Transistor

What is a semiconductor?

doped semiconductor

N-type semiconductor Phosphor: charge carrier are the electrons

P-type semiconductor Bore: charge carrier are the holes

P-N junction: depleted of charge carriers

positive charge on the N side

negative charge on the P side

No charge carrier at the junction: it is called the depletion region.

P-N junction allows current to flow in one direction: from the N to P. On the other side (P to N), the depletion region will go wider and the junction will behave like an insulator.

diode

Forward bias: + connected to P and - connected to N: current will flow

Reverse bias: + connected to N and - connected to P: insulator (very small current until depletion zone breakdown)