A model of surface ionization and complexation of surface hydroxyl groups on the gate insulator surface is adapted in conjunction with electronic device physics to arrive at a generalized theory for the current-voltage characteristics of an electrolyte-insulator-semiconductor field-effect transistor (EISFET) in electrolyte solutions. EISFET's that employ thermally grown silicon dioxide were tested in simple electrolytes that contain Na ⁺ , K ⁺ , and Li ⁺ ions titrated in a p H range from 2 to 9. Experimental results show good agreement with the theory. The model successfully explains p H sensitivity, as well as the ion interference effect, of the EISFET working as a p H sensor. From this model, it is conluded that, among all the electrolyte parameters associated with an EISFET, the surface site density of the hydroxyl groups N s and the separation of surface ionization constants \Delta pK are the primary factors to consider when employing EISFET's as p H sensors. For high sensitivity and good selectivity, large N s and small \Delta pK values are required.

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