SEMICONDUCTOR

Energy bands ∈solids :

Valance Band :
The range of energy possessed by valance electron is known as valance
bands. The electrons present in the outer most orbit of atoms called as valance
electrons having higher energy.
Conduction Band :
The range of energy possessed by conduction band electrons is known as
conduction band. All the electrons in conduction band are the free electrons. If a
substance has empty conduction bands it means current conduction is not
possible in that substance.
Forbidden Energy Band Gap :
The separation between conduction band and valance band on the energy level
diagram is known as forbidden energy band gap.
Classification of solids according ¿ energy band diagram :
According to band diagrams the solids are classified into three types
i) Conductor
ii) Semiconductor
iii) Insulator
i) Conductor :
a) Conductor is the substance which easily allows the passage of electric current
through them.
b) It’s because that there are a large number of free electrons available in a
conductor.
c) In terms of energy bands valance band and conduction band both overlaps each
other.
Ex :−¿All metals.

ii) Semiconductor :
a) Semiconductors are those substances whose electrical conductivity lies in
between conductors and insulators.
b) Semiconductors are those substances which partially flow electric current.
c) In terms of energy band the valance band is almost filled and conduction band
is almost empty.
d) Here the forbidden energy band gap is very small that is nearly 1 eV .
Ex :−¿Germanium

iii) Insulators:
a) These are the substances which don’t allow the passage of electric current in
them.
b) The energy gap between valance band and conduction band is very large i.e.
forbidden energy band gap is 1.5 eV .
Ex :−¿Glass, wood

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Semiconductors:
i) These are the substances whose electrical conductivity lies in between conductors and
insulators.
¿
These are the substances which partially flow electric current in them.
ii) Semiconductors have negative temperature coefficient of resistance. As the
temperature is increased the electrical conductivity of semiconductors also increases.
iii) At room temperature semiconductors behave as insulator.
iv) The most commonly used semiconductor is Germanium and silicon. For silicon the
forbidden energy band gap is 1.1 eV and in germanium the forbidden energy band gap
is 0.7 eV .
Semiconductors are of two types
a) Intrinsic Semiconductor
b) Extrinsic Semiconductor
a) INTRINSIC SEMICONDUCTOR:
i) The pure form of semiconductor is known as intrinsic
semiconductor.
ii) Under the influence of electric field the free electrons of
pure semiconductor constitute electric current of the same
time another current i.e. whole current also flow in the
semiconductor.
iii) When a covalent band is broken due to thermal energy the
removal of one electron leaves a vacancy this missing
electron is called a hole, which acts as positive charge.
iv) For 1 electron set free one hole is created therefore
thermal energy creates hole-electron pair.

b) EXTRINSIC SEMICONDUCTOR :
i) The intrinsic semiconductor has little current conduction capability to increase the
conducting properties; a small amount of suitable impurity is added to the
semiconductor.
ii) The addition of impurity is called as Doping and the doped semiconductor is called as
extrinsic semiconductor. The purpose of adding impurity is to increase either the
number of holes in the semiconductor crystal.
iii) Depending upon the type of impurity added the semiconductor is divided into two
types
 N- type semiconductor
 P- type semiconductor
i) N – type semiconductor :
 When a small amount of penta-valent impurity is added to the
pure semiconductor then it is known as N- type semiconductor.
Ex :−¿ Arsenic (33), Antimony (51)
 When penta-valent impurity like Arsenic is added to Germanium
crystal a large number of free electrons is available in the crystal,
the reason is Arsenic contains 5 electrons in the outermost orbit.
 The Arsenic atom fits in the germanium crystal in such a way that

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 its four valance electron form covalent bond with four germanium atoms and the
fifth valence electron finds no place in the covalent bond and becomes free. So for
each Arsenic atom added one free electron will be available in the germanium.
Energy Band :
The addition of donor impurity to an intrinsic semiconductor
creates extra energy level just below the conduction band with the
energy equal to 0.1 eV . In N -type semiconductor the majority charge
carriers are electrons and the minority charge carriers are holes.

