MODELING THE SHORT-TIME RESPONSE OF ISFET SENSORS

In this publication, a model for the short-time response of ISFET sensors is presented. In contrast to the static site-binding theory, this approach takes into account the kinetics of the electrochemical reactions occurring on the gate-insulator surface. As a result, a system of coupled non-linear differential equations is formulated, which is able to describe the pH-step response of an ISFET with an Si3N4 gate insulator. In the smalt-signal range (Delta pH <0.1), an increase of the response time with pH is found; time constants can be calculated between 1 ms (pH 2.6) and 600 ms (pH 8). Outside the small-signal regime, the behaviour of the sensor is non-linear and depends on the direction and the starting value of the pH step. For a confirmation of these results, measurements of the pH-step response have been performed in a specially designed ISFET/flow-injection set-up, which are in a good qualitative agreement with the theoretical model presented here.