A thyristor (/θaɪˈrɪstər/, from a combination of Greek language θύρα, meaning "door" or "valve", and transistor[1] ) is a solid-state semiconductor device which can be thought of as being a highly robust and switchable diode, allowing the passage of current in one direction but not the other, often under control of a gate electrode, that is used in high power applications like inverters and radar generators.

In a two-lead thyristor, conduction begins when the potential difference between the anode and cathode themselves is sufficiently large (breakdown voltage).

[3] Other sources define thyristors as more complex devices that incorporate at least four layers of alternating N-type and P-type substrate.

The thyristor is a four-layered, three-terminal semiconductor device, with each layer consisting of alternating N-type or P-type material, for example P-N-P-N.

The operation of a thyristor can be understood in terms of a pair of tightly coupled bipolar junction transistors, arranged to cause a self-latching action.

If a positive potential VG is applied at the gate terminal with respect to the cathode, the breakdown of the junction J2 occurs at a lower value of VAK.

A thyristor can be switched off if the external circuit causes the anode to become negatively biased (a method known as natural, or line, commutation).

Attempting to positively bias the anode within this time causes the thyristor to be self-triggered by the remaining charge carriers (holes and electrons) that have not yet recombined.

Such fast thyristors can be made by diffusing heavy metal ions such as gold or platinum which act as charge combination centers into the silicon.

Today, fast thyristors are more usually made by electron or proton irradiation of the silicon, or by ion implantation.

Irradiation is more versatile than heavy metal doping because it permits the dosage to be adjusted in fine steps, even at quite a late stage in the processing of the silicon.

Thyristor manufacturers generally specify a region of safe firing defining acceptable levels of voltage and current for a given operating temperature.

The boundary of this region is partly determined by the requirement that the maximum permissible gate power (PG), specified for a given trigger pulse duration, is not exceeded.

The first large-scale application of thyristors, with associated triggering diac, in consumer products related to stabilized power supplies within color television receivers in the early 1970s.

Thyristors have been used for decades as light dimmers in television, motion pictures, and theater, where they replaced inferior technologies such as autotransformers and rheostats.

[7] This is prevented by connecting a resistor-capacitor (RC) snubber circuit between the anode and cathode in order to limit the dV/dt (i.e., rate of voltage change over time).

In the realm of this and other very high-power applications,[2]: 12  both electrically triggered (ETT) and light-triggered (LTT) thyristors[8][9] are still the primary choice.

Because the TRIAC can conduct in both directions, reactive loads can cause it to fail to turn off during the zero-voltage instants of the AC power cycle.

In high-frequency applications, thyristors are poor candidates due to long switching times arising from bipolar conduction.

MOSFETs, on the other hand, have much faster switching capability because of their unipolar conduction (only majority carriers carry the current).