We predict the sensitivity of solution gate AlGaN/GaN field effect transistors to pH values of the electrolyte and to charged adsorbates at the semiconductor–electrolyte interface. Invoking the site-binding model for the chemical reactions at the oxidic semiconductor–electrolyte interface and taking into account the large polarization fields within the nitride heterostructure, the spatial charge and potential distribution have been calculated self-consistently both in the semiconductor and the electrolyte. In addition, the source–drain current is calculated and its sensitivity to the electrolyte’s pH value is studied systematically. Comparison with experiment shows good agreement. A significantly enhanced resolution is predicted for AlGaN/GaN structures of N-face polarity.