The thyristor (also called a silicon-controlled rectifier or SCR) is a solid-state semiconductor device similar to a diode, with an extra terminal which is used to turn it on. Once turned on, the thyristor will remain on (conducting) as long as there is a significant current flowing through it. If the current falls to zero, the device switches off.

The thyristor is a four-layer semiconducting device, with each layer consisting of an alternately N or P-type material, for example N-P-N-P. The main terminals, labelled anode and cathode, are across the full four layers, and the control terminal, called the gate, is attached to one of the middle layers. The operation of a thyristor can be understood in terms of a pair of tightly coupled transistors, arranged to cause the self-latching action.

Thyristors are mainly used where high currents and voltages are involved, and are often used to control alternating currents, where the change of sign of the current causes the device to automatically switch off. This is known as synchronous operation or Zero Cross operation. This principle is used to control the desired loading by adjusting the frequency of the sinusoidal input. The range of frequencies is great because there is no limit to the number of cycles a thyristor can perform, and exhibits no "wear out" modes. This is a frequency domain method of control.

With phase angle control a thyristor is turned on at a specific and adjustable portion of the cycle of the controlling sinusoidal input. Moving the point at which the thyristor is turned on regulates power output. An example of this method of control is a dimmer switch for lights. The turn on point of a thyristor is controlled to occur at a particular point on the sine curve of the AC supply. The thyristor stays on for the remainder of that cycle and the longer the thyristor stays on, the brighter the light. Fine resolution of output is possible with this method and is suitable for fast responding loads such as tungsten filament lamps or temperature variable resistance loads. Phase-angle control is also essential for inductive loads.

The drawback of a thyristor is that, like a diode, it only conducts in one direction. A similar self-latching 5-layer device, called a triac, is able to work in both directions.

An earlier gas filled tube device called a Thyratron provided a similar electronic switching capability, where a small control voltage could switch a large current.

Modern thyristors can switch large amounts of power (up to megawatts). In the realm of very high power applications, they are still the primary choice. However, in low and medium power (from few tens of watts to few tens of killowatts) they have almost been replaced by other devices with superior switching characteristics like MOSFETs or IGBT s. One major problem of thyristor is that is it not a fully controllable switch in the sense that triggering current direction need to be reversed to switch it off. GTO (Gate Turn-off Thyristor) is another related device which addresses this problem. In high-frequency applications, thyristors are poor candidates due to large switching times arising out of bipolar conduction. MOSFETs, on the other hand, has much faster switching capability because of its unipolar conduction (only majority carriers carry the current).