The thyristor is a solid-state semiconductor device with four layers of alternating N and P-type material. They act as a switch, conducting when their gate receives a current pulse, and continue to conduct for as long as they're forward biased (that is, as long as the voltage across the device hasn't reversed). Some sources define silicon controlled rectifiers and thyristors as synonymous; others define SCRs as a subset of thyristors, along with gate turn-off thyristor (GTO), triode ac switch (… (More on Thyristors)