ii) P−type semiconductor :

When a small amount of tri-valent impurity is added to the
pure semiconductor then it is known as P- type semiconductor.
Ex−¿Aluminum (13), Gallium (31), Indium (49) etc.
When trivalent impurity is added in germanium crystal a large
number of holes is produced in the crystal the reason is that
aluminum is trivalent i.e. the atoms has three valance
electrons.
Each atom of aluminum fits into the germanium crystal in such
a way that the three electrons form covalent bond with the
germanium atom.
In the fourth covalent bond only germanium atom contributes one valence electron
while aluminum has no valence electron the missing electron is called hole is created.

Energy Band :
The addition of acceptor impurity to an intrinsic
semiconductor creates an extra energy level called acceptor energy
level just above the valence band, small amount temperature
provides enough thermal energy to push the electrons in the valence
bond to the acceptor energy level so the conductivity of the P- type
semiconductor increase.

DIFFERENCE BETWEEN INTRINSIC SEMICONDUCTOR∧INTRINSIC SEMICONDUCTOR :

INTRINSIC SEMICONDUCTOR EXTRINSIC SEMICONDUCTOR
i) It is the pure form of semiconductor. i) It is an impure form of semiconductor i.e.
impurity is added in the semiconductor
like penta-valent or trivalent impurity.
ii) Its electrical conductivity is low. ii) Its electrical conductivity is high.
iii)The number of free electrons in the iii)In N-type semiconductor the number of
conduction band is equal to number of free electrons is more than that of holes.
holes in the valence band. In P-type semiconductor the number of
free holes is more than that of electrons.
iv)Conductivity depends upon temperature. iv) Its electrical conductivity depends on
temperature and also doping.
v) It has no practical use. v) It’s used in electric devices like radio, TV,
computers etc.

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DIFFERENCE BETWEEN N−TYPE SEMICONDUCTOR∧P−TYPE SEMICONDUCTOR:

N−TYPE SEMICONDUCTOR P−TYPE SEMICONDUCTOR

i) When a small amount of pentavalent i) When a small amount of tri-valent
impurity is added to the pure impurity is added to the pure
semiconductor then it is known as N- semiconductor then it is known as P-
type semiconductor. type semiconductor.
ii) Impurities are Arsenic, Antimony. ii) Impurities are Aluminum, Gallium, and
Indium.
iii)Here the majority charge carriers are iii) Here the majority charge carriers are
electrons and the minority charge holes and the minority charge carriers
carriers are holes. are electrons.
iv)Here the donor level is just below the iv) Acceptor enough level is just above the
conduction band. valance band.

PN −Junction Diode:
When P-type and N-type is connected electrically then they are called PN junction.
FORMATION OF DEPLETION LAYER∨POTENTIAL BARRIER :
i) In N-type semiconductor the concentration of
electrons is much greater as compared to
concentration of holes while in P-type semiconductor
the concentration of holes id much greater than the
concentration of electrons.
ii) When PN junction is formed then due to the
concentration gradient the holes diffuse forms P side
to N side and the electrons drifted from N-side to P-
side. These motions of charge carriers give rise to
diffusion constant across the junction.
iii) When an electron is drifted from N-region to P-region
it leaves behind a positive ion and on N side when a
hole diffuses from P-region N- region it leaves a negative ion on p- region.
iv) These negative and positive ions immobile in nature and get accumulated across the
junction. This layer contains no free charge carriers and called as Depletion layer.
CURRENT FLOW ∈ A PN −JUNCTION :
Forward biasing of pn− junction :
When the positive terminal of the battery in connected to P-type and negative
terminal of the battery is connected to N-type then the PN junction is said to be in forward
biased.

