Characteristics of reference electrodes using a polymer gate ISFET

Abstract

A new type of ISFET with a Parylene gate as a site-free and ion-blocked membrane has been proposed for a reference electrode. The reference ISFET should be insensitive to the variation of ion concentrations of all ions or a specific ion in the electrolyte and should have a constant gate surface potential. In this paper, the operation of this reference ISFET is analysed by the site binding model and the conditions required to make the gate surface potential constant are examined. In general, the surface potential of an electrolyte-insulator-semiconductor (EIS) system is determined by two factors, i.e., the available surface site density of the insulator and the total charge density of the semiconductor, and it can be made constant by controlling the total charge density of the semiconductor. The preliminary experimental results on the electrolyte-Parylene-Si diode and ISFET are in good agreement with the theoretical calculations and suggest that a reference ISFET could be realized by using the Parylene gate.