Junction Transistor-Current Components, Construction & Operation, Configurations-Common base, Common
Emitter and Common Collector. Comparison of CB, CE and CC characteristics, Transistor biasing, Transistor as an
amplifier, UJT operation & its Characteristics.

Unit- III: Field Effect Transistor and Number Systems

The Junction Field Effect Transistor (Construction &principle of operation), Volt-Ampere characteristics, MOSFET
(Construction & principle of operation), MOSFET Characteristics in Enhancement and Depletion modes.

Number Systems: Introduction to Number Systems, Base Conversion Methods, Complements of numbers, Codes –
binary codes, Binary Coded Decimal code and its properties, Gray Code, Alpha Numeric Codes, Error Detecting and
Correcting Codes.

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 ANALOG & DIGITAL ELECTRONICS (A30461)
B. Tech. (CSE) III-Semester
Unit- IV: Boolean Algebra and Combinational Circuits
Basic theorems and properties - Switching Functions, Canonical and Standard Forms-Algebraic simplification,
Digital Logic Gates, Properties of XOR gates &Universal Gates-Multilevel NAND/NOR realizations, The
Minimization of Boolean functions, Karnaugh Map method –four and five variable maps, Prime and Essential
Implications, Don ‘t Care Map Entries .

Combinational Circuits: Introduction, Arithmetic Circuits, Code-converters, Comparator, Multiplexer, Decoder

and Encoder.

Unit- V: Sequential Circuits Design

Types of Flip Flops- SR, JK, D and T. Realization using Flip-Flops, ripple counter, synchronous counter, shift
register, ring counter using shift register. Finite state machine-capabilities and limitations, Mealy and Moore
models-minimization of completely specified and incompletely specified sequential machines.
Text Books
1. Millman's Electronic Devices & Circuits-J. Millman, C.C. Halkais & Satyabrata Jit, 2 Ed., 1998, TMH.
2. Digital Design -Morris Mano, PHI, 3rd Edition, 2006.

Reference Books
1. Integrated Electronics- J. Millman and Christos C. Halkais, 1991 Ed., 2008, TMH.
2. Electronic Devices and Circuits- R.L. Boylstad and Louis Nashelsky, 9 Ed., 2006, PEI/PHI
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 N-Type Material
N-Type Material: ➢ When extra valence electrons are introduced into a material
such as silicon an n-type material is produced. The extra
valence electrons are introduced by putting impurities or
dopant into the silicon. The dopant used to create an n-type
material are Group V elements.

➢ The most commonly used dopant from Group V are arsenic,

antimony and phosphorus.

➢ The 2D diagram to the left shows the extra electron that will
be present when a Group V dopant is introduced to a material
such as silicon. This extra electron is very mobile.

Crystal lattice with a germanium atom displaced by

a pentavalent impurity atom.

Energy-band diagram of n-type semiconductor

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 P-Type Material
➢ P-type material is produced when the dopant that is introduced
P-Type Material: is from Group III. Group III elements have only 3 valence
electrons and therefore there is an electron missing. This
creates a hole (h+), or a positive charge that can move around
in the material.

➢ Commonly used Group III dopant are aluminum, boron, and

gallium.

➢ The 2D diagram to the left shows the hole that will be present
when a Group III dopant is introduced to a material such as
silicon. This hole is quite mobile in the same way the extra
electron is mobile in a n-type material.

Crystal lattice with a germanium atom

displaced by an atom of a trivalent

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Energy-band diagram of p-type semiconductor 4
 Conductivity of a Semiconductor
❑ With each hole-electron pair created, two charge-carrying “particles” are formed. One is negative (the
free electron), of mobility mn, and the other is positive (the hole), of mobility mp. These particles move
in opposite directions in an electric field e, but since they are of opposite sign, the current of each is in
the same direction. Hence the current density J is given by

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 Properties of germanium and silicon

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 Fermi Level in a Semiconductor Having Impurities
➢ In order to see how EF depends on temperature and impurity concentration, we recall that, in the case of no
impurities (an int r ins ic semiconductor), EF lies in the middle of the energy gap, indicating equal
concentrations of free electrons and holes.
➢ If a donor type impurity is added to the crystal, then, at a given temperature and assuming all donor atoms are
ionized, the first ND states in the conduction band will be filled. Hence it will be more difficult for the
electrons from the valence band to bridge the energy gap by thermal agitation.
➢ Consequently, the number of electron-hole pairs thermally generated for that temperature will be reduced.
Since the Fermi level is a measure of the probability of occupancy of the allowed energy states, it is clear that
EF must move closer to the conduction band to indicate that many of the energy states in that band are filled
by the donor electrons, and fewer holes exist in the valence band.

