SRI KRISHNA COLLEGE OF ENGINEERING AND TECHNOLOGY
(An Autonomous Institution, Accredited by NAAC with ‘A’ Grade)
Kuniamuthur, Coimbatore – 641008
Department of Electrical and Electronics Engineering
BASICS OF ELECTRICAL AND ELECTRONICS ENGINEERING
MODULE III – Basics of Analog and Digital
Electronics
3.1 - SEMICONDUCTOR, PN JUNCTION
DIODE, ZENER DIODE
1
Module III : Basics of Analog and
Digital Electronics
Semiconductor, PN junction diode, Zener diode, rectifier- Half
wave, full wave and Bridge rectifier, Introduction to Number
system, basic Boolean laws, reduction of Boolean expressions
and implementation with logic gates.
2
Introduction
• All the matters are composed of three fundamental
particles.
Fundamenta
l particle
Nature of
charge
Mass in kg
Neutron
Proton
no charge
positive
charge
negative
charge
1.675 x 10-27
1.675 x 10-27
Electron
9.107 x 10-31
3
Cont.,
 Every atom consists of a nucleus carrying a positive
charge around which electrons move in orbits/shells at a
certain distance from the nucleus.
 The maximum number of electrons in any orbit is 2(n)2.
 Atom structure of copper is,
4
Cont.,
 Electrons that are in shells close to nucleus are tightly
bound to the atom and have low energy.
 Electrons that are in shells away from the nucleus are
lightly bound to the atom and have high energy.
 Electrons with highest energy level exists in the
outermost orbit.
 The outer most orbit is called as valence shell and the
electrons are known as valence electron.
5
Energy Band
 The high energy outermost shell electrons can be easily
extracted and take part in chemical reactions.
 In solids, atoms are close together.
 The outermost electrons are shared by other atoms. It
forms a bond known as ‘covalent band’. As they are
shared by adjacent atoms, they are not free under normal
condition.
 Due to the coupling between the valence electrons, the
energy levels associated with the valence electrons
merge into each other. The merging forms an ‘energy
band’.
6
Cont.,
 Similarly, the energy levels of various electrons in
various orbit levels merge to form various energy bands.
 Out of all the energy bands, the most important are,
1. Valence band
2. Conduction band
3. Forbidden band
7
Cont.,
1. Valence band :
 The energy band formed due to merging of energy levels
associated with valence electrons.
 The electrons in this bond are not free under normal
condition.
 When certain energy is imparted, the electrons become
free.
8
Cont.,
2. Conduction band :
 The energy band formed due to merging of energy levels
associated with the free electrons.
 Under normal condition, conduction band does not have any
electrons.
 When certain energy is imparted, the electrons jump from
valence band to conduction band and become free.
 While jumping from valence band to conduction band, the
electrons have to cross an energy gap.
9
Cont.,
3. Forbidden band :
 The energy gap which is separating the conduction band and
valence band.
 The electrons cannot exist in the forbidden gap.
10
Classification of Materials
 Conductor – Ability to carry electricity is high. (Copper,
aluminum etc)
 Insulator - Ability to carry electricity is very low. (wood,
glass, mica etc)
 Semiconductor - Ability to carry electricity lies between
conductors and insulators. (Silicon and Germanium)
11
Cont.,
 Energy band level is,
12
Hole
 The absence of electron in a particular place in an atom
is called as hole.
 Hole is a electric charge carrier which has positive
charge. The electric charge of hole is equal to electric
charge of electron but have opposite polarity.
 When a small amount of external energy is applied, then
the electrons in the valence band moves in to conduction
band and leaves a vacancy in valence band. This
vacancy is called as hole.
13
Classification of Semiconductors
1. Intrinsic semiconductors
2. Extrinsic semiconductors
14
Cont.,
Intrinsic semiconductors:
 A pure semiconductor.
 Pure semiconductors have equal numbers of holes and
electrons.
 The number of electrons in the conduction band is equal
to the number of holes in the valence band. Therefore
the overall electric charge of a atom is neutral.
 It has limited number of free electrons at room
temperature.
 Hence, they do not conduct well at this temperature.
15
Cont.,
Extrinsic semiconductors :
 When the impurities are added to the intrinsic semiconductor,
it becomes an extrinsic semiconductor.
 The process of adding impurities to the semiconductor is
called doping. Doping increases the number of electrons.