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Working :
i) Under the forward biasing the applied potential difference causes a field which acts
opposite to the internal electric field ( Ei ) due to depletion layer these results in
reducing the potential barrier.
ii) In forward biasing the holes of the p-region are repelled by the positive terminals of
the battery and the electrons in the N-region are repelled by the negative terminals of
the battery. So the electrons and the holes are drifted towards the junction.
iii) If the external potential difference is more than the internal potential barrier then the
negative accepter ions and positive donor ions within the depletion layer region holes
and electrons respectively gets eliminated.
iv) Consequently the depletion layer is eliminated and the majority charge carriers flow
across the junction in opposite direction to cause forward current.
v) Due to forward biasing
a) Width of the depletion layer decreases
b) Resistance decreases
c) Current conduction increases
Reverse biasing of PN junction :
When the negative terminal of the battery in connected to P-type and positive
terminal of the battery is connected to N-type then the PN junction is said to be in
reversed biased.
Working :
i) In reversed biasing the majority charge carrier are
holes P-region and electrons in N-region are attracted
away from the junction.
ii) This increases the number of negative and positive
ions in P-region respectively. Thus the widening the
depletion layer and increasing of potential barrier.
iii) The majority carriers cannot flow across the junction
and no current flows through the external circuit.
iv) But actually a very small current flows in the reverse
direction. This is due to the minority charge carriers. This current is known as Reverse
current or leakage current.
v) Due to reverse biasing
a) Width of the depletion layer increases
b) Resistance increases
c) Current conduction decreases
Junction diode characteristics :
Variation of current through the circuit with a charge
in bias-voltage is shown in the above figure. Refers to the
forward biasing and R to the reverse biasing of the junction
diode. This curve is called as junction diode characteristics.
A forward biased junction is a low resistance
instrument while a reverse biased junction diode is a high
resistance instrument because when there is a small charge
in applied voltage, the current changes appreciably in
forward biasing but there is no effect in reverse biasing.

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Rectifier :
PN junction formed when P-type and N-type semiconductor kept in electrical
contact with each other. A rectifier is a device which converts the half cycles of AC to DC.

HALF−WAVE RECTIFIER :
It is the type of rectifier in which the positive half cycle of A.C. is converted into D.C.

Construction :
It consists of a single PN junction diode. The AC signal which to be rectified in
connected across the primary terminals P1 and P2 of a transformer.
One of the secondary terminals S 1 is connected to P- region of the junction which
the other secondary terminal S2 is connected to N- region with a load resistance R L in
series. The output is taken across the load resistance.
Principle:
The PN junction diode conducts current when it is forward biased and doesn’t
conduct current when it is reversed biased.
Working :
The AC input voltage during the positive half cycle of the input AC voltage the
terminal S1 is positive with respect to S2. During the half cycle of the diode is forward
biased, therefore the diode conducts current.
During the next half cycle the terminal S 1 is negative with respect to S 2 under this
condition the diode is reversed biased and conducts no current.
Thus, the output current in the load resistance flows only during positive half cycle
of input AC signal.

Efficiency of half wave rectifier :

output DC power
n= =40.6 %
input AC power

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FULL WAVE RECTIFIER :
It is an electric device which converts both the half cycle of i.e. positive and negative
half cycles of AC to DC.

Construction :
The full wave rectifier consists of two PN junction diodes D 1 and D2 whose N-region
terminals are connected together.
The AC signal to be rectified is connected across the primary terminals of P 1 and P2.
The P-region of the diode is connected to the ends S 1 and S2 of the secondary terminals of
the transformer.
The top of the secondary is connected to the load resistance (R L) and the output is
taken across load resistance.
Working :
During the first half cycle of input Ac signal the terminal S 1 is positive and S2 is
negative so that the diode D1 is forward biased and D2 is reversed biased. Therefore the
diode D1 conducts current while D2 does not conduct. The direction of current due the
diode D2 is in S1D1XC so the positive half cycle is rectified into DC.
In the negative half cycle of input Ac the diode D 1 is reversed biased whereas the
diode D2 is in forward biased that is the diode D1 doesn’t. The direction of current I2 due to
diode D2 is S2D2XC. So the negative half cycle is rectified into DC.
Thus the current in load resistance is in same direction for both the half cycle is
output AC voltage.