Positions of Fermi level in (a) n-type and (b) p-type semiconductors.

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 The PN Junction diode
Steady State
Metallurgical
Na Junction Nd

- - - - - + + + + +
- - - - - + + + + + When no external source is
P - - - - - + + + + + n
- - - - - + + + + + connected to the pn junction,
diffusion and drift balance each
ionized
Space Charge
Region
other out for both the holes and
ionized
acceptors donors electrons
E-Field
_ _
+ +
h+ drift == h+ diffusion e- diffusion == e- drift

Space Charge Region: Also called the depletion region. This region includes the net
positively and negatively charged regions. The space charge region does not have any free
carriers. The width of the space charge region is denoted by W in PN junction formula’s.

Metallurgical Junction: The interface where the p- and n-type materials meet.

Na & Nd: Represent the amount of negative and positive doping in number of carriers per
centimeter cubed. Usually in the range of 1015 to 1020.
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 The PN Junction diode

➢ If donor impurities are introduced into one side and acceptors

into the other side of a single crystal of a semiconductor, say,
germanium, a p-n junction is formed. Such a system is
illustrated in Fig. 5.1a.
➢ The donor ion is indicated schematically by a plus sign
because, after this impurity atom “donates” an electron, it
becomes a positive ion. The acceptor ion is indicated by a
minus sign because, after this atom “accepts” an electron, it
becomes a negative ion.
➢ Initially, there are nominally only p-type carriers to the left of
the junction and only n-type carriers to the right. Because
there is a density gradient across the junction, holes will
diffuse to the right across the junction, and electrons to the
left.

A schematic diagram of a p-n junction, including the charge density,

electric field intensity, and potential-energy barriers at the junction.
(Not drawn to scale.)
Band Structure of an Open-Circuited p-n Junction
➢ The energy-band diagram for a p-n junction appears as shown in Fig.
5.4, where a shift in energy levels Eo is indicated. Note that

Band diagram for a p-n junction under open-circuit

conditions. This sketch corresponds to Fig. 5.1e and
represents potential energy for electrons. The width of the
forbidden gap is EG in electron volts
 The Biased PN Junction
Metal
Contact
“Ohmic
Contact” _
(Rs~0) +
Applied Electric
P Field n

I
_
+

Vapplied
The pn junction is considered as biased when an external voltage is applied. There are two types of biasing:
1. Forward bias
2. Reverse bias.
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 The Biased PN Junction

Forward Bias: In forward bias the depletion region shrinks slightly in width. With this shrinking
the energy required for charge carriers to cross the depletion region decreases
exponentially. Therefore, as the applied voltage increases, current starts to flow
Vapplied > 0 across the junction. The barrier potential of the diode is the voltage at which
appreciable current starts to flow through the diode. The barrier potential varies
for different materials.

Reverse Bias: Under reverse bias the depletion region widens. This causes the electric field
produced by the ions to cancel out the applied reverse bias voltage. A small
leakage current, Is (saturation current) flows under reverse bias conditions. This
Vapplied < 0 saturation current is made up of electron-hole pairs being produced in the
depletion region. Saturation current is sometimes referred to as scale current
because of it’s relationship to junction temperature.

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 I-V Characteristics of P-N junction Diode

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 Properties of Diodes
The Shockley Equation
• The transconductance curve on the previous slide is characterized by the following
equation:
ID = IS(eVD/VT – 1)
• As described in the last slide, ID is the current through the diode, IS is the saturation
current and VD is the applied biasing voltage.
• VT is the thermal equivalent voltage and is approximately 26 mV at room
temperature. The equation to find VT at various temperatures is:
VT = kT
q
k = 1.38 x 10-23 J/K T = temperature in Kelvin q = 1.6 x 10-19 C

•  is the emission coefficient for the diode. It is determined by the way the diode is
constructed. It somewhat varies with diode current. For a silicon diode  is around 2
for low currents and goes down to about 1 at higher currents

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 Types of Diodes and Their Uses

PN Junction Are used to allow current to flow in one direction while blocking
current flow in the opposite direction. The pn junction diode is the
Diodes: typical diode that has been used in the previous circuits.

A K P n
Schematic Symbol for a PN Representative Structure for a
Junction Diode PN Junction Diode

Zener Diodes: Are specifically designed to operate under reverse breakdown

conditions. These diodes have a very accurate and specific reverse
breakdown voltage.

A K

Schematic Symbol for a Zener

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 Types of Diodes and Their Uses

Light-Emitting Light-emitting diodes are designed with a very large band gap so
movement of carriers across their depletion region emits photons of
Diodes: light energy. Lower band gap LEDs (Light-Emitting Diodes) emit
infrared radiation, while LEDs with higher band gap energy emit
visible light. Many stop lights are now starting to use LEDs because
they are extremely bright and last longer than regular bulbs for a
relatively low cost.

