The bipolar junction transistor is constructed with three doped semiconductor regions separated by two pn junctions.The three regions are emitter,base and collector.There are two types of bipolar junction transistors.
One type consists of two n regions separated by p region called npn.
Second type consists of two p regions separated by n region called pnp.
fig.1 ( npn )
fig.2 ( pnp )
The pn junction joining the base and emitter regions called base-emitter junction.Also the pn junction joining the base collector regions called base-collector junction.The three leads are connected to these regions and named as E, B and C for emitter ,base and collector respectively.
The base region is lightly doped and thin,emitter is heavily doped and collector is moderaftely doped.
The bipolar term is for both holes and electrons as carriers in transistor.
To operate the transistor properly as an amplifier, The two pn juctions must be correctly biased with proper dc voltage.For npn and pnp both the BE base-emitter junction is forward biased and BC base-collector junction is reverse biased known as forward-reverse bias.
Let's examine what happen when BE and BC junctions in npn transistor are forward-reverse biased.The forward bias for BE junction narrows the BE depletion region and reverse bias for BC junction widens the BC depletion region.