Efficiency of full wave rectifier :

output DC power
n= =81.2 %
input AC power

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Special purposeof PN − junction diode :
Light Emitting Diodes (LED ):
A LED is a simply a forward-biased P−N junction diode which emits light when energized.
Principle:
During a forward biased P−N junction the
electrons of N -region and holes form the P-region
are repelled towards the junction where electron–
hole recombination takes place. As the electrons
are in higher conduction band and holes of the
P−¿ region are in lower valence band. During the
process of recombination, some of this energy
difference is used as radiation (light and heat).
In case of germanium (¿) and silicon (Si)
junction greater percentage of energy lies in the
region of infra-red region, hence the emitted light
is insignificant. But in case of gallium (Ga) and phosphide (P), and gallium- arsenic-
phosphide (Ga As P) a greater percentage of light is released in the form of energy, which
is significant.
If the semiconductor material is translucent then light is emitted and the junction
acts a light-source. For visible light the semiconductor chosen as GaPand Ga As P. The color
of emitted light depends upon semiconducting material used, red or green light is
produced for GaP and Ga As P junction is used for mostly red and yellow light.And the LED
s emit no light when it is reversed biased.
Construction :
A P−N junction diode is formed and the metal contacts are made on P and N
regions. The metal contacts of P-region are made at the outer edges to allow more central
surface area open to escape light. The lower surface is coated with metallic film (let be
gold) to allow maximum reflection to the surface of device and to provide the cathode
connections.
When forward biased the diode emits light. Being made of a semiconductor
material, it is rugged, has a long life (about 10,000 hours), fast response time and good
contrast ratio for visibility.
Applications:
The application of LED is based on the following:
a. Wavelength of emitted light
b. Input power
c. Output power efficiency
d. Turn on and turn off time
e. Mounting arrangement
i. The LEDs operate at 1.5 V −3.3 V , they are used in solid state circuits (for solid state
video displays).
ii. Any desired number from 0 to 9 can be displayed by applying a forward bias to P−N
junction of suitable material (Typical valves for forward bias voltage and current are

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iii. 1.2V and 20mA respectively). Therefore, they are being widely used in hand-held
calculators.
iv. The LEDs are used for supplying power input to Laser and for entering information into
optical computer memories.
v. They are being used in burglar alarm systems.
PHOTODIODE :
A semiconductor photodiode is simply a reverse biased P−N junction, illuminated
by radiation.
Construction :
A P−N junction diode is embedded in a glass or clear plastic to form a photodiode.
Only one side of the package is kept transparent and the other side is either painted black
or enclosed in a metallic case. The entire unit is very small in dimension of the order of
2.5 mm. The P−N junction is reversed biased and a converging lens is used to focus
maximum light on the reverse biased junction.

Working :
When the P−N junction is reversed biased with sufficient amount of potential and
no light is made to fall on it, then a small amount of reverse current ( I s∨I 0) flows across
the junction. This current is due to electron-hole pairs and is called dark current. This
current is due to minority charge carries which falls on the potential hill at the junction,
whereas the junction barrier does not allow the majority charge carriers to cross the
junction.
Now if light is made incident of the surface, additional electron-hole pairs are
generated. As the concentration of majority charge carriers, is much higher than minority
charge-carriers, therefore, the percentage increases in majority carriers in much smaller
than that in minority carriers.
Hence we may neglect the increase in density of majority carriers and consider the
radiation solely as a minority injector. That is why photodiode is operated in reverse-
biased mode. These newly created or injected minority charge carriers
(electrons of P−region) diffuse to the junction, cross it and cause the current.
Characteristics :
Under a large reverse-bias the current is given by, I =I 0 + I s
Where, I 0=dark current
I s=short−circuit current , proprotional¿ light intensity
It is obvious that by changing the level of illuminations, the resistance of the diode can be
changed by a factor of nearly 20. A photodiode works as a fastest detector because it can
turn its current ON an OFF in nano –seconds.

