UNIT – III

Transistor:
A transistor is a device which transfers the current from low
resistance circuit into high resistance circuit.
Transistor is a three terminal, three layer (NPN or PNP) two Junction
(JE and JC) device. In transistor, both holes and electrons were involved in
carrying current. So, a transistor is a bipolar device.
Transistor terminals are Emitter, base and collector.

Emitter:
It is heavily doped region. It emits the charge carrier to the base.
Base:
It is lightly doped and very thin than compared to emitter and collector. It is
also controlthe collector current. It passes most of the injected carriers from the
emitter into the collector.
Collector:
This is moderately doped and large in size to dissipate the heat generated
in a transistor.
Construction:
The transistor consists of a Silicon (or Germanium) crystal in which a
thin layer of N – type Silicon is sandwiched between two layers of P – type
Silicon. This transistor is referred to as PNP. In a NPN transistor, a layer of
P – type material is sandwiched between two layers of N – type material.

Emitter P N P Collector Emitter N P N Collector

Base Base

(a) (b)

The symbolic representation of the two types of the transistors are shown in
below (a)& (b)
E C E C

B B
a)NPN Transistor b) PNP Transister
From fig (a) & (b) the arrow on the emitter specifies the direction of current
flowwhen the emitter Base (JE) Junction is forward biased.

Working of PN P Transistor:

E JE B JC C

0 0 0 0 0 0
IE IC
0P0 0N0 0P0

0 0 0 0 0 0

IB

+- +-
VEB VCB
(a)
T1 !PNP

VEB 5 VCB 5

(a)

Here the input Junction JE (i.e. Emitter – Base Junction) should be

forward biased andout put Junction JC should be reverse (Base – Collector
Junction) biased.

The holes in the emitter are repelled by the positive supply towards
the emitter Junction JC. The potential barrier at the Junction is reduced
due to the forward bias, hence holes cross the Junction JE and enters into
the N – type Base. But base is thin and lightly doped, majority of the holes
(about 95%) are able to drift across the base. The balance of (5%) holes is
lost in base region due to recombination with electrons to constitute a Base
current IB. The holes which after crossing the Junction JC and enter into
collector region and those are collected by the negative collector voltage
VC because base collector Junction is reverse bias. Hence, collector
current IC flows through the out put circuit. Thus the base and collector
current summed up gives the emitter current. I.e. IE = - (IC + IB).This
means, the direction of emitter is opposite the direction of both Base and
Collector currents.

Transistor Current Components:

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 E JE B JC C
IE IC
IPE IPC

P N P
(IPE – IPC)
InE IC0 2

IB

+- +-
VE B VC B

To understand the various current components which flow in a PNP

Transistor, whose emitter base Junction JE should be forwarded biased and
collector base Junction JC should be reverse – biased. Whenever JE is
forward, the emitter will deliver current IPE, due to holes crossing from
emitter to base and
The electron current InE, due to electrons crossing from base to
emitter. Since the direction of hole current IPE is positive, it is from left to
right. The electron current InE will be in the direction opposite to that of
movement of electrons (Since conventional current) and therefore InE will
also be from left to right.
The ratio of IPE / InE is proportional to the ratio conductivity of p
– type emitter to conductivity of N – Type base. Thus conductivity of P
– Type emitter is much more than that of N – Type material and hence IPE
is much more than InE. Out of all the holes crossing JE to enter the base
region, some of them recombine with electrons in the base region. Thus the
number of holes gets reduced. The remaining holes cross JC and reach the
collector. Let the hole current crossing JC be IPC. Hence the recombination
current is equal to (IPE - IPC).
So, the collector current IC is
IC = IC0 – IPC--- (1) (Therefore IE 0)
If the emitter were open – circuited, IE = 0 and then IPC would be Zero.
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 Under theseconditions, the collector current IC would equal the reverse
saturation current IC0.
From the current components we define various parameters.
Emitter Efficiency ( ) ): It is defined as

= Current of injected carriers at JE

Total emitter current.
In the case of a PNP Transistor,
IPE
IPE . ........... (2)
IE

Where IPE is the injected hole diffusion current at emitter junction and InE is the
injectedelectron diffusion current at emitter junction.
Transport Factor (β*):

The transport Factor β* is defined as

β* = Injected carrier current reaching JC
Injected carrier current at JE

In the case of a PNP Transistor,

β* = IPC / IPE ………………(3)

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 Large Signal Current Gain ( ) :

It is defined as the ratio of the negative of the collector current increment to

the emittercurrent change from Zero to IE.

