A Shockley diode (four-layer diode) is a semiconductor device with a "pnpn" structure, equivalent to a thyristor with a disconnected gate. It is named after its inventor, William Shockley, co-inventor of the transistor and Nobel laureate.
The Shockley diode is in a high-impedance state at low voltages. When a switching voltage is reached, which is in the range of 10 V to 250 V depending on the device, it triggers like a thyristor and switches into a conductive state where the residual device voltage falls to some 1-2 V.
The resultant negative resistance characteristic can be used for trigger circuits, flipflops or relaxation oscillators. Unlike tunnel diodes, Shockley diodes have high trigger voltages and typically also high capacitance, and are therefore limited to low-frequency applications.
Shockley diodes were among the first commercially available silicon semiconductor devices in the late 1950s. They are no longer produced except for the similar but bidirectional Diac often found in dimmer circuits.