The Silicon Unilateral Switch (SUS), also known as a four layer diode and as a Schokley diode, can be treated as a low-current SCR without a gate terminal. Figure 19-30 shows the Silicon Unilateral Switch circuit symbol and typical forward characteristics. The device triggers into conduction when a forward switching voltage (VS) is applied. At this point a minimum switching current (IS) must flow. Also, the voltage falls to a forward conduction voltage (VF) at switch on, and conduction continues until the current level falls below the holding current (IH). The 2N4988 SUS has VS ranging from 7.5 V to 9 V, Is = 150 μA, and IH = 0.5 mA. Silicon Unilateral Switch reverse characteristics are similar to SCR reverse characteristics; a very small reverse current flows until the reverse breakdown voltage is reached.
When an SCR is triggered into conduction by application of a gate current, the gate looses control and the device continues to conduct until the forward current falls below the holding current. A gate turn-off (GTO) device is essentially an SCR designed to be switched on and off by an applied gate signal The circuit symbol for a GTO is shown in Fig. 19-36(a), and the two-transistor equivalent circuit for the device is illustrated in Fig. 19-36(b) and (c). Note that at switch-on, the gate current has just got to be large enough to supply base current to transistor Q2. However, at switch-off, the Q1 collector current has to be diverted through the gate terminal in order to turn Q2 off. Consequently, for device turn-off relative large levels of gate current are involved; approaching half the GTO forward current.
The SIDAC is a two-terminal thyristor designed mainly for use in over-voltage protection situations. As a bilateral device with no gate terminal, it simply breaks down to its forward voltage drop when the applied terminal voltage (of either polarity) rises to the breakover voltage level. Like other thyristors, there is a minimum current that must flow to latch the SIDAC into an on state. Also, when switched on conduction continues until the current falls below a holding current level.
The circuit symbol and typical characteristics for a SIDAC are shown in Fig. 19-37. Available devices have breakover voltages ranging from 110 V to 280 V. Typically on state voltages is 1.1 V, rms current is 1 A, and holding current is 100 mA.
Figure 19-38 shows a SIDAC used to protect a dc power supply from ac line transients. Normally, the SIDAC will behave as an open-circuit. A voltage transient on the ac line will cause it to break down to its forward voltage level, so that it essentially short-circuits the transformer output. This will cause a fuse to blow or a circuit breaker to trip, thus interrupting the ac supply.