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 N N Conduction Band
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.
α=IE/IC
β=IC/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 α?
iv) what is β?