A diac is a form of solid-state switch used to switch AC voltage; it belongs to the class of switches known as thyristers. It is like a junction transistor without a base lead (it is a two-lead device) and accomplishes its switching action by breakdown at a certain voltage. There are also four layer devices with a similar mode of operation known as four-layer diodes.

The triode AC switch (TRIAC) is a power-switching device as is the SCR. The TRIAC conducts currents in both directions while the SCR allows current in only one direction. A common application is for lighting controllers. In response to a trigger, the triac conducts until the AC voltage applied reaches zero, then blocks flow until the next trigger occurs. Since a trigger can cause it to trigger current in either direction, it is an efficient power controller from essentially zero to full power.

The SCR is a power-switching device commonly used for lighting control, motor speed control and other variable power applications.