 Extrinsic semiconductor has high electrical conductivity than
intrinsic semiconductor.
 Hence the extrinsic semiconductors are used for the
manufacturing
of
electronic
devices
such
as
diodes,
transistors etc.
 The number of free electrons and holes in extrinsic
semiconductor are not equal.
16
Impurities
1.Pentavalent impurities
 Having five valence electrons
 When it is added with an intrinsic semiconductor, it
donates one electron to intrinsic material.
 It is called as ‘donor impurity’
 Ex : arsenic, bismuth, phosphorous
 The resulting extrinsic semiconductor with large
number of electrons is called n-type semiconductor.
17
Cont.,
2.Trivalent impurities
 Having three valence electrons
 When it is added with an intrinsic semiconductor, it
accepts one electron from intrinsic material and
creates hole in intrinsic material .
 It is called as ‘acceptor impurity’
 Ex : gallium, indium, boron
 The resulting extrinsic semiconductor with large
number of holes is called p-type semiconductor.
18
PN JUNCTION
 The p region has many holes (majority carriers) from the
impurity atoms and only a few thermally generated free
electrons (minority carriers).
 The n region has many free electrons (majority carriers)
from the impurity atoms and only a few thermally
generated holes (minority carriers).
19
Cont.,
 The p type and n type semiconductors are chemically
combined with a special fabrication technique to form a
p-n junction.
 It forms a semiconductor device called diode.
20
Cont.,
 The concentration of electrons at n-type semiconductor is
high while the concentration of electrons at p-type
semiconductor is low.
 According to coulomb’s law there exist an electrostatic force
of attraction between the opposite charges.
 Hence, the free electrons from the n-side are attracted
towards the holes at the p-side.
 Thus, the free electrons move from n-region (high
concentration
region)
to
p-region
(low
concentration
region).
21
Cont.,
 Therefore at the junction there is a tendency of free
electrons in the n region near the pn junction to diffuse
across the junction and fall into holes near the junction in
the p region. Similarly, holes from p side diffuses in to n
side. This process is called diffusion.
22
Cont.,
 Atoms on n side are donor atom.
 Holes diffused from p type, recombined with donor atom
and become positively charge immobile ions.
 Large number of positively charged immobile ions formed
near the junction on the n side.
23
Cont.,
 Atoms on p side are acceptor atom.
 Electrons diffused from n type, recombined with acceptor
atom and become negatively charge immobile ions.
 large number of negatively charged immobile ions formed
near the junction on the p side.
24
Cont.,
 In thermal equilibrium, in the region near the junction,
there exists a wall of negative immobile charges on p side
and positive immobile charges on n side.
 In the region, there is no mobile charge carriers.
 Such a region is depleted from the mobile charge carriers
and hence called depletion region or depletion layer.
25
Cont.,
 At the n-side–net positive charge. At the p-side–net negative
charge.
 The depletion region acts as a barrier to the further movement
of electrons and holes across the junction.
 According to Coulomb’s law, there exists a force between these
opposite charges.
 This force produces electric field between the charges.
 This electric field produces potential difference across the
junction.
 The direction of electric field from positive to negative charge.
26
Cont.,
 The potential difference has a fixed polarity.
 It acts as a barrier to the flow of electrons and holes across the
junction.
 Hence this potential difference is called as barrier potential or
junction potential or built-in potential barrier.
27
PN Junction Under ZERO BIAS
28
PN Junction Diode
 A p-n junction diode is two-terminal or two-electrode
semiconductor device, which allows the electric current in
only one direction while blocks the electric current in
opposite or reverse direction.
 Forward Bias - it allows the electric current flow.
 Reverse Bias - it blocks the electric current flow.
 Symbols:
Schematic
+ VD −
Internal View
Anode
Cathode
p
n
ID
29
Biasing of p-n junction diode
 The process of applying the external d.c voltage to a p-n
junction semiconductor diode is called biasing.
 External voltage to the p-n junction diode is applied in
any of the two methods:
1. Forward biasing
2. Reverse biasing.
30
PN Junction Under Forward Bias
 The p region terminal is connected to the positive of dc
voltage and the n region terminal is connected to the
negative of dc voltage.
 When p-n junction is forward biased, if the applied
voltage is less than the barrier potential, then there
cannot be any conduction.
