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1
PNP transistor
NPN transistor
Collector (n)
Collector (p)
Base (p)
Base (n)
Emitter(n)
Emitter (p)
Emitter is moderately wide and highly doped
Base is very narrow and lightly doped
Collector is very wide and moderately doped
2
How does a Transistor work?
Emitter
Base
Collector

Conventional current
Electron current
N
P
N
-
-
-
-

-

R2
IB
R1

IE
IC
Base emitter junction is forward biased
Base collector junction is reverse biased
3
View from energy band
Base
Emitter
Collector
P
From emitter source
N
To collector source
N
Conduction Band
Recombination path
Valence Band
4
Summary of transistor physics

• Magnitude of forward biased VBE controls the
  number of electrons entering the base emitter
  junction.

• Magnitude of reverse biased VCB has little
  control
• over the number of electrons entering the base
• collector junction.

• aIE/IC

• ßIC/IB

5
Question
Suppose that only 1 of the electrons injected
into The base recombine with base holes. If 1
million electrons enter the emitter in 1 µs, i)
how many electrons come out of the base lead in
this period? ii) how many come out of the
collector lead during this time? iii) what is
a? iv) what is ß?