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 A photodiode is used to switch light ON and OFF at maximum rate. To improve the
spectral response and get large photo currents multiple junction photodiodes have been
prepared.
SOLAR CELL :
A solar cell is photovoltaic device which converts solar energy to electrical energy.
Construction :
A P−N junction diode is embedded in a glass
or clear plastic package to form a solar cell. Only one
side of the package is kept transparent and the
other side is either painted black or enclosed in a
metallic case. The surface layer of P-type is
extremely thin so that the incident photons may
easily penetrate to reach the junction. Metal
contacts are made at whole N-side and at the ends
of P-side. The contact on P-side acts as anode while
on N-side acts as cathode.
The semiconductor with band gap nearly
1.5 eV are ideal materials for fabrication of solar cells.

Working :
An emf is produced in the solar cell when light falls on it, this is due to the following
reasons:
i. Breaking of covalent bonds, thus generating electron-hole pairs (whenhν> E g ) close to
the junction.
ii. The separation of electrons and holes due to electric field of the depletion region, the
electrons are swept to N-side and holes to P-side.
iii. The collection of electrons and holes: the electrons reaching the N-side are collected
by the front contact and holes, reaching the P-side are collected by the back contact.
Thus P-side becomes positive and N-side negative rise to photo voltage.
When external load ( R L ) is connected between front and back contacts, a photo current I L
flows through the load. This photo current is directly proportional to the intensity of
illumination and it also depends upon the surface are of the junction being illuminated.
The open circuit voltage V 0 c depends on the illumination. Hence power-input of a solar cell
depends on the intensity of incident light.
V −I Characteristics:
A V-I characteristic is drawn in fourth quadrant of the
coordinate axis. The reason is that the solar cell does not draw
current but supplies the current to the load.
Uses :
The solar cells are used in satellites to recharge their
batteries. It is being planned to orbit large banks of solar cells
outside earth’s atmosphere for converting solar energy to
electrical energy.

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ZENER DIODE :
A Zener diode is a reverse biased properly dipped crystal diode having a sharp
break-down voltage and operated in breakdown region.
It has already been pointed out that when the reverse bias on a crystal diode is
increased beyond a critical value called breakdown voltage, the reverse current increase
sharply to a high value. A zener diode is like an ordinary diode except that it is suitably
doped to have a sharp breakdown voltage, called Zener voltage.
The zener voltage depends upon the amount of doping. If the diode is heavily
doped, the depletion layer is thin and so the breakdown voltage or zener voltage is low. On
the other hand, a lightly doped diode has a higher breakdown voltage.
Zener diodes have been designed to operate from 1 to several hundred volts. In the
junction diodes which are operated below 6 volts, the breakdown of the junction is due to
zener effect, but in those diodes which are operated above 6V, the breakdown is due to
avalanche effect, but conventionally all diodes which are operated in the breakdown
region of their reverse characteristics are called zener diodes.
For normal operation of zener diode in breakdown region the current through the
diode should be limited by external resistance to suitable value such that the power
dissipation across the junction is within the tolerable limit of junction.
The symbol of zener diode is like an ordinary diode except that the bar is turned
into the shape of letter ‘Z’.
Characte rs of zener diode:
The circuit diagram of a zener diode is same as current-voltage characteristics of a PN
junction diode.
i. When forward biased, its characteristics are just like
those of ordinary diode.
ii. A zener diode is always reverse biased. When reverse
biased, a small reverse current fowls through it. This
current remains up to a certain critical voltage called
turn-over voltage and then current increases rapidly
and the reverse characteristics becomes nearly parallel
to current axis. The reverse voltage for which the
current corresponds to some point on linear portion of
reverse characteristic, is called the zener voltage.

