A semiconductor component that is unidirectional like a diode, but is switched like a transistor. It is widely used to control high currents and voltages in motors, lighting, heating and power applications. Once turned on at the gate, it allows current to flow until it falls below a certain threshold. In an AC circuit, it stops conducting each time the wave crosses over zero and can rectify AC to DC. See diode, PN junction and n-type silicon.