Unit-II

Element of Electronics Engineering

Presented By
Dr. Kundan Kumar
M.Tech. & Ph.D.|IIT, Dhanbad
Assistant Professor
Department of Electronics and Communication Engineering
SLIET, Longowal (Under MOE Govt. of India), Sangrur, Punjab India
Semiconductor Materials: Ge, Si, and GaAs
Semiconductors are a special class of elements having a
conductivity between that of a good conductor and
that of an insulator.
• They fall into two classes : single crystal and compound
• Single crystal : Germanium (Ge) and silicon (Si).
• Compound : gallium arsenide (GaAs),
cadmium sulfide (CdS),
gallium nitride (GaN),
gallium arsenide phosphide (GaAsP)
The three semiconductors used most frequently in the
construction of electronic devices are Ge, Si, and GaAs.
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Group → 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18

↓ Period

1 2
1 H He

3 4 5 6 7 8 9 10
2 Li Be B C N O F Ne

11 12
13 14 15 16 17 18
3 N M
Al Si P S Cl Ar
a g

21
19 20 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36
4 S
K Ca Ti V Cr Mn Fe Co Ni Cu Zn Ga Ge As Se Br Kr
c

37
38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54
5 R
Sr Y Zr Nb Mo Tc Ru Rh Pd Ag Cd In Sn Sb Te I Xe
b

55 56 * 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86
6
Cs Ba Hf Ta W Re Os Ir Pt Au Hg Tl Pb Bi Po At Rn

87 88 ** 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118
7
Fr Ra Rf Db Sg Bh Hs Mt Ds Rg Uub Uut Uuq Uup Uuh Uus Uuo

57 58 59 60 61 62 63 64 65 66 67 68 69 70 71
* Lanthanides
La Ce Pr Nd Pm Sm Eu Gd Tb Dy Ho Er Tm Yb Lu

89 90 91 92 93 94 95 96 97 98 99 100 101 102 103

** Actinides Ac Th Pa U Np Pu Am Cm Bk Cf Es Fm Md No Lr

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 History
• Diode , in 1939 was using Ge
• Transistor, in 1947 was using Ge
• In1954 Si was used in Transistor because Si is less
temperature sensitive and abundantly available.
• High speed transistor was using GaAs in 1970 (which is 5
times faster compared to Si)
• Si, Ge and GaAs are the semiconductor of choice

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 Atomic Structure
Valence shell (4 valence electrons) Valence shell (4 valence electrons)
Valence
shells electron

Valence
+ electron
+

Nucleus
orbiting
electrons
orbiting
Germanium electrons
Silicon
32 orbiting electrons 14 orbiting electrons
(tetravalent) (Tetravalent)

• Valence electrons: electrons in the outermost shell.

• Atoms with four valence electrons are called tetravalent.

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 Atomic Structure
Valence shell (3 valence electrons) Valence shell (5 valence electrons)
Valence Valence
shells electron electron
shells

+ +

Nucleus orbiting
electrons Nucleus orbiting
electrons
Gallium
Arsenic

31 orbiting electrons 33 orbiting electrons

(trivalent) (pentavalent)

• Atoms with three valence electrons are called trivalent, and

those with five are called pentavalent.

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 Covalent Bonding

Covalent bonding of Si crystal

This bonding of atoms, strengthened by the sharing of electrons,
is called covalent bonding
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 Covalent Bonding

There is sharing of
electrons, five electrons
provided by As atom and
three by the Ga atom.

Covalent bonding of GaAs crystal

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 Energy Levels

The farther an electron is from the nucleus, the higher is the

energy state.
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 Energy Levels

An electron in the valence band of silicon must absorb more energy than
one in the valence band of germanium to become a free carrier. [free
carriers are free electrons due only to external causes such as applied
electric fields established by voltage sources or potential difference.
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 n-Type and p-Type materials
n-Type Material
n-Type materials are created by
adding elements with five valence
electrons such as antimony, arsenic,
and phosphorous.

There is a fifth electron due to

the (Sb) atom that is relatively free
to move in the n-Type material.

The atoms (in this case is

antimony (Sb)) are called donor
atoms.
Doping with Sb, (antimony)
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 n-Type and p-Type materials
n-Type Material

The free electrons due to the added atoms have higher energy
levels and require less energy to move to conduction band.

