tyristor

A thyristor is a family of silicon-based semiconductor devices that function as high-power switches. It is a four-layer PNPN structure with three junctions and three terminals: anode, cathode, and gate. In forward-blocking mode, the anode-cathode path is off. A short gate current pulse injects carriers into the device, triggering a regenerative current that switches the device into a conducting state. Once on, the thyristor latches on until the current falls below a holding level or the supply is interrupted.

The most common member is the silicon controlled rectifier (SCR), which conducts current from anode to cathode when forward biased and is not turned off by the gate; it turns off only when the current falls below the holding value. Other forms include gate-turn-off thyristors (GTOs), which can be turned off by gate control, and bidirectional devices such as triacs—essentially two thyristors in inverse parallel. Light-triggered thyristors and protective variants provide triggering by optical or other mechanisms.

Thyristors are used in power electronics for controlled rectification and AC power control. Applications include phase-angle control of AC motors and heaters, rectifier bridges in switch-mode power supplies and HVDC links, and crowbar circuits for overvoltage protection. They offer high blocking voltages and currents, rugged operation, and relatively simple triggering compared with older vacuum-tube devices, contributing to widespread use in industrial and consumer equipment since the mid-20th century.