Ii bipolar junction transistor introduction

Base width modulation:-
As the applied base-collector voltage (VBC) varies, the base-collector depletion 
region varies in size. This variation causes the gain of the device to change, since 
the gain is related to the width of the effective base region. This effect is often called 
the "Early Effect" 
An NPN bipolar transistor can be considered as two diodes connected anode 
to anode. In normal operation, the emitter-base junction is forward biased and 
the base-collector junction is reverse biased. In an npn-type transistor for 
example, electrons from the emitter wander (or "diffuse") into the base 
• Base width has been assumed to be constant
• When bias voltages change, depletion widths change and the effective 
base width will be a function of the bias voltages
• Most of the effect comes from the C-B junction since the bias on the 
collector is usually larger than that on the E-B junction
• Base width gets smaller as applied voltages get larger

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Early Effect: Common Base Input Characteristic
I 
E

 I 
F
 
0
 
(
e
qV
EB
 
/
 
kT


1)
 


R
 I 
R
 
0
 
(
e
qV
CB
 
/
 
kT


1)
Ebers-Moll
Assuming 
–V
CB
> few kT/q and W/L
B
<< 
1


 W 



cosh




D
E
D
B



D
B

L
B 

I
F 
0

 qA
n
E 
0

p
B
0


 qA
p
B
0

L
E
L
B

 W 

W


sinh








L
B 


I 
E


 I 
F
 
0
e
qV
EB
 
/
 
kT


 qA 
D
W
B
p
B
0
e
qV
EB
 
/
 
kT
•
Exponential prefactor will increase as V
CB
increases (W decreases) 

---

Early Effect: Common Emitter Output Characteristic
I
C

 

dc 
I
 B

 
I
CE 
0

dc

1
1 

 W 

2
D W N
B
E



D
B
L
E
N
 E
2 



L
B 

W
eff
W
CB

W
 

W
EB 
Base
 

W
CB
Base
 

W
 

W
CB
Base

2
K
S

0

N 
A
 

 N
D


1
2



V

V

q
N
 D 
N
 A
bi
CB




N
C

W

 x

 W


CB
Base
n
Base 



N
C

 
N
B 

• If N
C
<< N
B
most of the depletion is in the collector and modulation of 
base width is minimized 
– reduced Early Effect
Avalanche Multiplication Breakdown
• 
Common Base
: Similar to single p-n junction V
CB0
 
?
 
V
BD
(B-C)
 
• 
Common Emitter
: more complicated
 
1. holes injected by FB emitter to base
2. holes generate e-p pairs in C-B depletion
3. e
-
drift back into base
4. e
-
injected to emitter

---

5. more holes into base…..
Transistor breakdown
Transistor breakdown mechanism:
Avalanche breakdown
: avalanche multiplication mechanism takes place
 
at 
CBJ or EBJ
Base punch-through effect
: the base width reduces to zero at high CBJ
 
reverse bias
In CB configuration, 
BV
CBO
is defined at 
i
E
= 0.
The breakdown voltage is smaller than 
BV
CBO
for 
i
E
> 0. 
In CE configuration, 
BV
CEO
is defined at 
i
B
=0.
The breakdown voltage is smaller than 
BV
CEO
for 
i
B
> 0. 
Typically, 
BV
CEO
is about half of 
BV
CBO
.
Breakdown of the BCJ is not destructive as long as the power dissipation is 
kept within safe limits
Breakdown of the EBJ is destructive because it will cause permanent 
degradation of Beta

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Transistor Biasing
The basic function of transistor is amplification. The process of raising the strength of 
weak signal without any change in its general shape is referred as faithful 
amplification. For faithful amplification it is essential that:- 
1. Emitter-Base junction is forward biased
2. Collector- Base junction is reversed biased
3. Proper zero signal collector current
The proper flow of zero signal collector current and the maintenance of 
proper collector emitter voltage during the passage of signal is called 
transistor biasing.
WHY BIASING?
If the transistor is not biased properly, it would work inefficiently and 
produce distortion in output signal. 
HOW A TRANSISTOR CAN BE BIASED?
A transistor is biased either with the help of battery or associating a circuit 
with the transistor. The later method is more efficient and is frequently used. The 
circuit used for transistor biasing is called the biasing circuit. 
BIAS STABILITY
Through proper biasing, a desired quiescent operating point of the transistor 
amplifier in the active region (linear region) of the characteristics is obtained. It 
is desired that once selected the operating point should remain stable. The 
maintenance of operating point stable is called Stabilisation. 
The selection of a proper quiescent point generally depends on the following 
factors: 
(a) The amplitude of the signal to be handled by the amplifier and 
distortion level in signal
(b) The load to which the amplifier is to work for a corresponding 
supply voltage
(c) The operating point of a transistor amplifier shifts mainly with changes 
in temperature, since the transistor parameters 
— 
β, I
CO
and V
BE
(where the symbols carry their usual meaning)
—are functions of 
temperature.

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