Transistors (abbreviated version of transfer resistor) are the fundamental building block in modern circuitry and are used either as switches or amplifiers of electronic signals and electrical power. It is composed of semiconductor material with at least three terminals for connection to an external circuit. A voltage or current applied to one pair of the transistor's terminals changes 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.
The terminals of a bipolar transistor are labeled as base, collector and emitter (whereas the terminals of a field-effect transistor are called gate, source, and drain). A small current at the base terminal can control or switch a much larger current between the collector and emitter terminals.