thyristors

Thyristor is a family of silicon-based four-layer PNPN semiconductor devices that function as bistable switches. The canonical member is the silicon-controlled rectifier (SCR). When forward biased and enough gate current is applied, the device switches from blocking to conducting and remains on (latched) until the current drops below a holding level.

In normal operation, the anode-cathode path is reverse-blocking until forward biased; a small gate pulse injects carriers at the emitter-gate region, initiating regenerative feedback between two internal minority-carrier transistors. This latches the device on, allowing large anode current to flow. To turn off, the current must fall below the holding current or be forcibly commuted.

Variants include the silicon-controlled rectifier (SCR), which conducts in one direction; the TRIAC, which conducts in both directions for AC control; and the gate-turn-off thyristor (GTO), which can be turned off by a gate signal. Modern power devices include equivalents and alternatives such as the IGBT, but thyristor-based devices remain common in high-power applications.

Key characteristics include high blocking voltages and high current handling, latch behavior, and sensitivity to dv/dt and temperature. They require protective measures such as snubbers, proper gate drive, and controlled commutation to prevent unintended triggering or turn-off.

Applications span controlled rectifiers, AC motor speed control, DC power supplies, HVDC links, and induction heating. Thyristors dominate high-power, high-voltage switching where efficient, rugged performance is valued, although newer devices such as MOS-controlled devices and IGBTs increasingly complement or replace them in some roles.