METAL OXIDE SEMICONDUCTOR FIELD EFFECT TRANSISTOR (MOSFET)

Since the 1980s the power MOSFET has superseded the BJT in inverters for drives. Like the BJT, the MOSFET is a three-terminal device and is available in two versions, the n-channel and the P-channel. The N-channel is the most widely used, and is shown in Figure 2.18. The main (load) current flows into the drain (D) and out of the source (S). (Confusingly, the load current in this case flows in the opposite direction to the arrow on the symbol.) Unlike the BJT, which is controlled by the base current, the MOSFET is controlled by the gate source voltage.

To turn the device on, the gate-source voltage must be comfortably above a threshold of a few volts. When the voltage is first applied to the gate, currents Flow in the parasitic gate-source and gate-drain capacitances, but once these capacitances have been charged the input current to the gate is negligible, so the steady-state gate drive power is minimal. To turn the device 0v, the parasitic capacitances must be discharged and the gate-source voltage must be held below the threshold level.

The principal advantage of the MOSFET is that it is a voltage controlled device which requires negligible power to hold it in the one state. The gate drive circuitry is thus less complex and costly than the base-drive circuitry of an equivalent bipolar device. The disadvantage of the MOSFET is that in the ‘on’ state the effective resistance of the drain source is higher than an equivalent bipolar device, so the power dissipation is higher and the device is rather less efficient as a power switch.

MOSFETs are used in low and medium power inverters up to a few kilowatts, with voltages generally not exceeding 700 V.