Thyristor - From Wikipedia,
A thyristor is a solid-state semiconductor device with four layers of alternating N and P-type material. They act as bistable switches, conducting when their gate receives a current trigger, and continue to conduct while they are forward biased (that is, while the voltage across the device is not reversed).
Some sources define silicon controlled rectifiers and thyristors as synonymous. Other sources define thyristors as a larger set of devices with at least four layers of alternating N and P-type material.
The first thyristor devices were released commercially in 1956. Because thyristors can control a relatively large amount of power and voltage with a small device, they find wide application in control of electric power, ranging from light dimmers and electric motor speed control to high-voltage direct current power transmission. Originally thyristors relied only on current reversal to turn them off, making them difficult to apply for direct current; newer device types can be turned on and off through the control gate signal. A thyristor is not a proportional control like a transistor but is only ever fully on or fully off, making them unsuitable for analog amplifiers.