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 n-Type and p-Type materials
p-Type Material p-Type materials are created by
adding atoms with three valence
electrons such as boron, gallium, and
indium.
In this case, an insufficient
number of electrons to complete the
covalent bonds.
The resulting vacancy is called a
“hole” represented by small circle or
plus sign indicating absence of a
negative charge.
The atoms (in this case boron(B))
Boron (B) are called acceptor atoms.
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 Majority and Minority carriers
Two currents through a diode:
Majority Carriers
•The majority carriers in n-type materials are electrons.
•The majority carriers in p-type materials are holes.
Minority Carriers
•The minority carriers in n-type materials are holes.
•The minority carriers in p-type materials are electrons.

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 p-n Junctions

One end of a silicon or germanium crystal can be doped as a p-

type material and the other end as an n-type material.

The result is a p-n junction.

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 p-n Junctions
At the p-n junction, the excess
conduction-band electrons on the
n-type side are attracted to the
valence-band holes on the p-type
side.

The electrons in the n-type

material migrate across the
junction to the p-type material
(electron flow).
The result is the formation of
The electron migration results in a a depletion region around
negative charge on the p-type side the junction.
of the junction and a positive
charge on the n-type side of the
junction.

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 Diodes
The diode is a 2-terminal device.

A diode ideally conducts in only

one direction.

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 • No bias
Diode Operating Conditions • Forward bias
• Reverse bias

Reverse bias Forward bias

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 Diode Operating Conditions
No Bias

• No external voltage is applied: VD = 0 V

• No current is flowing: ID = 0A
• Only a modest depletion region exists

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 Diode Operating Conditions
Reverse Bias
External voltage is applied across the p-n junction in
the opposite polarity of the p- and n-type materials.

• The reverse voltage causes the

depletion region to widen.
• The electrons in the n-type material
are attracted toward the positive
terminal of the voltage source.
• The holes in the p-type material are
attracted toward the negative
terminal of the voltage source.

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 Diode Operating Conditions
Forward Bias
External voltage is applied across the p-n junction in
the same polarity as the p- and n-type materials.

• The forward voltage causes the

depletion region to narrow.
• The electrons and holes are pushed
toward the p-n junction.
• The electrons and holes have
sufficient energy to cross the p-n
junction.

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 Actual Diode Characteristics
Note the regions for no
bias, reverse bias, and
forward bias conditions.
Carefully note the scale
for each of these
conditions.
The reverse saturation
current is seldom more
than a few microamperes.

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Diode equation

where
VT : is called the thermal voltage.
Is : is the reverse saturation current.
VD : is the applied forward-bias voltage across the diode.
n : is a factor function of operation conditions and physical
construction. It has range between 1 and 2. assume n=1 unless
otherwise noted.
K : is Boltzman’s constant =1.38 x 10-23
T: is temperature in kelvins = 273+temperature in C.
q : is the magnitude of electron charge = 1.6 x 10-19 C.

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 Load-Line
Analysis
The load line plots all possible
combinations of diode current (ID)
and voltage (VD) for a given circuit.
The maximum ID equals E/R, and
the maximum VD equals E.

The point where the load line and

the characteristic curve intersect is
the Q-point, which identifies ID and
VD for a particular diode in a given
circuit.

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 Series Diode
Configurations
Forward Bias
Constants
• Silicon Diode: VD = 0.7 V
• Germanium Diode: VD = 0.3 V

Analysis (for silicon)

• VD = 0.7 V (or VD = E if E < 0.7 V)
• VR = E – VD
• ID = IR = IT = VR / R

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 Series Diode
Configurations
Reverse Bias
Diodes ideally behave as open circuits

Analysis
• VD = E
• VR = 0 V
• ID = 0 A

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 Parallel
Configurations
V  0.7 V
D
V V  V  0.7 V
D1 D2 O
V  9.3 V
R
E V 10 V  .7 V
I D   28 mA
R R .33kΩ
28 mA
I I   14mA
D1 D2 2

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 Half-Wave
Rectification
The diode only
conducts when it is
forward biased,
therefore only half
of the AC cycle
passes through the
diode to the
output.

The DC output voltage is 0.318Vm, where Vm = the peak ACvoltage.

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 PIV
(PRV)
Because the diode is only forward biased for one-half of the AC cycle, it is
also reverse biased for one-half cycle.

It is important that the reverse breakdown voltage rating of the diode be

high enough to withstand the peak, reverse-biasing AC voltage.

PIV (or PRV) > Vm

• PIV = Peak inverse voltage

• PRV = Peak reverse voltage
• Vm = Peak AC voltage

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 Full-Wave
Rectification

The rectification process can be improved by

using a full-wave rectifier circuit.