A K The arrows in the LED

representation indicate
emitted light.
Schematic Symbol for a Light-
Emitting Diode

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 Types of Diodes and Their Uses

Photodiodes:
While LEDs emit light, Photodiodes are sensitive to received light.
They are constructed so their pn junction can be exposed to the
outside through a clear window or lens.

A K In Photoconductive mode the saturation current increases in

proportion to the intensity of the received light. This type of diode
is used in CD players.

 In Photovoltaic mode, when the pn junction is exposed to a certain

A K wavelength of light, the diode generates voltage and can be used as
an energy source. This type of diode is used in the production of
solar power.
Schematic Symbols for
Photodiodes

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 Ideal Diode

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 Temperature dependence of V-I characteristics of
P-N Junction diode
➢ The temperature is contained implicitly in VT and also in the reverse saturation current.
➢ The dependence of Io on temperature T is, where K is a constant and eVGO (e is the magnitude of
the electronic charge), VGO is the forbidden-gap
For Ge: h = 1 m = 2 VGO = 0.785 V
for Si: h = 2 m = 1.5 VGO = 1.21 V

Reverse saturation current at three different

Volt-ampere characteristics at three different temperatures temperatures for a silicon diode
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for a silicon diode
 Diode Resistance
❑ The static resistance R of a diode is defined as the ratio V/I of the voltage to the current. At any point on the volt-
ampere characteristic of the diode (Fig. 5.7), the resistance R is equal to the reciprocal of the slope of a line joining the
operating point to the origin. The static resistance varies widely with V and I and is not a useful parameter.

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 Static Resistance

➢ The static resistance R of a diode is defined as the ratio V/I of the voltage to the current.
➢ At any point on the volt-ampere characteristic of the diode, the resistance R is equal to the reciprocal of
the slope of a line joining the operating point to the origin.
➢ The static resistance varies widely with V and I and is not a useful parameter.

Linear characterization of a semiconductor diode.

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 Diode Resistance
For small-signal operation the dynamic, or incremental, resistance r is an important parameter, and is defined as the
reciprocal of the slope of the volt-ampere characteristic, r = dV/dI. The dynamic resistance is not a constant, but
depends upon the operating voltage. For example, for a semiconductor diode, we find from Eq. (5.33) that the dynamic
conductance g = 1/r is

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 Transition Capacitance

➢ A reverse bias causes majority carriers to move away from the junction, thereby uncovering more
immobile charges.
➢ Hence the thickness of the space-charge layer at the junction increases with reverse voltage.
➢ This increase in uncovered charge with applied voltage may be considered a capacitive effect. We may
define an incremental capacitance CT by

where dQ is the increase in charge caused by a change dV in voltage. It follows from this definition that
a change in voltage dV in a time dt will result in a current i = dQ/dt, given by

Therefore a knowledge of CT is important in considering a diode (or a transistor) as a circuit element.

The quantity CT is referred to as the transition-region, space-charge, barrier, or depletion-region,
capacitance.

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 Diffusion Capacitance
For a forward bias a capacitance which is much larger than that considered in the preceding section comes into play.
The origin of this capacitance is now discussed. If the bias is in the forward direction, the potential barrier at the
junction is lowered and holes from the p side enter the n side. Similarly, electrons from the n side move into the p side.
It is convenient to introduce an incremental capacitance, defined as the rate of change of injected charge with applied
voltage. This capacitance CD is called the diffusion, or storage, capacitance.

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 Zener Diode
➢ A Zener Diode, also known as a breakdown diode, is a heavily doped semiconductor device that is designed
to operate in the reverse direction.

➢ Voltage regulation is the most common application of a Zener diode.

➢ When the voltage across the terminals of a Zener diode is reversed and the potential reaches the Zener Voltage
(knee voltage), the junction breaks down and the current flows in the reverse direction. This effect is known as
the Zener Effect.

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 Types of breakdowns for a Zener Diode

❖ Avalanche Breakdown
❖ Zener Breakdown

➢ Avalanche breakdown occurs in normal diode and Zener Diode at high reverse voltage.

➢ When a high value of reverse voltage is applied to the PN junction, the free electrons gain sufficient energy

and accelerate at high velocities.

➢ These free electrons moving at high velocity collide with other atoms and knock off more electrons. Due to

this continuous collision, a large number of free electrons are generated as a result of electric current in the

diode rapidly increases.

