The base of a transistor is lightly doped than the emitter and is made narrow so that virtually all the electrons injected from the emitter (in an npn transistor) diffuse right across the base to the collector junction without recombining with holes. That is, the base width is kept less than the recombination distance.

Why the base of a transistor is lightly doped?

The base region in a transistor is doped lightly so that the number density of majority carriers (electrons in p-n and holes in n-p-n transistor) is low. When emitter is forward biased, the majority carriers move from emitter to collector through base.

Why base is lightly doped and emitter is heavily doped?

In most transistors, emitter is heavily doped. Its job is to emit or inject electrons into the base. These bases are lightly doped and very thin, it passes most of the emitter-injected electrons on to the collector. ... The collector is so named because it collects electrons from base.