BJT(Bipolar Junction Transistor)

Bipolar :- 

Conduction due to both holes and electron.

Diode is also a bipolar device. 

BJT is a bi-junction transistor i.e. it has two PN Junction inside.

BJT has three terminals:

   a) Emitter

   b) Base 

   c) Collector

BJT Diagram

• Emitter is most heavily doped because it is the source of all charge carries (NE > 10^18)
• Base is moderately doped but it is made thin so that charges  carriers entering from emitter are able to reach collector and very few charges recombined in base.
• Usually collector base junction is kept reverse biased therefor depletion layer exist.
• Due to less doping of collector region compare to base depletion layer extend more into collector.
• If (doping base) NB < NC then depletion layer extends more into base than collector and can completely depletion base due to less width called as punch through.
• But collector must also  be heavily doped to increase conductivity  so a small heavily doped region exists near collector terminal.
• collector region has highest area to radiate the heat generated in reverse biased CB junction.