Unit 2

Introduction
A Bipolar Junction Transistor (BJT) is a three terminal device whose operation is dependent on
the interaction of both majority and minority carriers and hence it is called bipolar device.
Transistor means transfer of resistor i.e, transfer of resistance from low resistance (input ) to high
resistance (output) circuit.

Construction of a BJT

1. n-p-n
The n – p –n transistor is formed by sandwiching a thin ‘p’ type semiconductor between two ‘n’
type semiconductors.(Fig 1)
Base comes between collector and emitter region.
Base is thin and lightly doped. Doping level of Emitter is intermediate between Emitter and
Base.
Emitter and collector are much wider than base and are heavily doped.
The emitter is heavily doped than the collector.

Fig 1

2. p-n-n
The p–n–p transistor is formed by sandwiching a thin ‘n’ type semiconductor between two ‘p’
type semiconductors.(Fig 2)
Base comes between collector and emitter region.
Base is thin and lightly doped.
Emitter and collector are much wider than base and are heavily doped.
The emitter is heavily doped than the collector.

Fig 2

Operation of p-n-p Transistor

Consider the fig 3. The Base Emitter junction is FORWARD BIASED collector base junction is
open circuited. The depletion region between base and emitter is reduced in width due to the
applied forward bias, resulting in a heavy flow of majority carriers from the p type emitter to the
n-type base.

SANJAY SINGH, H.C.S.T., MATHURA 1

 Note-
+ denotes Holes
- denotes electrons

Fig 3

Let us now remove the base-to-emitter bias of the p-n-p transistor and apply REVERSE BIAS
between Collector and base junction. (Fig 4)
Because of reverse bias, the depletion layer between Base and Collector junction increases in
width and only small current flows due to minority carriers.

Note-
+ denotes Holes
- denotes electrons

Fig 4

Now, the EBJ(Emitter Base junction) is Forward Biased and CBJ (Collector Base junction) is
reverse biased ,the resulting circuit is as shown in Fig 5.

A large number of majority carriers i.e, holes cross the emitter base junction and enters into
base. These injected majority carriers (holes) will appear as minority carriers in the n-type base.
A majority of these holes in base will be repelled by positive end of applied voltage between
base and collector into the p type collector. A small number of holes in base emitted by emitter
combine with small number of electrons present in base to form small base current.

Fig 5

SANJAY SINGH, H.C.S.T., MATHURA 2

Applying KCL, we obtain

IE = IB + IC

The collector current is composed of two components—the majority and minority carriers as
indicated in Fig 5. The minority-current component is called the leakage current and is given the
symbol ICO (Collector current with emitter terminal Open). The ICO is also called as ICBO i.e,
collector to base current with emitter open circuit.
The collector current is therefore defined as

IC= ICmajority + ICBOminority

IC is measured in milliamperes, while ICO is measured in microamperes or nanoamperes. ICO, like
(Is) for a reverse-biased diode, is temperature sensitive and must be examined carefully when
applications of wide temperature ranges are considered. It can severely affect the stability of a
system at hightemperature if not considered properly.

Operation of n-p-n Transistor

The working of n-p-n transistor is similar to p-n-p transistor discussed earlier. When the
EBJ(Emitter Base junction) is Forward Biased and CBJ (Collector Base junction) is reverse
biased ,the resulting circuit is as shown in Fig 6. A large number of majority carriers i.e,
electrons cross the emitter base junction and enters into base.
These injected majority carriers (electrons) will appear as minority carriers in the p-type base.
A majority of these electrons in base will be repelled by negative end of applied voltage between
base and collector into the n type collector. A small number of electrons in base emitted by
emitter combines with small number of holes present in base to form small base current.

Fig 6
Applying KCL ,we obtain
IE = IB + IC

Alpha (α)= IC/IE

Beta (β) = IC/IB

Lecture 2
Transistor Configurations
Depending on which terminal is made common to input and output port, there are 3 possible
configurations of the transistor.
1. Common Base Configuration

SANJAY SINGH, H.C.S.T., MATHURA 3

2. Common Emitter Configuration
3. Common Collector Configuration

Common Base Configuration

The common-base Configuration, the base is common to both the input and output sides of the
configuration. The fig 7 shows p-n-p and n-p-n transistor in Common base mode.

Fig 7

The Input Characteristics ( For n-p-n Common Base Transistor)

The input curve relate the input current (IE) to an input voltage (VBE) for various levels of output
voltage (VCB). They are similar to the v-I characteristics of forward biased p-n junction diode.

Fig 8 The Input Characteristics for CB configuration

The Output Characteristics ( For n-p-n Common Base Transistor)

The output characteristics relate the output current (IC) to an output voltage (VCB) for various
levels of input current (IE).

SANJAY SINGH, H.C.S.T., MATHURA 4

The figure 9 above, shows three regions
(a)Active Region- In active region the Emitter Base junction is Forward biased and Collector
base junction is Reverse Biased.
By biasing the transistor in this region, we can be operate it as Amplifier
(b)Saturation Region- In Saturation region the Emitter Base junction is forward biased and
Collector base junction is Forward Biased.
By biasing the transistor in this region, we can operate it as a Closed Switch.
(c)Cutoff Region- In Cutoff region the Emitter Base junction is Reverse biased and Collector
base junction is Reverse Biased.
By biasing the transistor in this region, we can be operate it as an Open Switch

Lecture 3
Common Emitter Configuration
The Emitter is a common terminal between input and output. Here, the input is applied at base
and output is taken at the collector.

Fig 10
The Input and Output characteristics (For Common Emitter n-p-n Transistor)
The input characteristics are a plot of the input current (IB) versus the input voltage (VBE) for a
range of values of output voltage (VCE) as shown in fig 11. The curve is similar to forward bias
diode.

Fig 11
For the common-emitter configuration the output characteristics are a plot of the output current
(IC) versus output voltage (VCE) for a range of values of input current (IB).
As shown in fig 12, for a fixed value of IB, as we increase the VCE, the collector current
increases.

SANJAY SINGH, H.C.S.T., MATHURA 5

 Fig 12 The output Characteristics of n-p-n Transistor in CE Configuration.

The figure 12 above, shows three regions

(a)Active Region- In active region the Emitter Base junction is Forward biased and Collector
base junction is Reverse Biased.
By biasing the transistor in this region, we can be operate it as Amplifier
(b)Saturation Region- In Saturation region the Emitter Base junction is forward biased and
Collector base junction is Forward Biased.
By biasing the transistor in this region, we can operate it as a Closed Switch.
(c)Cutoff Region- In Cutoff region the Emitter Base junction is Reverse biased and Collector
base junction is Reverse Biased.
By biasing the transistor in this region, we can be operate it as an Open Switch

Leakage currents and their relationship :

We know,
IC = ICmajority + ICO
IC= ICmajority + ICBO (Since, ICO is same as ICBO)
Replace ICmajority with α IE
We get

If IB =0 ,then IC is

Thus, we call

as ICEO i.e, Collector to Emitter Current with base open circuited.

SANJAY SINGH, H.C.S.T., MATHURA 6

Relation between α and β
We know

Relation between ICBO and ICEO

SANJAY SINGH, H.C.S.T., MATHURA 7