31
Cont.,
When applied voltage is more than the barrier voltage,
 Thus holes gets repelled by positive terminal and
electrons gets repelled by negative terminal and cross
the junction against the barrier potential.
 The negative terminal of the battery pushes the free
electrons in n region to p region against the barrier
potential.
 The positive terminal of the battery pushes the free
holes from p region to n region against the barrier
potential.
32
Cont.,
33
Cont.,
 Due to forward bias voltage,

More electron flow into the depletion region reduces the
number of positive ions.

More flow of holes into the depletion region reduces the
number of negative ions.
 This reduces the width of depletion region.
34
Cont.,
• The large number of majority carriers constitutes the
forward current.
• Once the conduction electron enters into p region , they
become valence electron.
• Then the valence electrons move from hole to hole towards
the positive terminal of the battery.
• Moment of valence electron means, the movement of holes.
• So the current in p region is movement of holes which are
majority carriers.
• This is the hole current.
35
Cont.,
 In n region, the current is movement of free electrons which
are the majority carriers.
 This is the electron current.
 The majority carriers travel around the closed circuit and
relatively large current flows.
 The direction of electron flow is from negative to positive of
the battery.
 The direction of flow of electron is opposite to the direction
of conventional current.
36
Cont.,
 The p region terminal is connected to the negative of dc
voltage and the n region terminal is connected to the
positive of dc voltage.
 When p-n junction is reverse biased, if the applied
voltage is less than the barrier potential, then there
cannot be any condition.
37
Cont.,
 When p-n diode is in reverse biased,
 The negative terminal of the battery attracts the holes
p region and positive terminal attracts the free
electrons in the n region.
 No charge carrier is able to cross the junction.
 The depletion region widens.
 This increases more positive ions in n region and
more negative ions in p region.
38
PN Junction Under Reverse Bias
39
Cont.,
 As depletion region widens, the barrier potential across
the junction also increased.
 The electrons on p side and holes on n side are the
minority charge carriers which constitute the current.
 Thus reverse conduction takes place.
40
Cont.,
 The reverse current flows due to minority charge carriers
which are small in number.
 Hence, the reverse current is always small.
 The generation of the minority carriers depends on the
temperature not with the applied reverse bias voltage.
 For a constant temperature, the reverse current is
constant through reverse voltage is increased up to
certain limit.
 Hence, it is called reverse saturation current.
41
V-I characteristics of p-n Junction
Diode
42
Cont.,
 Knee voltage: Forward voltage at which the current through
the PN junction starts increasing rapidly. Also called as cut
in or threshold voltage
 Knee voltage for,
 germanium – 0.3v
 silicon – 0.7v
 Reverse saturation current for,
 germanium – 50 mA
 silicon – 20 mA
43
V-I characteristics of p-n junction diode
44
Avalanche Breakdown
 The
avalanche
breakdown
occurs in both normal diodes
and zener diodes at high
reverse voltage.
 When high reverse voltage is
applied to the p-n junction
diode,
the
free
electrons
(minority carriers) gains large
amount of kinetic energy and
accelerated
velocities.
to
greater
45
Cont.,
 Because of this continuous collision with the atoms, a large
number of free electrons are generated.
 This is called carrier multiplication.
 The large number of minority carriers move across the
junction, breaking the p-n junction.
 As a result, electric current in the diode increases rapidly
due to the generation of avalanche charge carriers.
 This is called avalanche effect.
 The voltage at which the junction breakdown occurs is
called reverse breakdown voltage.
46
Diode Applications
 Rectifiers in dc power supplies
 Clampers in TV receivers.
 Clippers in for wave shaping.
 Voltage doublers in CRT.
 Switching in digital logic circuits.
 Demodulation circuits.
 Voltage regulators and Comparators.
 Laser diodes are used in optical communications.
 Light Emitting Diodes (LEDs) are used in digital displays.
47
ZENER DIODE
 The symbol of zener diode is similar to the normal p-n
junction diode, but with bend edges on the vertical bar.
 Zener diode consists of two terminals: anode and
cathode.
 In zener diode, electric current flows from both anode to
cathode and cathode to anode.
48
Cont.,
 A normal p-n junction diode. Forward bias - allows large
amount of electric current.
 Reverse Bias - blocks large amount of electric current.
 If reverse biased voltage applied is highly increased, It
causes a junction breakdown and
a sudden rise in
current occurs.
