A new analytical model for the response of AlGaN/GaN HEMT-based pH sensors

Abstract

pH sensors are monitoring devices with wide applications in biology, chemistry, medicine, and agriculture. To enhance their sensitivity and long-term stability, efficient study of such devices becomes imperative but is impossible without the aid of accurate analytical models. A new analytical model for the pH sensing characteristics of AlGaN/GaN-based high-electron-mobility transistors (HEMTs) is presented herein, as well as theoretical predictions and optimization of the charge sensitivity for ungated AlGaN/GaN HEMT-based pH sensors. The change in the drain current with the changing surface potential due to a variation in the pH of the electrolyte is calculated for devices with different Al mole fractions, AlGaN thicknesses, gate length spacings, and passivation layers. The numerical values for the drain current, threshold voltage, and surface potential obtained by using this new model show good agreement with available experimental results. It is demonstrated that the sensitivity of GaN HEMT-based pH sensors at lower pH values can be improved by applying a SiNx passivation layer to the HEMT. The calculated average root-mean-square error of our model is 0.018, being an order of magnitude lower than other models reported in literature.