Electrical – Transconductance in mosfets

The transconductance is the ratio of the relative change of current to the relative change of gate-source voltage.

Looking at the black line of your graph, the current at VGS=4.25 is 0.6A and at VGS=4.5V it is about 1.1A.

The transconductance is given by gm=(1.1A-0.6A)/(4.5V-4.25V)=2A/V.

So, it can be used to have a linear expression to calculate the change of current for a change of voltage around the point where the transconductance was calculated.

For integrated circuit design the backgate-transconductance can be found using design documents or simulation. For discrete circuit design the source and bulk are commonly connected so that it can be ignored.

Backgate effect in particular occurs because the backgate is connected to fixed potential while the source potential can change. If source and backgate are at the same potential (e.g. both at ground) then it can be disregarded.

NMOS Source-followers with the substrate connected to ground show a non-linear behavior because of the backgate effect. When the source and the backgate are connected and the current-source is a high output impedance, source followers work much better.