A transistor is a semiconductor device used to amplify and switch electronic signals and electrical power.It is composed of semiconductor material with at least three terminals for connection to an external circuit.ollector current is approximately β (common-emitter current gain) times the base current. It is typically greater than 100 for small-signal transistors but can be smaller in transistors designed for high-power applications. Unlike the field-effect transistor (see below), the BJT is a low–input-impedance device.