A CMOS-integrated 'ISFET-operational amplifier' chemical sensor employing differential sensing

Abstract

The ISFET (ion-sensitive field-effect transistor) pH sensor is first matched with a MOSFET at the differential input stage of a CMOS operational amplifier (called the ISFET-operational amplifier) to cancel out the temperature sensitivity. Then, the output of an ISFET-operational amplifier with a Ta/sub 2/O/sub 5//SiO/sub 2/ gate (58-59 mV/pH) ISFET is differentially amplified against the output of another on-chip ISFET-operational amplifier with a SiO/sub x/N/sub y//Si/sub 3/N/sub 4//SiO/sub 2/ gate ISFET (18-20-mV/pH). An on-chip noble metal counterelectrode serves as the electrical contact to define the electric potential of the electrolyte. No external reference electrode is required. The difference measurement technique achieves (1) common-mode rejection of the solution potential, and (2) relaxation of the requirement that the on-chip reference electrode be ideal. The CMOS-compatible ISFET process is modified from a standard self-aligned polysilicon gate CMOS process with minimal process redesign. The standard CMOS sequence is unaltered until the contact windows are opened. The complete sensor has 40-43-mV/pH pH sensitivity and demonstrates common-mode rejection to ambient light and noise from the electrolyte.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>