Question

what are the physical phenomena that determine the speed of the FET transistors

Answer 1

Answer : -

There are two types of FET ( feild effect transistor ) .

1 MOSFET

2 JFET

In general MOSFET is much popular for high speed switching application.

Speed Of FET depends on many factors, like gate suorce capcitor ( dimension of gate capasitor ) , current driven by gate , gate resistor etc. The switching speed or speed of transistor is nothing but the rate of change in logic state of given circuit by FET.

For switching speed measure measurement we have to provide a short pulse by signal generator with low duty cycle at some normal frequancy.

Now, out put connected to oscilloscope to measure  rise time , fall time and delay time.

By doing some math we can able to determine the speed of FET.

This is the set up for accurate measure ment of speed.

There are also one technique we can use is from FET characteristics.

The gate looks like a capacitor to the source. It actually has some capacitance to the drain too, but that has been taken into account in the total gate charge .

The time constant (RC) gives you some idea how fast the gate will slew given a step input on the other side of the gate resistor.

The FET won't suddenly go from full off to full on at a particular gate voltage, but there is a gate voltage at which a small incremental change will make the most difference in the FET output characteristic.

You have to decide how full on and how full off "switching" really means, and then decide what gate voltage range that represents.

Then you can use the equivalent R-C model to decide how quickly a step input will cause it to slew accross this region. So, we can estimate the speed of FET with some calculation

Gate capaaior and resistor is main parameters which is used to determaine speed of FET.