A PHYSICAL MODEL FOR THRESHOLD VOLTAGE INSTABILITY IN SI3N4-GATE H-SENSITIVE FETS (PH ISFETS)()

A physical model is presented which quantitatively describes the thres hold voltage instability, commonly known as drift, in n-channel Si3N4- gate pH ISFET's. The origin of the so-called drift is postulated to be associated with the relatively slow conversion of the silicon nitride surface to a hydrated SiO2 or oxynitride layer. The rate of hydration is modeled by a hopping and/or trap-limited transport mechanism known as dispersive transport. Hydration leads to a decrease in the overall insulator capacitance with time, which gives rise to a monotonic temp oral increase in the threshold voltage.