MODELING H+-SENSITIVE FETS WITH SPICE

A generalized physical model including two kinds of binding sites is presented for H+-sensitive ISFET devices. The model results in a set of equations which is introduced into a modified version of the electronic circuit simulation program SPICE. In this way, the effects induced on the device performances by varying several physico-chemical parameters are analyzed. The slope of V(out) versus pH curves is predicted for SiO2-, Al2O3-, and Si3N4-gate ISFET's. The program outputs are checked by comparing them with experimental data obtained with an Al2O3-gate ISFET. The model is then used to predict the behavior of a hypothetical, partially pH-insensitive (REFET) structure. Finally, the model is utilized to fit the slow response of the Al2O3-gate ISFET to a pH step.