- An active semiconductor device which can serve as a switch, amplifier, or oscillator, among other possible functions. The two main classes are bipolar junction transistors, and field effect transistors. Each of these has three regions or electrodes, and charge carriers flow between two of these terminals, while a third controls this flow. Transistors may be used as discrete devices or in components such as ICs, and can be found in countless items, such as computers, robots, TVs, cellular telephones, wireless transmitters, audio amplifiers, cars, and so on. A transistor may have more than three electrodes, as is the case with a tetrode junction transistor. Also called transistor.