Zener diode as a voltage regulator :

A zener diode is heavily doped PN-junction. It has a sharp breakdown voltage and is
used as a voltage regulator. When a zener diode is operated in the breakdown region, the
voltage across it remains practically constant for a large change in the current.
The zener diode is connected across load such that it is reverse biased. The series
resistance R absorbs the output voltage fluctuations so as to maintain constant voltage
across the load.
Working :
Let V ¿ be the unregulated input DC voltage and V 0 be
the output voltage across R L to be regulated and V Z be the
zener voltage of the diode. The value of the series resistance

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 is so chosen that the diode operates in the breakdown region under the zener
voltage V Z across it.
Let ‘ I ’ be the current drawn from supply, I Z the current through zener diode and I L
the current through the load.
Then, I =I Z + I L
 I Z =I −I L
If R Z zener resistance, then V 0=V Z =I Z RZ =I L RL
Applying Kirchhoff’s law to the mesh containing resistance R, zener diode and supply
voltage V ¿,
RI +V Z =V ¿
 V Z =V ¿ −RI −−−(i)
When the input voltage V ¿ is lower than the zener voltage V Z of diode, there is no
current conduction.
i.e., I Z =0 hence V 0=V ¿
As input voltage V ¿is increased so that it becomes equal to V Z , the breakdown point is
reached and the voltage across the diode V Z =V ¿ −RI becomes constant.
A further increase of input V ¿ does not result in the corresponding increase in
V 0∨V Z but merely increase the voltage drop across R.
Thus in breakdown region,
V 0=V Z =V ¿ −RI
Characteristics :
A graph is plotted by taking output voltage V 0 verses
input V ¿. From the graph it is clear that the output voltage
remains constant when the diode is in zener region.
It may be pointed that for maintaining constant regulated
output, the series resistance R for a given range of input
voltage be so chosen that:
i. The diode operates in zener region
ii. Current should not exceed a certain value to cause
out of diode.

TRANSISTOR :
A transistor consists of two PN junction formed by sandwiching either P-type or N-type
semiconductor between a pair of opposite types. A transistor is of two types
i) PNP transistor
ii) NPN transistor
i) PNP transistor :
It consists of a silicon or germanium crystal in which a
third layer of N-region is sandwiched between two P-
regions.

ii) NPN transistor :

It consists of a silicon or germanium crystal in which a
third layer of P-region is sandwiched between two N-
regions.

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PNP TRANSISTOR :
It consists of a silicon or germanium crystal in which a third layer of N-region is
sandwiched between two P-regions.
WORKING :
For the operation of PNP transistor the emitter-base is forward biased and the base-
collector is reversed biased in the junction, these three types of current flows in a PNP
transistor circuit i.e.
 Collector Current
 Base Current
 Emitter Current

Collector Current :
Under forward biased of emitter base junction holes in the emitter and electrons in
the base move towards the junction and get neutralized.
As the base region is very thin the holes from the emitter crossing the base and
enter into the collector due to reverse biasing of collector-base junction the holes are
attracted towards the collector terminal.
These holes on reaching the collector terminal are neutralized but an equal number
of electrons reaching from the negative terminal of the battery V CB into the collector.
At the same time an equal number of electrons flow from negative terminal of the
battery VEB and each the positive terminal of V CB.This flow to holes from the base to
collector results in collector current IC.
Base Current :
Due to forward biasing of emitter-base junction a large number of holes flow form
emitter to base but most of them reach to the collector and very few of them are
neutralized with the electrons of the base region.
These electrons are filled but the flow of an equal number of electrons from the
negative terminal of the battery VEB. This gives the base current IB.
Emitter Current :
Just when the electron enters the base from emitter batter V EB or electron enters
the collector from the battery V CB an equal number of electrons enters into the positive
terminal of the emitter battery VEB. This causes emitter current IE.
Relation Between Base , Emitter∧Collector Current :
I E =I B + I C
Where, I E =¿Emitter current
I B=¿Base current
I C =¿Collector current

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NPN Transistor :

Working :
For the operation of NPN transistor the emitter-base is forward biased and the base-
collector is reversed biased in the junction, these three types of current flows in a NPN
transistor circuit i.e.
 Collector Current
 Base Current
 Emitter Current

