A PHYSICAL MODEL FOR DRIFT IN PH ISFETS

A physical model is presented which quantitatively describes the thres hold 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 t ransport. The change in the chemical composition of the insulator surf ace leads to a decrease in the overall insulator capacitance with time , which gives rise to a monotonic temporal increase in the threshold v oltage. (C) 1998 Elsevier Science S.A. All rights reserved.