A voltage or current applied to one pair of the transistor's terminals controls the current through another pair of terminals. Because the controlled (output) power can be higher than the controlling (input) power, a transistor can amplify a signal.
A transistor works when the electrons and the holes start moving across the two junctions between the n-type and p-type silicon. Let's connect the transistor up to some power.
Transistor, semiconductor device for amplifying, controlling, and generating electrical signals.