Abstract: A power bipolar transistor has a MOSFET located within the base region, the MOSFET shorting the transistor emitter and base regions together when the bipolar transistor is in an off condition with a large base to collector voltage. The bipolar transistor is provided with floating guard rings, and the gate of the MOSFET is connected to one of these floating guard rings, so that when the depletion region from the base to collector junction reaches this floating guard ring, the MOSFET gate receives a voltage to turn on the MOSFET and provide the emitter to base short.

Abstract: The present invention provides a method for triggering a light triggered thyristor (LT.sup.2) which method comprises using a light source to provide background excitation of carriers in at least the amplifying gate areas of the LT.sup.2 prior to triggering the device. The background excitation is of a low level and will not trigger the device or cause excessive leakage current. At the desired time a second light signal is used to trigger the LT.sup.2.