Slow pH response effects of silicon nitride ISFET sensors

To examine the slow pH response of ion sensitive field effect transistors (ISFETs) with a silicon nitride gate-insulator, measurements were carried out in a thermostated measurement set-up by generating a stepwise increase and decrease of the pH in a range between pH 5 and 8. The slow response curves obtained were fitted to a multiexponential model to extract time constants and amplitudes of the various response effects. As a result, two exponential effects with different behaviour were found for the slow pH response. Moreover, a dependence of the response time from the direction of the pH-step was found. Based on these results, a buried layer beneath the gate-insulator surface with a heterogeneous site distribution is proposed as a phenomenological explanation for the slow response.