This BT169D Silicon Controlled Rectifier (SCR) or thyristor will conduct when voltage applied to gate. This continues even after this voltage is removed. Conduction stops only if supply voltage is removed. 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. Thyristors are synonymous to Silicon-Controlled Rectifier (SCR).