a thyristor that is switched to a high-conductivity state by the action of light. When a photothyristor is illuminated, charge-carrier pairs (electrons and holes) are generated in the semiconductor; the carrier pairs are separated by the electric field in the p-n junctions (see). As a result, photoelectric currents that serve as control currents begin to flow across the p-n junctions.

A photothyristor consists of a light-sensitive crystal with a p-n-p-n structure. The crystal, which is usually made of silicon, is placed on a copper base and enclosed by a hermetically sealed housing with a window that is transparent to light. The most widely used designs have an illuminated n-type emitter or an illuminated p-type base.

The sources of radiation that are suitable for controlling photothyristors include incandescent lamps, pulsed discharge lamps, light-emitting diodes, and lasers. The magnitude of the luminous flux required to switch a photothyristor to the high-conductivity state characterizes the sensitivity of the device and depends on the spectral composition of the radiation, the reflection and absorption coefficients of the single crystal, and the values of the electrical parameters of the device, such as the switching voltage and the risetime of the forward voltage.

Present-day photothyristors have currents of several milliamperes to 500 amperes and voltages of several tens of volts to 3 kilovolts. The power of the controlling optical radiation is of the order of 1–102 milliwatts for a wavelength of 0.9 micrometer.

Photothyristors are used in various automatic control and protection devices as well as in high-power high-voltage converters.