An MCT is a new device in the field of semiconductor-controlled devices. It is basically a thyristor with two MOSFETs built into the gate structure. One MOSFET is used for turning on the MCT and the other for turning off the device. An MCT is a high-frequency, high_:power, low-conduction drop switching device.

An MCT combines into it the features of both conventional four-layer thyristor having regenerative action and MOS-gate structure. However, in MCT, anode is the reference with  respect to which, all gate signals are applied. In a conventional SCR, cathode is the reference terminal for gate signals.

The basic structure of an MCT is shown in Fig. 2.20. A practical MCT consists of thousands of these basic cells connected in parallel, just like a power MOSFET (

7, 8). This is done in order to achieve a high-current carrying capacity of the device.

The equivalent circuit of MCT is shown in Fig. 2.21 (a). It consists of one on-FET, one off-FET and two transistors. The on-FET is ap-channel MOSFET and off-FET is an rc-channel MOSFET. An arrow towards the gate terminal indicates n-channel MOSFET and the arrow away from the gate terminal as the p-channel MOSFET. The two transistors in the equivalent circuit indicate that there is regenerative feedback in the MCT just as it is in an ordinary thyristor. Fig. 2.21 (6) gives the circuit symbol of an MCT

An MCT is turned-on by a negative voltage pulse at the gate with respect to the anode and is turned-off by a positive voltage pulse. Working of MCT can be understood better by referring to Fig. 2.21 (a).

Turn-on process. As stated above, MCT is turned on by applying a negative voltage pulse at the gate with respect to anode. In other words, for turning on MCT, gate is made negative with respect to anode by the voltage pulse between gate and anode. With the application of this negative voltage pulse, on-FET gets >turned-on and off-FET is off. With on-FET on, current begins to flow from anode A, through on-FET and then as the base current and emitter current of npn transistor and then to cathode C. This turns on npn transistor. As a result, collector current begins to flow in npn transistor. As off-FET is off, this collector current of npn transistor acts as the base current of pnp transistor. Subsequently, pnp transistor is also turned on. Once both the transistors are on, regenerative action of the connection scheme takes place and the thyristor or MCT is turned on.

Note that on-FET and pnp transistor are in parallel when thyristor is in conduction state. During the time MCT is on, base current of npn transistor flows mainly through pnp transistor because of its better conducting property.

Turn-off process. For turning-off the MCT, off-FET (or n -channel MOSFET) is energized by positive voltage pulse at the gate. With the application of positive voltage pulse, off-FET is turned on and on-FET is turned off. After off-FET is turned on, emitter-base terminals of pnp transistor are short circuited by off-FET So now anode current begins to flow through off’-FET and therefore base current of pnp transistor begins to decrease. Further, collector current of pnp transistor that forms the base current of npn transistor also begins to decrease.

As a consequence, base currents of both pnp and npn transistors, now devoid of stored charge in their n and p bases respectively, begin to decay. This regenerative action eventually turns off the MCT.

An MCT has the following merits :

(i)        Low forward conduction drop,

(ii)       fast turn-on and turn-off times,
(iii)   low switching losses and

(iv)    high gate input impedance, which allows simpler design of drive circuits.

An MCT is a brand-new device which is likely to be available commercially very soon. As it possesses highly adaptable features for its use as a switching device, it seems to have tremendous scope for its widespread applications. Its potential applications include dc and ac motor drives, UPS systems, induction heating, dc-dc converters, power line conditioners etc. It may, in the near future, challenge the existence of most of the available devices like -   resistors, GTOs, BJTs, IGBTs (7).

NEW SEMICONDUCTING MATERIALS

At present, silicon enjoys monopoly as a semiconductor material for the commercial production of power-control devices. This is because silicon is cheaply available and semiconductor devices of any size can be easily fabricated on a single silicon chip. There are, however, new types of materials like gallium arsenic (GaAs), silicon carbide and diamond which possess the desirable properties required for switching devices. At present, state-of-the-art technology for these materials is primitive compared with silicon, and many more years of research investment are required before these materials become commercially viable for the production of power-controlled devices. Superconductive materials may also be used in the manufacture of such devices, but work in this direction has not yet been reported.

Germanium is not used in the fabrication of thyristors because of the following reasons:

1. Germanium has much lower thermal conductivity; its thermal resistance is, there­fore, more. As a consequence, germanium thyristors suffer from more losses, more temperature rise and therefore lower operating life.
2. Its breakdown voltage is much less than that of silicon. It means that germanium thyristor can be built for small voltage ratings only.
3. Germanium is much costlier than silicon.

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