A transistor is a semiconductor containing a solid, non-moving part that passes a charge. A transistor can amplify and switch electronic signals and electrical power and is made of semiconductor material with at least three terminals that are used to connect to an external circuit. Transistors can amplify a signal because output power can be higher than input power. By applying a voltage or a current to one of the terminal pairs, there is a change in current through another pair of terminals.
There are several different kinds of transistors at Topdiode.

Small Signal Transistors