When breakdown occurs, internal positive feedback (impact ionization or two transistor feedback) ensures that the diode enters a region of negative dynamic resistance, leading to a sharp increase in current through the diode and a decrease in the voltage drop across it (typically full switch-on takes a few hundred nanoseconds to microseconds).

Its operation is similar to that of the DIAC, but a SIDAC is always a five-layer device with low-voltage drop in latched conducting state, more like a voltage triggered TRIAC without a gate.

The operation of the SIDAC is functionally similar to that of a spark gap, but is unable to reach its higher temperature ratings.

In many applications this function is now served by metal oxide varistors (MOVs), particularly for trapping voltage transients on the power mains.

They may also be used to sense over-voltage fault conditions to provide a 'crowbar' function to operate a fuse or a latching alarm that can only be reset by removing the supply.