Collector Current :
Under forward biased of emitter base junction electrons in the emitter and holes in
the base move towards the junction and get neutralized. Thus the depletion layer of
emitter-base junction is eliminated.
As the base region is very thin the electrons from the emitter cross (98 % ) the base
and enter into the collector.
The electrons crossing the base and entering the collector are attracted towards the
positive terminal of the collector battery under the forward biasing of emitter-base
junction holes on the emitter and electrons in the base move towards the junction. This
cause a current in collector circuit called as the collector current I C.
Base Current :
Due to forward biasing of emitter-base junction a large number of electrons flow
form emitter to base but most of them reach to the collector and very few of them are
neutralized with the holes of the base region.
As soon as a hole combines with an electrons of emitter region an electron is
released from the base region is attracted by the positive terminal of the battery V EB giving
rise to feeble base current IB.
Emitter Current :
When the electrons enters the emitter battery VEB form the base or electrons enter the
collector battery VCB form the collector an equal number of electrons enters into the
emitter from emitter battery VEB causing the emitter current.
Relation between base , emitter∧collector current :
I E =I B + I C
Where, I E =¿Emitter current
I B=¿Base current
I C =¿Collector current

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Common base connection for PNP∧NPN transistor :
In this circuit arrangement the input is applied between emitter and base and
output is taken from collector and base. Here the base of the transistor is common to both
the input and output circuit and hence the name common base connection.

Characteristics of common base connection:

Input Characteristics :
It is the curve between emitter current I e and emitter base voltageV eb at constant
collector base voltageV cb. The emitter current is generally taken along y-axis and emitter
base voltage is taken along x-axis.
Input resistance is the ratio of change in emitter base voltage( ∆V eb ) to the resulting
change in emitter current( ∆ I e ) at constant collector base voltage( V cb ).
∆ V eb
Input resistance r i = at constant V cb
∆ Ie

The following points can be noted from the characteristics

i) The emitter current I e increase rapidly with small increase in emitter base voltage V eb.
It means that resistance is very small.
ii) The emitter current is almost independent of collector base voltage. This leads to the
condition that emitter current (and hence collector current) is almost independent of
collector voltage.

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Output Characteristics :
It is the curve between collector current I c and collector base voltage( V cb )at
constant emitter current I e. Generally collector current is taken along y-axis and collector
base voltage along x-axis.
Output resistance is the ratio of change in collector base voltage ( V cb ) to the
resulting change in collector current voltage ( ∆ I c ) at constant emitter current.

∆ V cb
Output resistance r o = at constant I e
∆ Ic
The following points can be noted from the characteristics
i) The collector current I c varies with collector base voltage V cb only at a very low voltage
( V <1 V ). The transistor never operated in this region.
ii) When the value of V cbis raised above to 1−2 V , the collector current becomes constant
as inductive by straight horizontal curves. It means that now I c is independent of V cb
and depends upon I eonly. This is constant with the theory that the emitter current flow
almost entirely to the collector terminal. The transistor is always operated in this
region.
iii) A very large change in collector base voltage produces only a tiny change in collector
this means that output resistance is very high.
COMMON EMITTER CONNECTION :
In this circuit diagram input is applied between base and emitter and output is taken
from collector and emitter. Here the emitter of transformer is common to both the input
and output circuit and hence the name is common emitter connection in figure (i) a
common emitter NPN transistor circuit is shown in figure (ii) which shows the emitter PNP
transistor circuit.

