Question

why is base lightly doped, and collector moderately doped?

Answer 1

Base provides the proper junction for interconnection between emitter and collector.The Emitter is highly doped because all the free charges ( conduction electrons in Ntype and holes in P type) starts from Emitter. Base is lightly doped because we want that the base current should be small.Now the collector is moderately doped may be because we dont want a crowd of electrons in the collector otherwise the electrons coming from the Emitter-Base path may repelled and collector current may decrease.

Answer 2

The base lightly doped, and collector moderately doped because we want base current should be small.

=width of the base is very thin to

increase the majority carrier

concentration gradient in the base

region thereby enhancing the diffusion

current and also to reduce the number

of majority carriers lost due to

recombination in the base.