Question

Why is the Base very thin in a transistor?

Answer 1

The base of a transistor is kept very thin compared to the collector and emitter so that most of the carriers (carriers means electrons in case of npn transistors and holes in case of pnp transistors) pass to the collector injected from the emitter. The transistor action needs that a small base current control a large current flowing from emitter to collector thus base is made to absorb a very small number of such carriers to constitute the base current.