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BASE CURRENT AMPLIFICATION FACTOR∨CURRENT GAIN (β ):
The ratio of change in collector current ( ∆ I c ) to the change in base current (∆ I B ) is known
as current amplification factor.
∆ Ic
β=
∆ IB
Characteristics of common emitter connection :
Input characteristics:
It is the curve between base current I B and base emitter voltage V BC at constant
collector emitter voltage VCE. The base current is taken along y-axis and base emitter
voltage is taken in x-axis.
Input resistance is the ratio between the changes of base emitter voltage ( ∆ VBE) to
the change in base current (∆ IB) at constant collector emitter voltage (VCE).
∆ V BE
Input resistance, r i = at constant VCE
∆ IB

The following point can be noted from the characteristics

i) The characteristics resemble that the of a forward biased diode curve. This is expected
since the base emitter section of a transistor is a diode and it is forward biased.
ii) As compared to common base arrangement I B increases rapidly with VBE. Therefore
input resistance of common emitter is higher than that of common base circuit.
Output characteristics :
It is the curve between the collector current I C and collector emitter voltage V CE at
constant base current IB. Collector current is taken along y-axis and emitter voltage is along
x-axis.
Output resistance is the ratio of change in collector emitter voltage ( ∆ VCE) to the change in
collector current (∆ IC) at constant base current.
∆ V CE
I.e. output resistance r o = at constant IB
∆ IC

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The following point can be noted from the characteristics
i) The collector current IC varies with VCE for VCE between 0V to 1V only. After this
collector current becomes almost constant and independent of V CE. This value of VCE up
to which collector current I C changes with VCE is called the knee voltage(V knee). The
transistor is always operated in the region above knee voltage.
ii) Above the knee voltage IC almost constant however a small increase in I C with
increasing VCE is caused by the collector depletion layer getting wider and capturing a
few more majority charge carriers before electron-hole combinations occurs in the
base region.
iii) For any value of VCE above knee voltage the collector current is approximately equal to
β x IB.
Transistor const ant∨ parameter :
i) Current amplification factor (α ):
It is the ratio between the changes in collector current to change in emitter current at
constant collector voltage.
∆ Ic
α= at constant V CB
∆ IE
ii) Base current amplificationfactor ∨current gains(β ):
It is the ratio between changes in collector current to change in base current at
constant collector emitter voltage.
∆ Ic
β= at constant V CE
∆ IB
Rel ationbetween α ∧β :
From the relation I E =I B + I C
The above equation can be written as
 ∆ I E=∆ I B + ∆ I C
 ∆ I B =∆ I E −∆ I C −−−−−(i)
∆ Ic
We know that α = and
∆ IE
∆ Ic
β¿ ∆ I −−−−−(ii)
B

Using equation (ii) in equation (i)

∆ Ic
 β¿
∆ IB
∆ Ic
 β¿ −−−−−(iii)
∆ I E −∆ I C
∆ Ic
∆ IE
 β ¿ ∆I ∆ I
E C
−
∆ I E ∆ IE
α
 β ¿ 1−α
 γ =1+ β

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Transistor as an amplifier :
Amplifier is a device which used to increase the strength of a weak signal.
Output voltage
Voltage gain, A v =
Input voltge
In a transistor one of the three terminals (base, emitter, collector) is kept common
to both the input and output circuits. Hence we have three basic transistor circuits such as
the common base, common emitter or common collector amplifier.

Common emitter transistor amplifier :

An NPN transistor has the signal
connected in the emitter base circuit while the
load resistance is connected in the collector
emitter circuit. Imposition of signals results in
the variation of base current and hence
variation in collector current. This results in a
large variation in the voltage across the load
resistance, causing amplification.

Voltage gain:
Voltage input ¿ δ I b x Ri [δ I b=change∈base cur rent ]
Voltage output¿ δ I c x Rl [δ I c =change ∈collector current]

Where, Ri=input resistance

Rl=output resistance

output voltage
Voltage gain=
input voltge
δ I c x Rl
 Voltage gain=
δ I b x Ri
Rl
 Voltage gain=β x
Ri
 Voltage gain=β x R esistance gain
Power gain:
Power input=E ixδ I b
Power output =Eo xδ I c
power output
Power gain=
power input
E xδ I δ I x R x δ I
 Power gain= Eo x δ I c = δ I c x Rl x δ I b
i b b i c

( )
2
δ Ic Rl
 Power gain= δ I x R
b i
R
 Power gain=β 2x R
l

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 Power gain=β 2x Resistance gain

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