When we utilize a biomolecule with higher dielectric constant compared to the air, it enhances the electrostatic control of the gate, resulting in a higher drain current. The rate of this increment highly depends on the working principle of your device and the dimension of the cavities carved in the gate oxide.
Generally, FET gain or transconductance = delta of drain current divided by delta of gate-source voltage grows as square root of drain current. e.g. https://sound-au.com/articles/fet-f1.gif
Taken from https://sound-au.com/articles/fet-applications.htm