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 Types of breakdowns for a Zener Diode
❑ Avalanche Breakdown: This sudden increase in electric current may
permanently destroy the normal diode. However, a Zener diode is designed to
operate under avalanche breakdown and can sustain the sudden spike of
current.
❑ Avalanche breakdown occurs in Zener diodes with Zener voltage (Vz) greater
than 6V.

❑ Zener Breakdown: When the applied reverse bias voltage reaches closer to the
Zener voltage, the electric field in the depletion region gets strong enough to
pull electrons from their valence band. The valence electrons that gain
sufficient energy from the strong electric field of the depletion region break
free from the parent atom.
❑ At the Zener breakdown region, a small increase in the voltage results in the
rapid increase of the electric current.

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 V-I Characteristics of Zener Diode

When reverse-biased voltage is applied to a Zener diode, it allows only a small amount of
leakage current until the voltage is less than Zener voltage.

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 Zener Diode
➢ They are specially designed p-n junction diodes with adequate power dissipation capabilities to operate in the
breakdown region which can be employed as voltage reference or constant voltage devices in the electronic circuits.
➢ In other words, a Zener diode maintains nearly a constant voltage across its terminals in the breakdown region
irrespective of the current flowing through the diode in its operating regions. This important property of the Zener
diodes is used to minimize the voltage fluctuation of a dc power supply obtained by the rectifier-filter combination
discussed earlier.
➢ This is why; a Zener diode is sometimes called a voltage regulator diode and the diode circuit in which the Zener
diodes are used as voltage regulator is called a Zener voltage regulator or simply a Zener regulator

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 Zener Diode as Voltage regulator

➢ Zener diodes are designed to operate at voltages greater than the breakdown voltage (peak reverse voltage)
➢ The breakdown voltage of a Zener diode is determined by the resistivity of the diode
➢ Zener diodes are used to stabilize or regulate voltage

Equivalent representation of part (b) where the VZ and r are the

breakdown voltage and dynamic resistance of the Zener diode
respectively and RS is a current limiting series resistance.
Zener diode voltage regulator with load.

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 Zener Diode as Voltage regulator

Clearly, when the effect of r is neglected, Eq.

(6.67) suggests VL a VZ which is normally
considered in most of the practical cases for
the designing of Zener-diode circuits. In this
case, Eq. (6.66) and Eq. (6.69) can be modified
by replacing VL by VZ.

We may note from Eq. (6.71) that if there is

any fluctuation in VS due to the fluctuation in
the power line voltage, the Zener current IZ is
changed accordingly, whereas the load current
IL and series current IS are unchanged.

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 Rectifier
A rectifier is an electrical device that converts alternating current (AC), which periodically
reverses direction, to direct current (DC), which is in only one direction, a process known
as rectification

➢ In this section, we will consider some basic applications of

Types of Rectifiers
diode in the electronic circuits and systems. One of the
important applications of the diode considered in details is the
rectifier circuits.
❖ Half wave Rectifier ➢ These circuits are used to convert the ac input of the normal
available power supply into a dc output. The dc source of
❖ Full wave Rectifier power is an important requirement in almost all electronic
systems like the televisions, stereos and computers.
❖ Bridge Rectifier ➢ However, the output of a rectifier circuit always contains
some ac components. The details of the filtering techniques
employed to remove the harmonic ac components from the
rectifier output to obtain a more accurate dc output.

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 Half Wave rectifier
➢ Any electrical device which offers a low resistance to the current in one direction but a high
resistance to the current in the opposite direction is called a rectifier.

➢ Such a device is capable of converting a sinusoidal input waveform, whose average value is zero,
into a unidirectional (though not constant) waveform, with a nonzero average component.

(a) Basic circuit of half-wave rectifier. (b) Transformer sinusoidal secondary voltage vi.
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 Half Wave rectifier

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 Half wave rectifier

The effective or rms value squared of a periodic function of time is given by the area of one cycle
of the curve which represents the square of the function, divided by the base. Expressed
mathematically,

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 Half wave Rectifier
When the diode is conducting, it has a resistance Rf, and the voltage across it is iRf. When the device is
nonconducting, the current is zero, and from Fig. 6.1a it is seen that the transformer secondary voltage vi appears
across the diode. Thus

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 Half wave rectifier
Wattmeter: This instrument is built to indicate the average value of the product of the instantaneous current
through its current coil and the instantaneous voltage across its potential coil.
➢ Hence the power read by a wattmeter, whose voltage coil is placed across the transformer secondary

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 Half wave rectifier
Peak Inverse Voltage (PIV): For each rectifier circuit there is a maximum voltage to which the diode is subjected.
This potential is called the peak inverse voltage, because it occurs during that part of the cycle when the diode is
nonconducting. From Fig. 6.2 it is clear that for the half-wave circuit (without a filter) the peak inverse voltage is Vm,
the peak transformer secondary voltage

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 Half wave rectifier
A measure of the fluctuating components is given by the ripple factor r, which is defined as Ripple factor

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 Half wave rectifier

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 Full wave rectifier
This circuit is seen to comprise two half-wave circuits which are so connected that conduction takes place through one
diode during one half of the power cycle and through the other diode during the second half of the power cycle.

