GTO
Appearance
A gate turn-off thyristor (GTO) is a special type of thyristor, which is a highpower semiconductor device. It acts as a bistable switch, conducting when the gate receives a current trigger, and continuing to conduct until the voltage across the device is reversed biased, or until the voltage is removed. GTOs, as opposed to normal thyristors, are fully controllable switches which can be turned on and off by their third lead, the gate lead.
For more information, see Wikipedia.