Role of gate oxide in AlGaN/GaN high-electron-mobility transistor pH sensors

We report on a comparison of different gate oxides for AlGaN/GaN high-electron-mobility transistor (HEMT) pH sensors. The HEMTs show a linear increase in drain-source current as the pH of the electrolyte solutions introduced to the gate region is decreased. Three different gate oxides were examined, namely the native oxide on the AlGaN surface, a UV-ozone-induced oxide and an Sc ₂O ₃ gate deposited by molecular beam epitaxy. The Sc ₂O ₃ produced superior results in terms of resolution in measuring small changes in pH. The devices with Sc ₂O ₃ in the gate region exhibited a linear change in current between pH 3 and 10 of 37 μA/pH with a resolution of <0.1 pH over the entire pH range. In contrast, the native oxide devices showed a larger change in current, ̃ 70 μA/pH, but with a degraded resolution of ̃ 0.4 pH. Results for the UV-ozone oxide were intermediate in resolution, 0.2 pH. These HEMTs have promise for detecting pH changes in biological samples and can be readily integrated into a standard package for wireless data transmission.