1 A MOSFET is characterized by a very high input gate resist

1. A MOSFET is characterized by a very high input (gate) resistance. Does it mean that no appreciable current ever flows to the gate? Explain. 2. Draw schematics of circuits for experiments with a MOSFET described in sections 1, 2, and 3, below

Solution

1)

To DC, the resistance of the gate is so high it is practically infinite, and no current will flow in steady state. For AC or transients (like a step, say when you first connect dc power), the gate looks like a capacitor. Its value is pretty small, but it becomes quite important at moderate frequencies. So while the DC current into the gate might be practically zero, the AC current may be quite large: For a power supply running at 200 kHz, using large power mosfets. The current required to drive the gates at this high frequency was around 6 amps! The gate capactiance is non-linear too, so most data sheets specify the total gate charge at a certain voltage, for example, 20 nC at 10 volts. This tells you how much charge you need to push into the gate to make it 10 volts. This charge has to get there by a flow of current. It will get there faster with a higher current.