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The Shockley diode (named after physicist William Shockley) is a four-layer semiconductor diode, which was one of the first semiconductor devices invented. It is a PNPN diode with alternating layers of P-type and N-type material. It is equivalent to a thyristor with a disconnected gate. Shockley diodes were manufactured and marketed by Shockley Semiconductor Laboratory in the late 1950s. The Shockley diode has a negative resistance characteristic.[1] It was largely superseded by the diac.
The Shockley diode remains in an OFF state, with a very high resistance, until a voltage greater than the trigger voltage is applied across its terminals. When the voltage exceeds the trigger value, the resistance drops to an extremely low value and the device switches ON. The constituent transistors help in maintaining the ON and OFF states. Since the construction resembles a pair of interconnected bipolar transistors, one PNP and other NPN, neither transistor can turn ON until the other is turned ON due to the absence of any current through the base-emitter junction.
Common applications:
Trigger switch for silicon controlled rectifier
Relaxation oscillator / sawtooth oscillator
Niche applications:
Audio amplifier
Small-signal Shockley diodes are no longer manufactured, but the unidirectional thyristor breakover diode, also known as the dynistor, is a functionally equivalent power device. An early publication about dynistors was published in 1958.[5] In 1988 the first dynistor using silicon carbide was made.[6] Dynistors can be used as switches in micro- and nanosecond power pulse generators.
https://en.wikipedia.org/wiki/Shockley_diode