A substrate's embedded substrate contact electrode forms a reference voltage point. A gate insulator is spaced outwardly from the substrate and has an exposed outer surface configured for contact with a fluid analyte. A device region is intermediate the substrate and the gate insulator; source and drain regions are adjacent the device region; and a field insulator is spaced outwardly of the drain region, the source region, and the substrate away from the device region. The gate insulator and the field oxide are formed of different materials having different chemical sensitivities to the fluid analyte. The field insulator is coupled to the substrate through the field insulator capacitance. The gate insulator capacitance is much smaller than the field insulator capacitance. The embedded substrate contact electrode can be connected to a separate voltage so that the electrical potential between the substrate and the source region can be controlled.