Full-wave rectification produces a greater

DC output:

• Half-wave: Vdc = 0.318Vm

• Full-wave: Vdc = 0.636Vm

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 Full-Wave
Rectification

Bridge Rectifier

• Four diodes are connected in a

bridge configuration
• VDC = 0.636Vm

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 Full-Wave Rectification

Center-Tapped Transformer
Rectifier
Requires
• Two diodes
• Center-tapped transformer

VDC = 0.636Vm
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 Summary of Rectifier Circuits
Rectifier Ideal VDC Realistic VDC

Half Wave Rectifier VDC = 0.318Vm VDC = 0.318Vm – 0.7

Bridge Rectifier VDC = 0.636Vm VDC = 0.636Vm – 2(0.7 V)

Center-Tapped Transformer
VDC = 0.636Vm VDC = 0.636Vm – 0.7 V
Rectifier

Vm = peak of the AC voltage.

In the center tapped transformer rectifier circuit, the peak AC voltage

is the transformer secondary voltage to the tap.

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 Diode Clippers
The diode in a series clipper “clips”
any voltage that does not forward
bias it:
• •A reverse-biasing polarity
•A forward-biasing polarity less than
0.7 V (for a silicon diode)

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 Biased Clippers

Adding a DC source in
series with the clipping
diode changes the
effective forward bias of
the diode.

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 Parallel Clippers

The diode in a parallel clipper

circuit “clips” any voltage that
forward bias it.

DC biasing can be added in

series with the diode to change
the clipping level.

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 Summary of Clipper Circuits

more…

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 Summary of Clipper Circuits

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 Clampers

A diode and capacitor can be

combined to “clamp” an AC
signal to a specific DC level.

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 Biased Clamper Circuits

The input signal can be any type

of waveform such as sine, square,
and triangle waves.

The DC source lets you adjust

the DC camping level.

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 Summary of Clamper Circuits

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ZENER DIODES
Zener Diode
 Introduction
The zener diode is a silicon pn junction devices that differs from rectifier
diodes because it is designed for operation in the reverse-breakdown
region. The breakdown voltage of a zener diode is set by carefully
controlling the level during manufacture. The basic function of zener
diode is to maintain a specific voltage across it’s terminals within given
limits of line or load change. Typically it is used for providing a stable
reference voltage for use in power supplies and other equipment.
 Construction of Zener
Zener diodes are designed to operate in reverse breakdown. Two types of reverse
breakdown in a zener diode are avalanche and zener. The avalanche break down
occurs in both rectifier and zener diodes at a sufficiently high reverse voltage. Zener
breakdown occurs in a zener diode at low reverse voltages.
A zener diode is heavily doped to reduced the breakdown
voltage. This causes a very thin depletion region.
The zener diodes breakdown characteristics are determined by
the doping process
Zeners are commercially available with voltage breakdowns of 1.8
V to 200 V.
 Working of Zener
A zener diode is much like a normal diode. The exception being is that
it is placed in the circuit in reverse bias and operates in reverse
breakdown. This typical characteristic curve illustrates the operating
range for a zener.
Note that it’s forward characteristics are just like a normal diode.
 Breakdown Characteristics
Figure shows the reverse portion of a zener diode’s characteristic
curve. As the reverse voltage (VR) is increased, the reverse current (IR)
remains extremely small up to the “knee” of the curve. The reverse
current is also called the zener current, IZ. At this point, the breakdown
effect begins; the internal zener resistance, also called zener impedance
(ZZ), begins to decrease as reverse current increases rapidly.
 ZENER BREAKDOWN

• Zener and avalanche effects are responsible

for such a dramatic increase in the value of
current at the breakdown voltage.

• If the impurity concentration is very high, then

the width of depletion region is very less.
Less width of depletion region will cause high
intensity of electric field to develop in the
depletion region at low voltages.
• Large number of electrons gets separated from
their atoms, resulting in sudden increase in the
value of reverse current.
• This explanation was given by scientist C. E.
Zener. Such diodes are called Zener diodes.
• Zener effect predominates in diodes whose
breakdown voltage is below 6 V.
 AVALANCHE BEAKDOWN

• Zener effect predominates on diodes whose

breakdown voltage is below 6 V. The breakdown
voltage can be obtained at a large value by reducing
the concentration of impurity atom.

• We know that very little amount of current flows in

the reverse biased diode. This current is due to the
flow of minority charge carriers i.e., electrons in the
p type semiconductor and holes in the n type
semiconductor.
• When a reverse bias voltage is applied across the
terminals of the diode, the electrons from the p type
material and holes from the n-type materials
accelerates through the depletion region.
• This results in collision of intrinsic particles
(electrons and holes) with the bound electrons in the
depletion region. With the increase in reverse bias
voltage the acceleration of electrons and holes also
increases.
• Now the intrinsic particles collides with bound
electrons with enough energy to break its covalent
bond and create an electron-hole pair. This is shown
in the figure.
Avalanche Breakdown
Mechanism
• The collision of electrons with the atom creates an
electron-hole pair.
.