 It leads to high power dissipation which will result in the
diode to become destructive.
 A normal p-n junction diode does not operate in
breakdown
region
because
permanently damages the diode.
the
excess
current
49
Cont.,
 A zener diode is a special type of p-n junction
semiconductor device designed to operate in the reverse
breakdown region.
 The breakdown voltage of a zener diode is carefully set
by controlling the doping level during manufacture.
 The zener diodes have breakdown voltage range from
3V to 200V.
50
V-I characteristics of Zener Diode
 The dc voltage can be applied to the zener diode so as
to make it forward biased or reverse biased.
Forward bias
Reverse bias
51
Cont.,
52
Cont.,
 In the forward biased condition, the normal diode and
the zener diode operated in similar fashion.
 But the zener diode is designed to be operated in the
reverse biased condition.
 In reversed biased condition, the diode carries
reverse saturation current till the reverse voltage
applied is less than the reverse breakdown voltage.
53
Cont.,
 When the reverse voltage exceeds reverse breakdown
voltage, the current through it changes drastically but the
voltage across it remains almost constant.
 Such a breakdown region is a normal operating region
for a zener diode.
 In the reverse biasing, reverse voltage (VR) starts to
increases the reverse current (IZ) remains negligibly
small up to the 'knee' of the curve.
54
Cont.,
 At this knee point the voltage is called zener breakdown
voltage VZ, remains constant. There is a maximum value of
zener current designated as IZ(Max) above which diode may
be damaged.
 The value of this current is given by the maximum power
dissipation of zener diode.
 There is a minimum value of zener current called break over
current designated as IZ(min) which must be maintained in
order to keep the diode in breakdown.
55
Zener Diode Breakdown Voltage
 There are two distinct mechanism due to which
breakdown may occur in the zener diode.
1. Zener breakdown
2. Avalanche breakdown
 The different breakdowns are usually differentiated on
the basis of doping concentration.
 When the PN- junction is highly doped, the zener
breakdown occurs while avalanche breakdown occurs
only when the PN- junction is very lightly doped.
56
Zener Breakdown
 When the reverse bias voltage is 6V or less causes a
field across the depletion region of the order of 3 x 105
V/cm.
 Such high magnitude of electric field exerts a large force
on the valence electrons of the atom, tending to separate
them from the nucleus.
 Hence, electron-hole pairs are generated in large
numbers and a sudden increase of current is observed.
 This process is referred to as Zener breakdown.
 The diode may be destroyed due to the excessive heat
at the junction.
57
Avalanche Breakdown
 Avalanche breakdown occurs when the breakdown voltage
of zener diode is more than 6V.
 It occurs in both normal diodes and zener diodes at high
reverse voltage.
 When high reverse voltage is applied to the p-n junction
diode, the free electrons (minority carriers)
gains large
amount of kinetic energy and accelerated to greater
velocities.
58
Cont.,
 It collides with the atoms and knock off more electrons.
These electrons are again accelerated and collide with
other atoms.
 Because of this continuous collision with the atoms, a large
number of free electrons are generated.
 As a result, electric current in the diode increases rapidly
due to the generation of avalanche charge carriers.
 At this stage, the junction is said to be in breakdown and
current starts increasing rapidly.
59
Advantages of Zener Diode
 Power dissipation capacity is very high
 High accuracy
 Small size
 Low cost
60
Applications
 As a voltage regulating element in voltage regulators.
 In various protection circuits.
 In zener limiters. i.e. clipping circuits which are used to
clipp off the unwanted portion of the voltage waveform.
61
Zener Breakdown Vs Avalanche Breakdown
Zener breakdown
Avalanche breakdown
Breaking of covalent bonds is due
to intense electric field across the
narrow depletion region. This
generates large number of free
electrons to cause breakdown
Breaking of covalent bonds is due
to collision of accelerated charge
carriers having large velocities and
kinetic energy with adjacent atoms.
Occurs when reverse voltage less Occurs when
than 6V
greater than 6V
Temperature coefficient is negative
reverse
voltage
Temperature coefficient is positive
The breakdown voltage decreases The breakdown voltage increases
as junction temperature increases
as junction temperature increases
The V-I characteristics is very sharp The V-I characteristics is not as
in breakdown region
sharp as in zener breakdown
region
62
THANK YOU
63