The thyristor's 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 pulse, and continue to conduct for as long as they're forward biased (that's, as long as the voltage across the device hasn't reversed). Some sources define silicon controlled rectifiers and thyristors as synonymous. Other sources define thyristors as larger set of devices with at least four layers of alternating N… (More on Thyristors)