Operation of chemically sensitive field-effect sensors as a function of the insulator-electrolyte interface

All chemical sensors based upon the field-effect principle share a common quality. Their measurable properties can be described in terms of a flat-band voltage. The various terms in the expression of the flat-band voltage are described and discussed, and in particular the voltage drop at the insulator-electrolyte interface. It is shown that this voltage drop depends on the
pH
of the electrolyte and is determined by two parameters, the
pH
at the point of zero charge, and a sensitivity parameter which is introduced in this paper. These parameters are obtained from the site-dissociation model of the insulator-electrolyte interface, combined with the Gouy-Chapmann-Stern theory of the electrical double layer at this interface. The theoretical description is used to interpret experimental results obtained from insulators with widely different properties, namely SiO
2
and Al
2
O
3
.