Equivalent Circuit and Microwave Performance: An equivalent circuit for common-source connectionis shown in Fig. given below. The gate resistance RG is associated with the gate contact material over the oxide.
The input resistance i.e. Rin 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.
The Rs affects effective gate bias, and the extrinsic transconductance is calculated with help of
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,
In ideal case where there is zero parasitics, it can be illustrated that
For very large source and drain resistances, the comprehensive expression is given by
The other figure-of-merit