Introduction to MOSFET

Introduction: MOSFET is an ideal transconductance amplifier which has infinite input resistance and a current generator at the output. However, in practice, there are several other non-ideal circuit elements.

Equivalent Circuit and Microwave Performance:  An equivalent circuit for common-source connectionis shown in Fig. given below. The gate resistance R_G is associated with the gate contact material over the oxide.

The input resistance i.e. R_in is an outcome of tunneling current through the thin gate insulator, and also includes conductance through defects.This is nothing but a function of oxide thickness.

For a silicon dioxide layer i.e. thermally grown, the leakage current between gate and the channel is trivial; so the input resistance is very high, i.e. one of the main advantages of a MOSFET.

For oxides below thickness of 5 nm, tunneling current becomes a significant factor.

In the saturation region, V_D and thus R_D has little effect on the drain saturation current.

The R_s affects effective gate bias, and the extrinsic transconductance is calculated with help of
equation (1)

When the microwave performance is analyzed, and the cutoff frequency fTi.e. defined as the frequency for unity current gain (i.e. the ratio of drain current to gate current) is attained,

equation (2)

In ideal case where there is zero parasitics, it can be illustrated that

equation (3)

For very large source and drain resistances, the comprehensive expression is  given by

equation (4)

The other figure-of-merit