Silicon Controlled Switch

If we take the equivalent circuit for an SCR and add another external terminal, connected to the base of the top transistor and the collector of the bottom transistor, we have a device known as a silicon-controlled-switch, or SCS:

This extra terminal allows more control to be exerted over the device, particularly in the mode of forced commutation, where an external signal forces it to turn off while the main current through the device has not yet fallen below the holding current value. Consider the following circuit:

When the “on” pushbutton switch is actuated, there is a voltage applied between the cathode gate and the cathode, forward-biasing the lower transistor’s base-emitter junction, and turning it on. The top transistor of the SCS is ready to conduct, having been supplied with a current path from its emitter terminal (the SCS’s anode terminal) through resistor R2 to the positive side of the power supply. As in the case of the SCR, both transistors turn on and maintain each other in the “on” mode. When the lower transistor turns on, it conducts the motor’s load current, and the motor starts and runs.
The motor may be stopped by interrupting the power supply, as with an SCR, and this is called natural commutation. However, the SCS provides us with another means of turning off: forced commutation by shorting the anode terminal to the cathode. If this is done (by actuating the “off” pushbutton switch), the upper transistor within the SCS will lose its emitter current, thus halting current through the base of the lower transistor. When the lower transistor turns off, it breaks the circuit for base current through the top transistor (securing its “off” state), and the motor (making it stop). The SCS will remain in the off condition until such time that the “on” pushbutton switch is re-actuated.

Field-effect-controlled thyristors

Two relatively recent technologies designed to reduce the “driving” (gate trigger current) requirements of classic thyristor devices are the MOS-gated thyristor and the MOS Controlled Thyristor, or MCT.
The MOS-gated thyristor uses a MOSFET to initiate conduction through the upper (PNP) transistor of a normal thyristor structure, thus triggering the device. Since a MOSFET requires negligible current to “drive” (cause it to saturate), this makes the thyristor as a whole very easy to trigger:

Given the fact that ordinary SCRs are quite easy to “drive” as it is, the practical advantage of using an even more sensitive device (a MOSFET) to initiate triggering is debatable. Also, placing a MOSFET at the gate input of the thyristor now makes it impossible to turn it off by a reverse-triggering signal. Only low-current dropout can make this device stop conducting after it has been latched.
A device of arguably greater value would be a fully-controllable thyristor, whereby a small gate signal could both trigger the thyristor and force it to turn off. Such a device does exist, and it is called the MOS Controlled Thyristor, or MCT. It uses a pair of MOSFETs connected to a common gate terminal, one to trigger the thyristor and the other to “untrigger” it:

A positive gate voltage (with respect to the cathode) turns on the upper (N-channel) MOSFET, allowing base current through the upper (PNP) transistor, which latches the transistor pair in an “on” state. Once both transistors are fully latched, there will be little voltage dropped between anode and cathode, and the thyristor will remain latched so long as the controlled current exceeds the minimum (holding) current value. However, if a negative gate voltage is applied (with respect to the anode, which is at nearly the same voltage as the cathode in the latched state), the lower MOSFET will turn on and “short” between the lower (NPN) transistor’s base and emitter terminals, thus forcing it into cutoff. Once the NPN transistor cuts off, the PNP transistor will drop out of conduction, and the whole thyristor turns off. Gate voltage has full control over conduction through the MCT: to turn it on and to turn it off.
This device is still a thyristor, though. If there is zero voltage applied between gate and cathode, neither MOSFET will turn on. Consequently, the bipolar transistor pair will remain in whatever state it was last in (hysteresis). So, a brief positive pulse to the gate turns the MCT on, a brief negative pulse forces it off, and no applied gate voltage lets it remain in whatever state it is already in. In essence, the MCT is a latching version of the IGBT (Insulated Gate Bipolar Transistor).