MOS-Controlled thyristors—A new class of power devices

Abstract

A new class of power devices is described that is based on an optimal combination of MOS and thyristor elements. Devices of this class function in the ON-state and OFF-state in a manner indistinguishable from a thyristor yet can switch from on-to-off or off-to-on by applying a voltage to its MOS gate. Thus, the devices exhibit extremely low forward drop, high surge current capability, and enjoy negative thermal feedback. To turn off the device, one activates the gate so that FET's are turned on to effectively short one of the emitting junctions of the thyristor. These FET's need only block a maximum of about 1 V when off and carry a sizable current for about 1 µs when on. To turn on the device, any of the normal methods may be employed. However, it is most convenient to use the same MOS gate electrode (and polysilicon layer) and a voltage of the opposite polarity to turn on the thyristor with another FET-just as if it were a normal MOS gated thyristor. The current density that can be turned off depends on the density and effective resistance of the turn-off FET's while turn-on speed and di/ dt rating depend on the initial turn-on area, which in turn depends on the density of the ON-FET's. If the OFF-gate voltage is maintained during the desired OFF-state period, the device has, effectively, an infinite dv/dt capability. Switching speed is most similar to, but somewhat faster than, that of gate turn-off thyristors (GTO's) and, as in other bipolar devices, depends chiefly on carrier recombination time, device thickness, and turn-offdi/dt.