I I
   C C …………… (4) (Therefore IC and IE have opposite signs)
O
IE

From equation (1)

IC - IC 0 = -IPC , So
I
  PC
IE
IPC IPE I I I
   PC  PE E
IPE IPE IE
    

‘  ’ is always positive. The value of ‘  ’ lies in the range of 0.90 to 0.995.

From equation …. (4)

In general IC =  IE + ICO

VC
IC   IE + IC 0 (1 – e VT )

Where VC is Collector voltage, VC is negative.

So,

IC  IE  ICO

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Transistor Configurations:

Transistors are used as amplifier. In any amplifier, for giving input we

need two terminals and for taking out put, we need two terminals. Thus we
need a total of four terminals. But a transistor has only three terminals. So,
it is necessary to make any one of terminals common to both the input side
and the output side. Depending on which terminal is made common, there
are three different possible configurations.
(a) Common – base configuration.
(b) Common – emitter configuration
(c) Common – Collector configuration.

The Early effect:

An increase in magnitude of collector voltage increases the space –
charge width at the out put Junction i.e., the effective base width “w” will
decreases with increase in collection voltage. This phenomenon is known
as the “Early effect”. This decrease in “w” has two consequences (1) there
is less chance for recombination within the base region. (2) The transport
factor “β*” and “” increases.

E C

w'

(a) Common – Base Configuration:

 IE  IC E JE B JC C

VEE - + VCC
+ VEB VC B -

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 N P N

- + - +
VEE VCC

(a) (b)

To analyze the characteristics of transistor in C.B configuration, it must

be operated in active region i.e., JE should be forwarded bias and Jc
should be reverse bias. In this configuration the base is made common to
both the input and output i.e., emitter acts as i/p terminal and o/p is
collector terminal.

Input characteristics:
The input characteristics is a plot of input V EB to input IEB to input
IE for a constant output voltage “VCB “.

IE VCB = 2v
VCB = 1v
VCB = 0
VCB open

Vr VEB

When the collector is open i.e. VCB open, the input

characteristic is same as that of forward biased PN Junction diode, when the
collector is shorted to the base (VCB = 0), the emitter current increases for a
given VEB. Since the collector now removes minority carriers from the base and
hence the base attract more carriers from the emitter. Then emitter current IE
increases. When VCB is increased, due to “early effect”, there is an increase in
IE and so the IE may reach the maximum value a little earlier than the previous
case. Thus, VCB is increased IE reaches the maximum for a lesser value of
VEB and so characteristics shifts towards the left for increasing of VCB.

Out put Characteristics:-

The output characteristics are a plot of o/p voltage “VCB” to o/p
current “IC “for a constant i/p current “IE”.

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The output characteristics are divided into three Active region
regions.
They are
IE=4ma
(1) cut – off region IE=3ma
(2) Saturation region IE=2ma
(3) Active region. IE=1ma
IE=0(ICO)
Saturation region

0 VCB
Cut – off region
Fig (b)

(1) Cut – off region:

In this region both Junctions JE & JC (i/p & o/p) are reverse biased. If
IE =0 the collectorcurrent IC is equal to IC0 (Since due to minority carriers
IC0 flows through the collector Junction).
(2) Saturation region:
The region to the left of the ordinate, VCB=0 and above IE=0 characteristics
in the above fig (b) is saturation region. In this region both Junctions (JE &
JC) are forwarded biased. The forward biasing of the JC accounts for large
charge in IC with small charges in VCB. For a forward bias, IC increases
exponentially with voltage. A forward bias means that collector is made
negative (n – region) w.r.t the P – type base and hence electrons current
flows from N – side across the collector Junction to the P – type base.
Hence collector current IC increases rapidly.
(3) Active region:
In this region the collector Junction “J C” is reverse biased and emitter
Junction is forward biased. Suppose that an emitter current IE is caused to
flow in the emitter circuit due to forward bias of “JE”. Then a “ IE “of this
current will reach the collector. i.e., IC is almost equal to the IE when 1.
In this region, the collector current IC is independent of collector voltage
VCB and depends only upon the input current “IE “. So input current “IE” is
constant then IC also constant.
IC = -  IE + IC0 IC IC
 , A.C  .
The current gain IE IE
d .c output current
is 
Input current
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C.E Configuration:
The characteristic behavior of transistor in C.E configuration must
be analyzed in active region. In this configuration “Emitter is common to
both input and output i.e., Base acts asinput terminal and output is collector
terminal. The C.E configuration is also called as grounded emitter
configuration.
Input characteristics:
It is the plot of input voltage VBE to input current IB for a constant output voltage “VCE

