Influence of Gate Material, Geometry, and Temperature on ISFET Performance in pH Sensing Applications

Ion-sensitive field-effect transistors (ISFET) are used in various sensing applications including electrochemical biosensing. In this study, analytical models of Ta2O5, Si3N4, and SiO2-gated ISFET-based sensors were developed using MATLAB® and LTspice. The site binding, surface potential, threshold voltage, and drain current transport models were used for analysis of ISFET. This work aims to address the effect of sensing gate material, gate thickness, aspect ratio, temperature, and matrix on ISFET’s performance. The thin-layer, gate degradation, and short-channel effects were also analysed. An optimum geometry of ISFET was obtained satisfying the required leakage current and gate capacitance (COX). The role of isothermal point and temperature on ISFETs were also investigated. Additionally, the efficacy of the matrix on ISFET was examined in water, blood, and urine. From the simulation results, a high current sensitivity of 1.97 (mA)1/2/pH and near-Nernstian voltage sensitivity of 56.84 mV/pH were obtained for Ta2O5-gated ISFET. However, Si3N4 and SiO2-gated ISFETs have non-uniform sensitivity characteristics to changes in the pH level. A significant change in pH sensitivity was observed for the Si3N4-gated ISFET in all matrices, whereas the Ta2O5 and SiO2-gated ISFETs were found to be less sensitive to matrix changes. Finally, this work suggests the usage of Ta2O5-gated ISFETs for sensitive sensing applications.