Because all terms are constant except ψo, it is this term which makes the ISFET sensitive to the electrolyte pH, which is controlling the dissociation of the oxide surface groups. Designing a pH sensitive ISFET with a maximum sensitivity and
selectivity needs therefore a detailed investigation of the oxide-electrolyte interface in order to be able to choose the best oxide, which is not a priori the silicon dioxide as used for MOSFETs.