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

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2 Author(s)
H. -S. Wong ; Sherman Fairchild Center, Lehigh Univ., Bethlehem, PA, USA ; M. H. White

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 Ta2O5/SiO2 gate (58-59 mV/pH) ISFET is differentially amplified against the output of another on-chip ISFET-operational amplifier with a SiOx Ny/Si3N4/SiO2 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

Published in:

IEEE Transactions on Electron Devices  (Volume:36 ,  Issue: 3 )