IC IB
IB VCE =2v
VCE =4v
VCE open
C + C
N P N VCE
VBB -
E E
VBE +
B 0
VBE vr VBE
- +
(a) (b) (c)
-VCC

When VCE is zero, if emitter – base Junction is forward biased, the

common emitter input characteristics are same as that of forward biased
PN Junction diode.

When VCE is increased, due to “early effect” there is a decrease in

IB. So the IB may reach the maximum value a little later voltage of VBE
than the previous case. Thus as VCE is increased, IB reaches the
maximum for a larger value of VBE. So, the characteristics shift towards the
right for increasing values of VCE. This is shown in above fig (c).

Output Characteristics:

It is the plot of output voltage “VCE “to output current ‘IC‘ for a
constant input current ‘IB’.

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 IC Active region
IB = 25 µA

IB = 20 µA
Saturation IB = 15 µA
region IB = 10 µA

IB = 0 (ICE0)

0.2v

Cut – off region

Fig (d)
Cut – off region:
In this region both Junctions (input & output) are reverse biased. Only
reverse saturation current flows through the collector due to minority
carriers. i.e., IC = ICEO. The region below the IB = 0 is cut – off region as
shown in above figure.

Active region:
In the output characteristics, the region above the cut – off region and
beyond the saturation region is active region. In the active region, the
Emitter – base Junction is forward biased and collector Junction is reverse
biased. In C.B configuration collector current is given by

IC = - α IE + ICO-----(1)
But we know that IE = - (IC + IB )------ (2)
From equation (1) & (2), we
getIC = - α (IC + IB )
+ ICO
IC (1- α) = αIB + ICO

Therefore IC = I  ICO
1   B 1   1
IC = β IB + (1+ β) ICO or (  ,  1 )
 1  
IC = β IB + ICEO (.(1+ β) ICO = ICEO)

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Where β = Current gain in C.E
ICEO = Reverse saturation current in C.E.
outputcurrent IC
But we have βd.c =  , I

 C
.
IB
A.C
inputcurrent IB
 >>1 and βIB >> ICEO, so
IC ~ β IB.

In this region, the transistor output collector current ‘IC’is most sensitive to
the input base current ‘IB’i.e.,for a small change in base current there exists a large
in the collector current by a factor ‘β’. Due to early effect ‘α ‘ increases. Thus the
increase in‘α’ is fractional change in ‘β’ is prominent.

Since β = . So the collector current IC = β IB increases with increase in the
1
reverse voltage ‘VCE’. Thus for a constant value of IB, IC increases with
increasing “VCE”.

Saturation Region:
In this region both the collector Junction (J C) and (JE) Emitter Junctions are
forward biased. The forward biasing the collector Junction accounts injections of
majority carrier on either side causes collector current ‘IC’ increases exponentially
with applied voltage “VCE”. The required conditions for the transistor saturation
are VCE < 0.2V and base current must be i.e. IB > IC / β.
Common collector configuration
The characteristics of common collector configuration of a transistor
are same as that of the characteristics of a C.E Configuration.

Input characteristics Output characteristics

Active region
IB IC
IB = 25 µA
VEC open
VEC=4v IB = 20 µA
VEC = 2v Saturation
IB = 15 µA
region IB = 10 µA

IB = 0 (ICEO)
Vr VBC
0 Cut – off region
0.2v

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 Transistor as an amplifier:

BC 107 !NPN
RL 1k

VEB 5 VCC 5

Fig (1)

A load resistor RL is in series with the collector supply voltage VCC. A small change in
input volta between emitter and base causes a relatively large emitter current ge Vi
change IE.
This result a fraction of current change passes through R L. Then the change in output
voltage ao cross tLhe Eload resistor V   ' R I, which is many tiimes the change in input
voltage V.
Then the voltage amplification A= Vo will be greater unity and the transistor acts as an
amplifier. V i

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