A thyristor is a solid-state semiconductor device with four layers of alternating p-type and n-type materials used for high-power applications. It acts exclusively as a bistable switch (or a latch), conducting when the gate receives a current trigger, and continuing to conduct until the voltage across the device is reversed-bias, or until the voltage is removed (by some other means).
Thyristors (PNPN Devices)
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Triac
As with other thyristors, the triac ceases to conduct when the anode current drops below the specified value of the holding current \(I_H\). The only way to turn off the triac is to reduce the current to a sufficiently low level.
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Diac
A diac (diode for alternating current) is a two-terminal thyristor that is equivalent to back-to-back inverse four-layer diodes that can conduct current in either direction when activated. The right side of the stack can be regarded as a pnpn structure with the same characteristics as a four-layer diode, while the left side is an inverted four-layer diode having an npnp structure. The two terminals are both anodes (\(A_1\) and \(A_2\)).