Download - Bipolar Junction Transistor (BJT)
Operation Theory
BIPOLAR JUNCTION TRANSISTOR (BJT)
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 P
Two Junctions
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
N NPE C
B
Base-Emitter Junction Base-Collector Junction
BJT CIRCUIT SYMBOLS
N NPE C
B B
CE
C
E
B
P PNE C
B B
CE
C
E
B
FORWARD BIASED JUNCTION OPERATION
+
+
+
-
-
-
E
NP
+ -
Opposing field due to battery
Electrons Push electrons to the other side
Positive terminal attracts electrons
Flow of electron current; but in semiconductor electronics the flow of positive or conventional current is considered
REVERSE BIASED JUNCTION OPERATION
+
+
+
-
-
-
E
NP
+-
Supporting field due to battery Electrons Push Minority Carrier electrons to the other side
Positive terminal attracts Minority Carrier electrons
Current flows due to minority carriers
BJT BIASING
N P N
Emitter Collector
Base- +
- +
VEE VCC
- +
+-
ON
OFF
BJT CURRENTS
N P N
Emitter Collector
Base- +
- +
VEE VCC
Negative Terminal pushes electrons to the other side
When electrons come to Base, they become minority carriers
Minority carriers are attracted to the positive terminal of the battery
Some electrons escape to the ground
BJT CURRENTS
N NP
EC
B
Conventional Current
Electron Current
-+
IB
IE IC (INJ)
IE = IB + IC(INJ)
COLLECTOR-BASE REVERSE CURRENT
N NP
EC
B
ICBO
Conventional Current
Electron Current
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
Total Collector current is = IC = IC(INJ) + ICBO
Α OF BJT
α = IC(INJ)/IE
IC = IC(INJ) + ICBO ICBO<<IC(INJ)
IC = IC(INJ)
α = IC/IE
HOME WORK• Example: 4.1
• End Problems: 4.1 4.5