MOS controlled Thyristors (MCTs)

MOS controlled Thyristors (MCTs) have the combination of thyristor current and voltage capability with MOS gated turn-on and turn-off. Various sub-classes of MCTs can be made: P-type or N-type, symmetric or asymmetric blocking, one or two-sided off-FET gate control, and various turn-on alternatives including direct turn-on with light.

All of these sub-classes have one thing in common; turn-off is accomplished by turning on a highly interdigitated off-FET to short out one or both of the thyristor’s emitter-base junctions. Harris Company is the only present (1977) supplier of MCTs, however ABB has introduced a new device called “Insulated Gate Commutated Thyristor” (IGCT) which is the same family of devices.

The MCT turns on simultaneously over the entire device area giving the MCT excellent di/dt capability. The MCT offers a lower specific on-resistance at high voltage than any other gate-driven technology.

Figure depicts the MCT equivalent circuit. MCT closely approximates a bipolar thyristors (the two transistor model is shown) with two opposite polarity MOSFET transistors connected between its anode and the proper layer to turn it on and off. Since MCT is a NPNP device rather than a PNPN device and output terminal or cathode must be negatively biased.

Driving the gate terminal negative with respect to the common terminal or anode turns the P channel FET on, firing the bipolar SCR and thus MCT turns on. Driving the gate terminal positive with respect to the anode turn on the N channel FET on shunting the base drive to PNP bipolar transistor making up part of the SCR, causing the SCR to turn off and thus MCT turn off. It is obvious from the equivalent circuit that when no gate to anode voltage is applied to the gate terminal of the device, the input terminals of the bipolar SCR are un-terminated. Operation without gate bias is not recommended.