A thyristor is a four-layer semiconductor that is often used for handling large amounts of power. While a thyristor can be turned on or off, it can also regulate power using something called phase angle control. This allows the amount of power output to be controlled by adjusting the angle of the current input. An example of this is the a dimmer switch for a light.
While thyristors have the advantage of using phase angle control and handling large amounts of power, they are not as suitable for low power applications. This is because they can only be turned off by switching the direction of the current. For this reason, a thyristor may take longer to turn on or off that other semiconductors. Also, thyristors can only conduct in one direction, making them impractical for applications that require current to be conducted to and from each device.