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 Full wave rectifier
The current to the load which is the sum of these two currents, has the form shown in Fig. 6.3b. The
dc and rms values of the load current in such a system are readily found, from the definitions (6.3)
and (6.5), to be

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 Efficiency for HWR and FWR

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 Efficiency for HWR and FWR

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 Regulation for HWR and FWR

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 Regulation for HWR and FWR

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 Bridge rectifier

(a) A bridge rectifier circuit, (b) Equivalent diode circuit for the positive half-cycle of vi, (c) Equivalent
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 Bridge rectifier

(d) Typical waveforms of the input voltage vi, currents i1, i2 and the resultant current i = i1 + i2 flowing
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through the load resistor R. The output voltage is v = iR. 49
Copyright ©
 Bridge rectifier
It is observed that the waveform of the current i is similar to that of Fig. 6.3b corresponding to the full-wave rectifier
circuit. However, since two diodes conduct simultaneously for both the positive and negative cycles of the input,
we can simply replace Rf by 2Rf in Eq. (6.18) to obtain

As the output of the bridge rectifier is similar to that of the full-wave rectifier, we can easily substitute for Im from Eq.
(6.22) in Eq. (6.17) to obtain Idc and Irms for this case.

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 Bipolar Junction Transistor
The symbols VEB, VCB, and VCE are the emitter-base, collector-base, and collector-emitter voltages,
respectively. (More specifically, VEB represents the voltage drop from emitter to base)

a) A p-n-p and an n-p-n-transistor. The emitter (collector) junction is JE (JC ), (b) Circuit representation
of the two transistor types
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 Potential Distribution through a Transistor
The dashed curve applies to the case before the application of external biasing voltages, and the
solid curve to the case after the biasing voltages are applied.

(a) A p-n-p transistor with biasing voltages. (b) The potential barriers at the junction of the unbiased transistor.
(c) The potential variation through the transistor under biased conditions. As the reverse-bias collector junction
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voltage |VCB | is increased, the effective base width W decreases 52
 Transistor Current Components
In this Fig. we show the various current components which flow across the forward-biased emitter junction
and the reverse-biased collector junction. The emitter current IE consists of hole current IpE (holes crossing
from emitter into base) and electron current InE (electrons crossing from base into the emitter).

Transistor current components for a forward-biased emitter junction and a reversed-biased collector junction

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 Transistor Construction
In Fig. 7.3 we show the various current components which flow across the forward-biased emitter
junction and the reverse-biased collector junction. The emitter current IE consists of hole current
IpE (holes crossing from emitter into base) and electron current InE (electrons crossing from base

Construction of transistors. (a) Grown, (b) alloy, and (c) diffused, or epitaxial, planar types. (The dimensions are
approximate, and the figures are not drawn to scale. The base width is given in microns, where 1 m = 1026 m = 1023 mm.)

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 The Common-Base Configuration
We may completely describe the transistor of Fig. 7.1a or b by the following two relations, which give the
input voltage VEB and output current IC in terms of the output voltage VCB and input current IE:

➢ In the case of the transistor, it turns out to be most useful to select the input
current and output voltage as the independent variables. The output current and
input voltage are expressed graphically in terms of the independent variables.

➢ In Fig. 7.2a, a p-n-p transistor is shown in a grounded-base configuration. This

circuit is also referred to as a common base, or CB, configuration, since the
base is common to the input and output circuits.

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 The Common-Base Configuration
The relation of Eq. (7.34) is given in Fig. 7.5 for a typical p-n-p germanium transistor and is a plot of collector
current IC versus collector-to-base voltage drop VCB, with emitter current IE as a parameter. The curves of Fig.
7.5 are known as the output, or collector, static characteristics.:

Typical common-base output characteristics of a p-n-p transistor. The cutoff,

active, and saturation regions are indicated.
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 The Common-Base Configuration
Input Characteristics A qualitative understanding of the form of the input and output characteristics is not
difficult if we consider the fact that the transistor consists of two diodes placed in series “back to back” (with
the two cathodes connected together). In the active region the input diode (emitter-to-base) is biased in the
forward direction.

Common-base input characteristics of a typical p-n-p germanium junction transistor

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 Common-Emitter Configuration
Most transistor circuits have the emitter, rather than the base, as the terminal common to both input
and output. Such a common-emitter CE, or grounded-emitter, configuration is indicated in Fig. 7.7.

