At first glance, MOSFETs are simple devices. We like to think of them as rather idealized voltage-controlled switches: you use a microcontroller pin to apply a bit of voltage to the gate and current flows freely through the channel. What could be better?

The details, of course, paint a different picture. When we are working in the real world, we have to think about such things as on-state resistance, channel-length modulation, and thermal resistance. Another issue is that the gate is actually a capacitor. This means that we have to provide nontrivial amounts of charge in order to raise the gate voltage and thus activate the device; consequently, the rate at which the driver circuitry can deliver charge determines the speed with which we can turn on the FET.

Another perpetually irksome MOSFET characteristic is that the gate voltage itself actually has no ability to bring the device into conduction. Rather, it is the gate-to-source voltage that must exceed the threshold voltage (VTH). This detail is easily ignored when the source is connected to the ground node, but we frequently use MOSFETs in applications where one or two of the FETs cannot possibly have a source voltage that remains at ground. Here I’m thinking specifically of the half-bridge and full-bridge circuits that we so frequently use for driving motors.

For More Details:
CRM Video