CBSE Class 12-science Answered

NPN transistor has a layer of P-doped semiconductor (the "base") sandwiched between two N-doped layers. 

 

The emitter-base circuit of the transistor is always forward biased while the collector -base circuit is always reversed biased.

 

When the emitter-base circuit is forward biased with certain voltage, electrons in the emitter section are repelled and are injected into base. Since p-region (base) is not highly doped and is very thin, most of the electrons diffuse through it and reach the collector section. About 5% of the electrons are lost as charge carriers because of combination with holes in p-region. Remaining electrons are are rapidly swept over to the the collector region by the collector voltage. For each electron flowing out of the collector and entering the positive terminal of the battery, an electron enters the emitter from the negative terminal which again moves towards the base and the process is repeated.