• This newly created electron also gets accelerated

due to electric field and breaks many more
covalent bond to further create more electron-hole
pair.
• This process keeps on repeating and it is
called carrier multiplication.
• The newly created electrons and holes contribute
to the rise in reverse current.
• The process of carrier multiplication occurs very
quickly and in very large numbers that there is
apparently an avalanche of charge carriers.
Thus the breakdown is called avalanche
breakdown.
 DIFFERENCE BETWEEN ZENER
AND AVALANCHE BREAKDOWN
Zener Breakdown Avalanche breakdown
1. This occurs at junctions which being 1. This occurs at junctions which
heavily doped have narrow depletion being lightly doped have wide depletion layers.
layers
2. This breakdown voltage sets a 2.Here electric field is not strong
very strong electric field across enough to produce Zener breakdown.
this narrow layer.
3.Her minority carriers collide with semi
3.Here electric field is very strong conductor atoms in the depletion region, which
to rupture the covalent bonds breaks the covalent bonds and electron-hole
pairs are generated. Newly generated charge
thereby generating electron-hole
carriers are accelerated by the electric field
pairs. So even a small increase in which results in more collision and generates
reverse voltage is capable of producing avalanche of charge carriers. This results in
Large number of current carriers. avalanche breakdown.

4.Zener diode exhibits negative temp: 4.Avalanche diodes exhibits positive temp:
coefficient. Ie. breakdown voltage coefficient. i.e breakdown voltage increases
with increase in temperature.
decreases as temperature increases.
Conduction direction: (a) Zener diode;
(b) semiconductor diode.

Zener equivalent
circuit: (a) complete; (b) approximate.
 Zener Diodes

The Zener is a diode operated

in reverse bias at the Zener
Voltage (Vz).

• When Vi  VZ
– The Zener is on
– Voltage across the Zener is VZ
– Zener current: IZ = IR – IRL
– The Zener Power: PZ = VZIZ

• When Vi < VZ
– The Zener is off
– The Zener acts as an open circuit

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 Zener Resistor Values
If R is too large, the Zener diode cannot conduct because the available amount of
current is less than the minimum current rating, IZK. The minimum current is
given by:
ILmin  IR IZK

The maximum value of resistance is:

VZ
R Lmax 
I Lmin

If R is too small, the Zener current exceeds the maximum current

rating, IZM . The maximum current for the circuit is given by:
VL V
I Lmax 
 Z
RL RLmin
The minimum value of resistance is:
RVZ
RLmin 
Vi V Z
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LIGHT-EMITTING DIODES
 What is an LED?

• Light-emitting diode
• Semiconductor
• Has polarity
• The increasing use of digital displays in calculators, watches, and all forms
of instrumentation has contributed to the current extensive interest in
structures that will
emit light when properly biased.
• The two types in common use today to perform this
function are the light-emitting diode (LED) and the liquid-crystal display
(LCD).
• Since the LED falls within the family of p-n junction devices.
• As the name implies, the light-emitting diode (LED) is a diode that will give off
visible light when it is energized.
• In any forward-biased p-n junction there is, within the structure and primarily
close to the junction, a recombination of holes and electrons. This
recombination requires that the energy possessed by the unbound free electron
be transferred to another state.
• In all semiconductor p-n junctions some of this energy will be given off as heat
and some in the form of photons. In silicon and germanium the greater
percentage is given up in the form of heat and the emitted light is insignificant.
• In other materials, such as gallium arsenide phosphide (GaAsP) or gallium
phosphide (GaP), the number of photons of light energy emitted is sufficient to
create a very visible light source.
• The process of giving off light by applying an electrical source
of energy is
called electroluminescence
• Graphic symbol, the conducting surface connected to the p-material is
much smaller, to permit the emergence of the maximum number of
photons of light energy.
• Note in the figure that the recombination of the injected carriers due to
the forward-biased junction results in emitted light at the site of
recombination. There may, of course, be some absorption of the
packages of photon energy in the structure itself, but a very large
percentage are able to leave, as shown in the figure.
 Practical
Applications
• Rectifier Circuits
– Conversions of AC to DC for DC operated circuits
– Battery Charging Circuits

• Simple Diode Circuits

– Protective Circuits against
– Overcurrent
– Polarity Reversal
– Currents caused by an inductive kick in a relay circuit

• Zener Circuits
– Overvoltage Protection
– Setting Reference Voltages

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