A thyristor is a solid-state semiconductor device with four layers of alternating P- and N-type materials. It acts exclusively as a bistable switch, conducting when the gate receives a current trigger, and continuing to conduct until the voltage across the device is reversed biased, or until the voltage is removed (by some other means)

Thyristors have been around since the 1950s and the name thyristor is a combination of the words thyratron and transistor. These bistable devices are very useful to control power circuits and they are fairly cheap and easy to make.

If we had to categorize them, we would end up with four main types of thyristors

• Silicon controlled rectifiers (SCR)
• Gate turn off and Integrated Gate commutated thyristors (GTO - IGCT)
• MOS Controlled thyristors (MCT)
• Static induction thyristors (SIT)

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