Question

In digital circuits, MOSFETs are used as switches. What are the key features of an ideal switch? Provide your own argument as to whether MOSFETs are ideal switches based on their I-V curves and why.

Answer 1

The key features of an ideal switch are,

1)The switch should have infinite reverse breakdown voltage,

2)When the switch position is off, there is zero current passing through the switch

3)When the switch position is on, there is zero voltage across the two terminals of the switch,

4)The turn-on and turn-off delay times of switch are zero seconds,

5)Since the voltage at ON position or the current at OFF position is always zero, the instantaneous power dissipation is always zero.

MOSFETs are close to Ideal Switch but they are ideal switches:

It is said that an Ideal switch must have Zero Forward Voltage when it is in ON state.

For a particular value of Vgs, the voltage drop across the Drain and Source is not zero in the in V-I characteristics.

Even though it is much smaller when compared with JFET, we considerably can observe the drain to source voltage.

When the MOSFET is in OFF position it draws 250uA which has to 0 in the ideal switch case.

The MOSFETs have also a small transition time.

They will dissipate little power while they are operating as Switches.

They have a finite reverse breakdown voltage also.

Hence MOSFETs are not completely ideal switches but they are very effieicent that previos semiconductor tecchnologies and thus they are mostly used in digital circuits.