A silicon-controlled rectifier (SCR) is a unidirectional current control device like a Shockley diode. However, the SCR has a third terminal capable of supporting a digital gate connection, which adds another means of controlling the current flow. The SCR switches on when the forward bias voltage exceeds the forward-breakover voltage or when a current pulse is applied to the gate terminal.

The SCR is triggered into conduction by applying a gate-cathode voltage (VGK), which causes a specific level of gate current (IG). The gate current triggers the SCR into conduction. The device is returned to its nonconducting state by either anode current interruption or forced commutation. When the SCR is turned off, it stays in a non-conducting state until it receives another trigger.