Silicon Controlled Rectifier / Thrysistors: The thyristor is a solid-state semiconductor device similar to a diode, with an extra control terminal (gate). A thyristor is functionally different than a diode in that even if the device is forward biased, it will not conduct ("turn on") until the gate is pulsed. Rather than a continuous gate requirment as in BJTs and MOSFETs, thyristors are turned on by a pulsed signal. For SCRs, the control signal is a current pulse. GTO thyristors utilize a voltage pulse and LASCRs require a light pulse. Once turned on, the thyristor will remain on as long as the device is forward biased.

Thyristors are mainly used where high currents and voltages are involved, and are often used to control alternating currents, where the change of polarity of the current causes the device to automatically switch off; refered to as Zero Cross operation. The device can be said to operate synchronously as, once the device is open, it conducts current in phase with the voltage applied over its cathode to anode junction with no futher gate modulation being required to replicate; the device is biased fully on. This is not to be confused with symmetrical operation, as the output is unidirectional, flowing only from cathode to anode, and so is asymmetrical in nature. Thyristors can be used as the control elements for phase angle triggered controllers, also known as phase fired controllers.

Thyristors can also be found in power supplies for digital circuits, where they can be used as a sort of "circuit breaker" or "crowbar" to prevent a failure in the power supply from damaging downstream components. The thyristor is used in conjunction with a zener diode attached to its gate, and when the output voltage of the supply rises above the zener voltage, the thyristor opens, shorting the power supply output to ground (and in general blowing an upstream fuse)