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 Output Characteristics of Common-Emitter Configuration
In Fig. 7.8 the abscissa is the collectorto-emitter voltage VCE, the ordinate is the collector current IC,
and the curves are given for various values of base current IB.

Fig. 7.8 Typical common-emitter output characteristics of a p-n-p germanium

junction transistor. A load line corresponding to VCC = 10 V and RL = 500 Ω is
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 Input Characteristics of Common-Emitter Configuration
If VBE become zero, then IB will be zero, since under these conditions both emitter and collector junctions will
be short-circuited. For any other value of VCE, the base current for VBE = 0 is not actually zero but is too small
(Sec. 7.15) to be observed in Fig. 7.9. in general, increasing |VCE| with constant VBE causes a decrease in base
width W (the Early effect) and results in a decreasing recombination base current.

Fig. 7.9 Typical common-emitter input characteristics of thep-

n-p germanium junction transistor of Fig. 7.8
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 Common-Emitter Configuration

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 Graphical Analysis of the CE Configuration
The collector and emitter current and voltage component variations from the corresponding quiescent
values are

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 Graphical Analysis of the CE Configuration

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 Graphical Analysis of the CE Configuration

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 The Common-Collector Configuration

➢ The circuit is basically the same as the circuit

of Fig. 7.7, with the exception that the load
resistor is in the emitter circuit rather than in
the collector circuit.
➢ If we continue to specify the operation of the
circuit in terms of the currents which flow, the
operation for the common-collector is much the
same as for the common emitter configuration.
➢ When the base current is ICO, the emitter
current will be zero, and no current will flow in
the load.

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 Transistor Current Components
If the emitter were open-circuited so that IE = 0, then IpC would be zero. Under these circumstances, the base
and collector would act as a reverse-biased diode, and the collector current IC would equal the reverse
saturation current ICO. If IE ≠ 0, then from Fig. 7.3, we note that

➢ For a p-n-p transistor, ICO consists of

holes moving across JC from left to
right (base to collector) and
electrons crossing JC in the opposite
direction.
➢ Since the assumed reference
direction for ICO in Fig. 7.3 is from
right to left, then for a p-n-p
transistor, ICO is negative. For an n-p-
n transistor, ICO is positive

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 Transistor Current Components

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 Transistor Current Components

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 Transistor Current Components

The CE cutoff Region

The actual collector current with collector junction reverse-biased and base open-circuited is designated by
the symbol ICEO. Since, even in the neighborhood of cutoff, a may be as large as 0.9 for germanium, then IC
≈10ICO at zero base current. Accordingly, in order to cut off the transistor, it is not enough to reduce IB to
05-12-2022zero. Instead, it is necessary to reverse-bias the emitter junction slightly. 69
 Transistor Current Components

The CE cutoff Region

The actual collector current with collector junction reverse-biased and base open-circuited is designated by
the symbol ICEO. Since, even in the neighborhood of cutoff, a may be as large as 0.9 for germanium, then IC
≈10ICO at zero base current. Accordingly, in order to cut off the transistor, it is not enough to reduce IB to
05-12-2022zero. Instead, it is necessary to reverse-bias the emitter junction slightly. 70
 Transistor Current Components
➢ The actual collector current with collector junction reverse-biased and base open-circuited is designated
by the symbol ICEO. Since, even in the neighborhood of cutoff, a may be as large as 0.9 for germanium,
then IC ≈10ICO at zero base current.
➢ Accordingly, in order to cut off the transistor, it is not enough to reduce IB to zero. Instead, it is
necessary to reverse-bias the emitter junction slightly.

Hence, even with IB = 0, we find, from Eq. (7.40), that IC = ICO = -IE, so that the transistor is still very
close to cutoff. We verify in Sec. 7.15 that, in silicon, cutoff occurs at VBE ≈ 0 V, corresponding to a base
short-circuited to the emitter. In summary, cutoff means that IE = 0, IC = ICO, IB = -IC = -ICO, and VBE is a
reverse voltage whose magnitude is of the order of 0.1 V for germanium and 0 V for a silicon transistor

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 Reverse Collector Saturation Current ICBO

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 Large-Signal, dc, and Small-Signal CE Values of Current Gain

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 Transistor as an Amplifier

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05-12-2022 75
 Ebers-Moll Model

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 Comparison of CB, CE and CC characteristics
Common collector
Characteristic Common base (CB) Common emitter (CE)
(CC)
Input Dynamic Resistance Very Low (less than 100 ohm) Low (less than 1K) Very High(750K)
Output Dynamic Resistance Very High High Low
Less than 1
Current Gain High Very High

Greater than CC but less than

Voltage gain CE Highest Lowest(less than 1)

Medium
Power gain Highest Medium

Very small
Leakage current Very large Very large

Relationship between I/p and

In phase Out of phase(180º) In phase
o/p
For Audio freq. For impedance
Application For High freq. applications
Applications Matching Applications

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 Transistor Biasing and Thermal Stabilization
❑ Methods for establishing the quiescent operating point of a transistor amplifier in the active region of the
characteristics.
❑ The operating point shifts with changes in temperature T because the transistor parameters (β, ICO, etc.) are
functions of T. A criterion is established for comparing the stability of different biasing circuits.

