MOS Controlled thyristors (MCT´s)

The feasibility of a new class of power devices which is based on an optimal combination of MOS and thyristor elements has been shown. 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 IV when off and carry a sizable current for about a microsecond 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 GTO´s (Gate Turn-Off thyristors) and, as in other bipolar devices, depends chiefly on carrier recombination time, device thickness and turn-off di/dt.