A diac is a  two-electrode, three-layer bidirectional avalanche diode that can be switched from the off state to the on state for either polarity of applied voltage. Fig. 18 shows the junction diagram and schematic symbol for a diac; Fig. 19 shows the voltage-current characteristic.

This three-layer trigger diode is similar in construction to a bipolar transistor, but differs from it in that the doping concentrations at the two junctions are approximately the same and there is no contact made to the base layer. The equal doping levels result in a symmetrical  bidirectional  switching characteristic, as shown in Fig. 19. When. an  increasing positive or negative voltage is applied across the terminals of the diac, a minimum (leakage) current I _(BO)flows through the device until the voltage reaches the breakover point V(BO). The reverse-biased junction then undergoes avalanche breakdown and, beyond this point, the device exhibits a negative-resistance characteristic, i.e., current through the device increases substantially with decreasing  voltage.

Diacs are primarily used as triggering devices in thyristor phase-control circuits used for light dimming, universal motor-speed control, heat control, and similar applications. Fig. 20 shows the  general circuit diagram for a diac/triac phase-control circuit.

On to Field Effect Transistors

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