Bidirectional p-n-p-n switches

Summary

The p-n-p-n bidirectional switches conduct current in the positive or negative direction and are, therefore, useful in ac applications. The devices that belong to this class of switches are the p-n-p-n diode switch and triac (acronym for triode, ac switch). A triac can be triggered by either the positive or negative pulses applied to the gate which acts, depending on the switching mode, as a conventional gate, a junction gate, or as a remote gate. There are four possible triggering modes of a triac. Two in the first quadrant of the I–V characteristics and two in the third quadrant of the I–V characteristics. Since each of these switching modes depends on a different physical basis, the triggering current is different for each mode. Table 11.1 summarizes the state of various emitter and base layers and junctions of a triac during the turn-on transient.