Tuning a potential sensor using a source-follower MOSFET
Your graph would seem to show a p-enhancement fet.(Vgs is -ve for increasing IDD)

Perhaps the polarity of the gate has some effect in attracting/repelling the ions from the active gate selective coating area. 

Yes the fet will be in saturation, i.e. operating as a constant current source.

I would expect that you would have a pair of matched fets on the die, with one exposed to ions and the other shielded from them. By doing that you get first order thermal compensation.

An actual IC circuit probably looks like an opamp input stage, and quite different to how you would do it outside of an IC.

This is a pfet version. Pay attention to the polarities. The opamp creates a "virtual earth" i.e. it always hold the source/-ve input at 0V. So VGS is not affected by the current







simulate this circuit &ndash; Schematic created using CircuitLab