Continuous Monitoring of pH using Ion-selective Field Effect Transistors (ISFET’s) Operating with a Fixed Bias Applied to the Reference Electrode

A method for correction of threshold voltage drift in pH-sensitive ion-selective field effect transistors (ISFET’s) is presented. The proposed method allows accurate continuous monitoring of pH under clinical conditions corresponding to rapid pH changes such as those encountered in acute metabolic acidosis and cardiopulmonary bypass operations. This method, which involves monitoring the differential of drain current in the current mode of ISFET operation with a constant potential applied to the reference electrode, can also be of advantage in telemetry applications requiring low-power consumption. In the current mode the inherent device instability resulting from a drift in the threshold voltage of the ISFET manifests as a relatively slow, monotonic decrease in the measuring signal, namely the drain current. Ordinarily, over sufficiently small intervals of time the variation in the measuring signal due to drift is essentially negligible in applications where changes in pH are abrupt. Consequently, the differential of the measuring signal represents only the changes in the pH if the device sensitivity is adequately high. The validity of the proposed method is examined using a Si3N4-gate pH-sensitive ISFET to monitor pH variations in the 3.5-10.0 range. Key-Words: ISFET, pH, Drift, Instability, Continuous monitoring