When forward biased, MOSFET can be turned on by applying a positive voltage between gate and source. The gate voltage must be maintained for as long as the switch is to be on.
The switch can be turned off by applying a negative voltage between gate and source. The gate voltage must be maintained for as long as the switch is to be off.
Unlike BJT, which is current controlled, MOSFET is voltage controlled. This implies that a very small amount of power is required from the drive circuit to turn the device on and off.
Fig. 7-8 shows the symbol for a MOSFET and the - characteristic of MOSFET.
MOSFET is not capable of withstanding reverse voltages. MOSFETs have the advantages of low power demand on the driver circuit and high-speed switching (100 kHz and beyond), and the disadvantage of relatively low-power rating.
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