Various circuits and combinations of radiant energy responsive transducer means such as photovoltaic diodes connected to the gate of a depletion mode FET whose source and drain are connected to the gate and cathode of a thyristor, are disclosed to provide a semiconductor switch which is triggered into conduction solely by a small amount of radiant energy, without the need for a second triggering energy source, and which also affords immunity to unwanted dv/dt and temperature induced turn-on. Various modes of operation are disclosed, including the thyristor self-triggering into conduction, and/or being of the light-activated type itself and being directly triggered by impinging light, and/or being triggered by the light-responsive diode bias, and/or being triggered by a small bias supplied from a second set of photovoltaic diodes connected to the thyristor gate. Other combinations are disclosed providing zero-cross firing.