pH response of GaN surfaces and its application for pH-sensitive field-effect transistors

The pH-sensitivity of GaN surfaces in electrolyte solutions has been determined. For this purpose, GaN field-effect transistors and AlGaN/GaN high-electron-mobility transistor (HEMT) structures were used to measure the response of nonmetallized GaN gate regions to changes of the H+-concentration in an ambient electrolyte. We found a linear response to changes in the pH between pH=2 and pH=12 for both as-deposited and thermally oxidized GaN surfaces. Both surfaces showed an almost Nernstian behavior with sensitivities of 57.3 mV/pH for GaN:Si/GaN:Mg and 56.0 mV/pH for GaN/AlGaN/GaN HEMT structures. This suggests that the native metal oxide on the III-nitride surface is responsible for pH-sensitivity. The investigated devices showed stable operation with a resolution better than 0.05 pH over the entire pH range.