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BIPOLAR JUNCTION TRANSISTOR (BJT)
Operation Theory
Engr.Usman Ali khan
CONTENTS
1.
BJT Formation
2.
BJT Terminals
3.
Biasing of BJT
4.
BJT Currents
5.
α of BJT
FORMATION OF BJT
P
N
Two Junctions
P
N
P
N
Two Junctions
N-Type material has excessive free electrons and P-Type material has
excessive holes
NPN and PNP BJTs have same characteristics. The only difference is in their
polarities of applied voltages. The polarities are reversed for the two types of
transistor.
BJT TERMINALS
N
P
N
Emitter
Collector
Base-Emitter Junction
Base-Collector Junction
Base
C
E
N
P
B
N
BJT CIRCUIT SYMBOLS
C
C
E
N
P
E
C
N
B
B
B
E
C
C
E
P
N
B
E
C
P
B
B
E
FORWARD BIASED JUNCTION OPERATION
Opposing field due to battery
E
Positive terminal
attracts electrons
-
+
-
+
-
+
Electrons Push
electrons to the
other side
N
P
+
-
Flow of electron current; but in semiconductor
electronics the flow of positive or conventional
current is considered
REVERSE BIASED JUNCTION OPERATION
Supporting field due to battery
Electrons Push
Minority Carrier
electrons to the
other side
E
-
+
-
+
-
+
N
P
-
+
Positive
terminal
attracts
Minority Carrier
electrons
Current
flows due to
minority
carriers
BJT BIASING
N
P
N
Emitter
Collector
-
+
Base
+
VEE
ON
VCC
OFF
-
+
-
+
BJT CURRENTS
When electrons come to
Base, they become
minority carriers
Negative
Terminal pushes
electrons to the
other side
N
P
Minority carriers are
attracted to the
positive terminal of
the battery
N
Emitter
Collector
-
+
Base
+
VEE
VCC
Some electrons
escape to the
ground
BJT CURRENTS
C
E
IE
IC (INJ)
N
P
N
IB
-
Conventional
Current
+
B
IE = IB + IC(INJ)
Electron Current
COLLECTOR-BASE REVERSE CURRENT
C
E
ICBO
N
When Collector-Base
Junction is reverse biased
and Emitter-Base Junction is
Open, a reverse current flows
in this reverse biased junction
which is called ICBO
P
N
Conventional
Current
B
Electron Current
Total Collector current is = IC = IC(INJ) + ICBO
Α OF BJT
α = IC(INJ)/IE
IC = IC(INJ) + ICBO
IC = IC(INJ)
α = IC/IE
ICBO<<IC(INJ)
HOME WORK
•
Example: 4.1
•
End Problems: 4.1  4.5