Analytical Correction of Threshold Voltage Drift in Ion-selective Field Effect Transistors (ISFETs)

A method for correction of threshold voltage drift in pH-sensitive ion-selective field effect transistors (ISFETs) is presented. The proposed method involves monitoring the differential of drain current with a constant potential applied to the reference electrode. Device instability, manifesting as thresholdvoltage drift is associated with a relatively slow, monotonic change in the measuring signal, namely the drain current, under operating conditions corresponding to a fixed bias applied to the reference electrode. Ordinarily, over sufficiently small intervals of time the variation in the measuring signaldue to drift is essentially negligible in applications, such as cardiopulmonary bypass, 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. In the feedback mode of ISFET operation thepercent change in the measuring signal resulting from threshold voltage drift over small intervals of time is smaller than the corresponding change in the current mode of ISFET operation. 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.

Keywords:

ISFET , pH , Drift , Instability , Continuous monitoring