The term ‘thyristor’ refers to a group of semiconductor devices that have a very rapid switch-on characteristic. They are all basically four-layer devices, being made up as a p n p n sandwich, and their ‘snap-on’ switching action is referred to as ‘p n p n regenerative feedback’. They can have two, three or four terminals, depending on the type of device. Some devices pass current only one way (unidirectional), while others pass current both ways (bidirectional).
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