A gate turn-off switch, also known as a gate-controlled switch (GCS) or gate turn-off thyristor (GTO), is similar to an SCR but can be turned off by a negative signal on the gate terminal. GTOs generally handle much lower currents than SCRs.

GTOs have many characteristics of SCRs and transistors, and in some ways are superior to both for power-switching applications. GTOs switch dc current without the auxiliary components that SCRs require, resulting in reduced cost and lower electrical and electromagnetic noise. Also, GTOs latch on or off with a single pulse.

Despite their advantages, GTOs are not as widely used as once seemed possible. Their low use is probably because peak current that could be reliably turned off in early GTOs is limited to relatively low values. The limit is imposed by current filaments that produce localized hot spots during turn-off.

Newer GTOs, however, turn off much higher current than previous models. Higher peak-controllable ratings are obtained with new shorted-anode structures, precise doping, and finely interdigitated geometry. The new GTOs, moreover, switch faster than previous versions, exhibit higher ratios of peak-to-average current, and greater on-state gain. Also, peak voltage ratings are higher than those for bipolars and Darlingtons.

GTOs generally cannot be turned off successfully when conducting current in the range between the maximum turn-off rating and the maximum surge rating. However, properly selected fuses may protect new devices from damage due to current in this range.

GTOs are similar to SCRs in that both are four-layer devices. However, the average current rating of GTOs, due to an interdigitated construction, is appreciably lower than that for SCRs of corresponding size.

Average current ratings for GTOs generally are quite close to those for Darlingtons of identical dimensions because similar interdigitation techniques are used for both. But GTOs generally are capable of turning off higher current because Darlingtons go out of saturation at high levels of current.

Silicon-controlled switch (SCS): SCSs are also called tetrode thyristors. They are similar to SCRs having two gate terminals: an anode gate and a cathode gate.

The two gates can be used in several ways. Putting a negative signal on the anode gate fires the SCS so that the device acts like a complementary SCR (conducting a positive current with a negative trigger). If the SCS is turned on with the cathode-gate signal like a conventional SCR, a sufficiently positive signal on the anode gate will subsequently turn it off. A positive signal applied to both gates simultaneously turns the SCS on faster than an equivalent SCR. The SCS can also be turned off like a gate-controlled switch by applying a negative signal on the cathode.

Bilateral triode (triac): Triacs can withstand high current and are often used to control ac power. They are similar to SCRs, but capable of conducting current in two directions. With a small forward voltage applied between the anode and cathode, a triac is normally off and conducts no current. If forward voltage increases sufficiently, the triac begins to conduct. A positive voltage applied to the gate terminal controls conduction threshold. The higher the gate signal, the lower the triac voltage when conduction occurs. Triacs operate similarly for current flowing in the opposite direction. In this case, gate-signal polarity must be reversed; the more negative the gate signal, the lower the conduction point.

Unijunction transistor (UJT): Unijunction transistors have three terminals called the emitter, base 1, and base 2. The emitter material is p-type semiconductor and the base material is n-type. In complementary UJTs, base material is p-type and emitter n-type.

A signal applied to the UJT emitter terminal controls resistance between the base terminals. With no emitter signal, resistance between base 1 and base 2 is high and very little current flows. As the emitter voltage increases, the resistance between the base terminals remains high until emitter voltage reaches a point called the peak voltage Vp. At this point, resistance between the base terminals starts to decrease as current flowing into the emitter terminal increases. Emitter voltage decreases as emitter current increases until a voltage valley point Vv. After the valley point, increasing emitter currents cause an increase in emitter voltage.

Unijunction transistors can be used as negative resistors when operated between Vp and Vv. This characteristic and a low firing current make them useful in oscillator and timing circuits, and in triggering SCRs.

Diacs: Diacs consist of ac trigger diodes and bilateral diodes. Each has two terminals and behaves the same for current in either direction. When a small voltage of either polarity is applied across the diac, little current flows. Diac resistance remains high until applied voltage reaches a breakdown voltage point. At this point, diac resistance drops and high current begins to flow. Since this action occurs in both directions of current flow, the device is sometimes called an ac switch. It is often used for triggering triacs.

Four-layer diode: Four-layer diodes are also called Shockley diodes and have two terminals. When a small forward voltage is applied across the four-layer diode, little current flows and the diode resistance is high. Resistance remains high until the forward voltage increases to a switch point Vs, then high current begins to flow and the voltage drops. Once the diode is forward conducting, it can be turned off by reducing the current or voltage below a holding value.

In the forward direction, current encounters two forward-biased junctions and one reverse-biased junction with a low breakdown voltage. For voltages applied in the opposite direction, the diode blocks current and behaves like a conventional reverse-biased diode. In this direction, current encounters one forward-biased junction and two reverse-biased junctions, each with large breakdown voltage.

MOS-controlled thyristor: The MOS-controlled thyristor (MCT) adds speed to power switching. It is similar to the gate turn-off (GTO) thyristor, but much less drive power is needed to turn off the MCT because little current flows into the gate. MCTs can handle 100 A at 1,000 V with turn off times under 2 \#181>sec and turn-on times of 200 nsec. In addition, they also have a low holding voltage in the on state.