In a transistor base is made thin and doped with little impurity atoms. Why?

Explanation

The base region in a transistor is made very thin so that there is a better conduction of majority carriers from emitter to collector through base. Due to it, the base current is quite weak, the collector current is nealy equal to the emitter current. As result of it, the transistor can give good power gain and voltage gain.
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. Since base is thin and lightly doped, only a small (about5%) electron-hole combination will take place giving weak base current and remaining majority carriers will be colleced by collector giving collector current nearly equal to emitter current.