The Fixed-bias Circuit The point Q2 can be

established by noting the required current IB2 in Fig. 8.2
and choosing the resistance Rb in Fig 8.1 so that the base
current is equal to IB2. Therefore

The current IB is constant, and the network of Fig 8.1 is called the fixed-
bias circuit 78
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 Transistor Biasing and Thermal Stabilization

Note that even if we are free to

choose Rc, RL, Rb, and VCC, we may not
operate the transistor everywhere in
the active region because the various
transistor ratings limit the range of
useful operation.

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 Typical value for silicon and germanium transistor

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 Bias Stability
➢ Let us refer to the biasing circuit of Fig. 8.1. In this
circuit the base current IB is kept constant since IB a
VCC/Rb. Let us assume that the transistor of Fig. 8.1 is
replaced by another of the sametype.

Thermal Instability A second very

important cause for bias instability is a variation in
temperature. In Sec. 7.9 we note that the reverse
saturation current ICO† changes greatly with temperature.
Specifically, ICO doubles for every 10°C rise in
temperature. This fact may cause considerable practical
difficulty in using a transistor as a circuit element. For
example, the collector current IC causes the collector-
junction temperature to rise, which in turn increases ICO

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 Bias Stability
➢ Let us refer to the biasing circuit of Fig. 8.1. In this
circuit the base current IB is kept constant since IB a
VCC/Rb. Let us assume that the transistor of Fig. 8.1 is
replaced by another of the sametype.

Thermal Instability A second very

important cause for bias instability is a variation in
temperature. In Sec. 7.9 we note that the reverse
saturation current ICO† changes greatly with temperature.
Specifically, ICO doubles for every 10°C rise in
temperature. This fact may cause considerable practical
difficulty in using a transistor as a circuit element. For
example, the collector current IC causes the collector-
junction temperature to rise, which in turn increases ICO

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 Bias Stability

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 Bias Stability

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 Stability factors of BJT

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 Collector-to-Base Bias or Collector-Feedback Bias

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 Collector-to-Base Bias or Collector-Feedback Bias

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 Collector-to-Base Bias or Collector-Feedback Bias

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 Emitter-Feedback Bias

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 Emitter-Feedback Bias

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 Collector-Emitter Feedback Bias

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 Self-Bias, Emitter Bias, or Voltage-Divide Bias

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 Bias Compensation

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 Unijunction Transistor (UJT)
A unijunction transistor (UJT) is a three-lead electronic semiconductor device with only one junction that
acts exclusively as an electrically controlled switch.

The UJT is not used as a linear amplifier. It is used in free-running oscillators, synchronized or triggered
oscillators, and pulse generation circuits at low to moderate frequencies (hundreds of kilohertz). It is
widely used in the triggering circuits for silicon controlled rectifiers.

The unijunction transistor was invented as a byproduct of research on germanium tetrode transistors at
General Electric. It was patented in 1953. Commercially, silicon devices were manufactured.

Graph of UJT characteristic curve, emitter-base1 voltage as a function of emitter

05-12-2022 current, showing current-controlled negative resistance (downward-sloping region 94
 Unijunction Transistor (UJT)
This device was originally described in the literature as the double-base diode, but is now commercially
available under the designation unijunction transistor (UJT).

Another device whose construction is similar to that of the FET is indicated in Fig. 12.33.

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 Unijunction Transistor (UJT)
➢ The principal constructional difference between the FET and the UJT is that the gate surface of the former is
much larger than the emitter junction of the latter.

➢ The main operational difference between the two devices is that the FET is normally operated with the gate
junction reverse-biased, whereas the useful behaviour of the UJT occurs when the emitter is forward-biased.

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 Field-effect transistor
➢ The field-effect transistor is a semiconductor device which depends for its operation on the
control of current by an electric field.

➢ There are two types of field effect transistors, the junction field-effect transistor (abbreviated
JFET, or simply FET) and the insulated-gate field-effect transistor (IGFET), more commonly
called the metal-oxide-semi conductor (MOS) transistor (MOST or MOSFET).

