BIPOLAR JUNCTION TRANSISTORS

Bipolar Junction Transistors
(BJTs)
❑ BJT is constructed with three doped
semiconductor regions separated by two pn
junctions
❑ The three regions are called emitter, base and
collector
Base Base


Collector Emitter Collector Emitter
n p n p n p
Base-collector Base-emitter
junction junction PNP transistor
NPN transistor
Department of EE 1

, Bipolar Junction Transistors
(BJTs)….
❑ The base region is lightly doped and very narrow
compared to the heavily doped emitter and
collector regions
❑ The term Bipolar refers to the use of both holes
and electrons as carriers in the transistor
❑ In order for the transistor to function, the base-
emitter and base-collector pn junctions have to be
correctly biased (i.e. Forward or Reverse Bias)


Department of EE 2

, NPN BJTs
❑ We shall use the NPN transistor for illustration

Emitter Collector
n p n
Base-Emitter Base Base-Collector
junction junction +
- (Reverse
(Forward
Biased) Biased) -
+

❑ The transistor pn junctions are forward-reverse
biased
Department of EE 3

, NPN BJTs….
❑ We shall use the NPN transistor for illustration

n p n
Emitter Collector
IE IC
e e
+
- Base-Emitter Base-Collector
junction e junction
(Forward (Reverse
IB
-
+ Biased) Biased)




Department of EE 4