BJT_1 - iitk.ac.in

EE210: Microelectronics-I
Lecture-7 : Bipolar Junction Transistor-1
B. Mazhari
Dept. of EE, IIT Kanpur
B. Mazhari, IITK
1G-Number
Invention of the Transistor
By late November 1947, Bardeen and Brattain managed to make a working transistor. It must be
said that it was very crude, but they improved it from late November through the first part of
December. By December 16, 1947, they had a working point-contact transistor. They were able to
gradually improve it and actually make a circuit to demonstrate to Bell Labs management on
Christmas Eve.
Eve Of course,
course this was a very big event.
event During the next six months at Bell Labs,
Labs
Bardeen and Brattain spent a lot of time making sure they had patents filed and then clearing it for
release to the public with the military. It was interesting that at one stage, the military was
threatening to classify this discovery as top secret. Fortunately, Bell Labs management worked
around that. By June 30, 1948, Bell Labs had a press conference in New York
City which was quite elaborate.
B. Mazhari, IITK
https://www.youtube.com/watch?v=kLBII5x43P0
2G-Number
3Qs
What is transistor action and how does it occur in a
BJT?
 Is BJT two back-to-back diodes connected together?
g
How does a BJT amplify?
B. Mazhari, IITK
3G-Number
Transistor
Trans-resistor
IO
+
+
VIN
Vo
-
-
Current IO is much more sensitive to VIN than VO
“Ideal Transistor”
I0
+
+
gm v i
Vi
V0
-
-
I0
I0
Vi3
Vi
Vi2
Vi1
V0
Voltage controlled Current Source (VCCS)
Transistors can be used for AMPLIFICATION
V0
VS
vS
+
vi
RL
v0
g m vi
RL
Vo
AV 
  g m  RL
Vs
By choosing sufficiently larger load resistance, voltage gain can be obtained
Bipolar Junction Transistor (BJT)
E
N
P
N
C
E
P
P
N
B
B
C
B
E
NPN
C
E: E
E
Emitter
itt
B: Base
C: Collector
B
E
PNP
E: Emitter
B: Base
C: Collector
C
More Realistic View
LE
E
WE
n+
P
N
P
WB
C
N DE ~ 1019 cm 3
N AB ~ 1017 cm 3
N DC ~ 1016 cm 3
N
-
B
+
W B ~ 2000 A
W E ~ 1000 A
LE ~ 1m
BJT is not symmetric: emitter and collector cannot be
simply interchanged
E
B
n+
P
N
N
P
-
+
C
Top View
N
E
P
P
-
B
C
N
Background
N
P
I  IN  IP
V
I N ; I P  exp(
p( )
VT
If doping in N region is much larger than doping in p region
then IN >> IP
Basic Transistor Operation
N+
P
P
N
C
E
B
B
I  IN  IP
We will assume that doping in emitter
is much more than base so that
electron
l
current iis much
h llarger than
h
hole current
I N  I P
In the reverse biased junction
current is small because there
are very few electrons in P
and holes in N-region
Basic Transistor Operation
E
I  IN  IP
N+
P
N+
P
P
P
B
N
N
C
IE
N+
P
N
IC
C
E
IB
IE  IN  IP
Current Gain :  
B
IC  I N
IB  IP
IC I N

 1
IB IP
N+
P
IC
N
C
E
5V
B
0.7V
Same operation as before
I C  I N  exp(
VBE
)
VT
T
Transistor
i t action
ti
Current is affected by base-emitter voltage and not by collector-base voltage
I0
+
Vi
-
B
+
g m vi
V0
-
IC
+
VBE
+
f(VBE)
E
C
VCE
-
Alternative representation
IB
IC
B
C
F.IB
IC

IB
I B  I BS


V BE
)  1
 exp(
VT




V BE
)  1
 exp(
VT


IC  IS
IB 
IC

E
F
Forward Active Mode
Transistor Characteristics
P
N
IB
N
IC
IC
IB3
IB2
IB1
VBE
VBE
VCB
Output Characteristics of the transistor
IC
IB3
IB2
IB1
VCB
IB4
IB3
IB2
IB1
VCE = VCB + VBE
= VBE - VBC
VCE  0.7  VBC

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