A physical model for drift in pH ISFETs

A physical model is presented which quantitatively describes the threshold voltage instability, commonly known as drift, in n-channel Si3N4-gate and as well as Al2O3-gate pH ISFETs. The origin of the so-called drift is postulated to be associated with the relatively slow chemical modification of the gate insulator surface as a result of exposure to the electrolyte. The chemical modification of the surface is assumed to result from a transport-limited reaction whose rate is modeled by a hopping and/or trap-limited transport mechanism known as dispersive transport. The change in the chemical composition of the insulator surface leads to a decrease in the overall insulator capacitance with time, which gives rise to a monotonic temporal increase in the threshold voltage. (C) 1998 Elsevier Science S.A. All rights reserved.