How It work semiconductor

Many materials, such as most metals, allow electrical current to flow through them.

These are known as conductors.‡‡Materials that do not allow electrical current to

flow through them are called insulators. ‡‡ Pure silicon, the base material of most transistors, is considered a semiconductor because its conductivity can be modulated by the introduction of impurities. 

Here you can find some semiconductor devices interactive animations.It shows how it work.

Diode

N-channel enhancement MOSFET

N-channel JFET