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.
The oxide-electrolyte interface
The surface of any metal oxide always contains hydroxyl groups, in the case of silicon
dioxide SiOH groups. These groups may donate or accept a proton from the solution,
leaving a negatively charged or a positively charged surface group respectively, as
schematically represented by figure 3.