Silicon Diode for Alternating Current, SIDAC, is a multilayer silicon semiconductor switch. This component is triggered by voltage and can be operated as bidirectional swtch. Usually, this SIDAC is used in cheap high voltage power supply or ignition circuits. Figure below shows SIDAC block construction, schematic symbol and geometric construction :

SIDAC Basic Operation circuit schematic
The SIDAC has leakage current(Idrm) less than 5 µA during off state. When the SIDAC receive a supply voltage greater than SIDAC Vbo, the device will turn to a negative resistance switching mode with characteristic like an avalanche diode. The SIDAC will turn on when it supplied with enough current(Is), it allows high current to flow. The magnitude of the current flow affect the voltage drop when the voltage accros the SIDAC.

The SIDAC is still on as long as holding current is less than maximum value(150mA). The switching current (Is) is very near the holding current (Ih). A discharging small capacitor can generate current of 10A to 100A to primary or small, very high-voltage transformers for 10 µs to 20 µs when the SIDAC switches. [Circuit’s schematic diagram source: littlefuse.com]