The FET enjoys several advantages over the conventional transistors:

1. Its operation depends upon the flow of majority carriers only. It is therefore a unipolar (one type of
carrier) device. The vacuum tube is another example of a unipolar device. The conventional transistor is a
bipolar device.
2. It is relatively immune to radiation.
3. It exhibits a high input resistance, typically many meg-ohms.
4. It is less noisy than a tube or a bipolar transistor.
5. It exhibits no offset voltage at zero drain current, and hence makes an excellent signal chopper.2
6. It has thermal stability.

The main disadvantage of the FET is its relatively small gain-bandwidth product in comparison with that
which can be obtained with a conventional transistor. 97
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 The Junction Field-Effect Transistor

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 Field-Effect Transistor

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 Field-Effect Transistor

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 Junction Field-Effect Transistor

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 N-Channel JFET

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 P-Channel JFET

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 The Junction Field-Effect Transistor
FET Static Characteristics The circuit, symbol, and polarity conventions for a FET are indicated in
Fig. 12.2. The direction of the arrow at the gate of the junction FET in Fig. 12.2 indicates the direction in which gate
current would flow if the gate junction were forward-biased. The common-source drain characteristics for a typical n-
channel FET shown in Fig. 12.3 give ID against VDS, with VGS as a parameter.

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 The JFET Volt-Ampere Characteristics
Assume, first, that a small voltage VDS is applied between drain and source. The resulting small drain current ID
will then have no appreciable effect on the channel profile. Under these conditions we may consider the effective
channel cross section A to be constant throughout its length.

Hence A = 2bw, where 2b is the channel width corresponding to zero drain current as given by Eq. (12.3) for a
specified VGS, and w is the channel dimension perpendicular to the b direction, as indicated in Fig. 12.1.

Since no current flows in the depletion region, then, using Ohm’s law [Eq. (4.1)], we obtain for the drain current

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 The JFET Volt-Ampere Characteristics

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 Junction Field-Effect Transistor

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 Junction Field-Effect Transistor

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 Junction Field-Effect Transistor

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 Junction Field-Effect Transistor

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 Junction Field-Effect Transistor

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 Junction Field-Effect Transistor

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 Junction Field-Effect Transistor

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 Junction Field-Effect Transistor

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 Metal-oxide-semiconductor (MOS) transistor (MOSFET)

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 The JFET Volt-Ampere Characteristics

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 The Junction Field-Effect Transistor

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 MOSFET

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 MOSFET

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 MOSFET

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 MOSFET

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 Enhancement MOSFET

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 Enhancement MOSFET

The Enhancement MOSFET If we ground the substrate for the structure of Fig. 12.10
and apply a positive voltage at the gate, an electric field will be directed perpendicularly through the
oxide.This field will end on “induced” negative charges on the semiconductor site, as shown in Fig. 12.10.

The negative charge of electrons which are minority carriers in the p-type substrate forms an “inversion
layer.” As the positive voltage on the gate increases, the induced negative charge in the semiconductor
increases.

The region beneath the oxide now has n-type carriers, the conductivity increases, and current
flows from source to drain through the induced channel.

Thus the drain current is “enhanced” by the positive gate voltage, and such a device is called an
enhancement-type MOS.

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 Enhancement MOSFET
The current IDSS at VGS ≤ 0 is very small, being of the order of a few nanoamperes. As VGS is made
positive, the current ID increases slowly at first, and then much more rapidly with an increase in VGS.

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 Depletion MOSFET
➢ A second type of MOSFET can be made if, to the basic structure of Fig. 12.10, an n channel is diffused between
the source and the drain, as shown in Fig. 12.12a. With this device an appreciable drain current IDSS flows for zero
gate-to-source voltage, VGS = 0.
➢ If the gate voltage is made negative, positive charges are induced in the channel through the SiO2 of the gate
capacitor. Since the current in a FET is due to majority carriers (electrons for an n-type material), the induced
positive charges make the channel less conductive, and the drain current drops as VGS is made more negative.
➢ The redistribution of charge in the channel causes an effective depletion of majority carriers, which accounts for
the designation depletion MOSFET.

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 Depletion MOSFET
➢ A MOSFET of the depletion type just described may also be operated in an enhancement mode.
➢ It is only necessary to apply a positive gate voltage so that negative charges are induced into the n-type
channel. In this manner the conductivity of the channel increases and the current rises above IDSS.
➢ The volt-ampere characteristics of this device are indicated in Fig. 12.13a, and the transfer curve is
given in Fig. 12.13b.
➢ The depletion and enhancement regions, corresponding to VGS negative and positive, respectively,
should be noted.

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