The Shockley diode (named after physicist William Shockley) is a four layer semiconductor diode which was one of the first semiconductor devices invented. It was a "pnpn" diode. It is equivalent to a thyristor with a disconnected gate.
Small signal Shockley diodes are no longer manufactured, but the unidirectional thyristor breakover diode, also known as the dynistor, is a functionaly equivalent power device.