Enhanced pH Sensitivity of AlGaN/GaN Ion-Sensitive Field-Effect Transistor by Recess Process and Ammonium Hydroxide Treatment

AlGaN/gallium nitride (GaN) ion-sensitive field-effect transistors (ISFETs) were fabricated as pH sensors. The sensitivity of the AlGaN/GaN ISFETs was evolved with gate recess process and ammonium hydroxide (NH ₄ OH) treatment. By performing the gate recess process, the threshold voltage (V _T ) of the ISFET increased from -3.33 to -0.31 V and the maximum conductance (G _M ) of the ISFET increased from 0.8 to 2 mS, with the current sensitivity of the pH sensor improving from 52.25 to 78.86 μA/pH. Further, after performing the ammonium hydroxide treatment, the V _T of the ISFET increased from -0.33 to -0.14 V, with the current sensitivity of the pH sensor improving from 78.86 to 84.39 μA/pH. To characterize the surface conditions the X-ray photoelectron spectroscopy (XPS) was deployed. The results indicated that many nitrogen vacancies (V _N ) were introduced during the recess process, leading to a negative V _T shift and a smaller potential sensitivity (S _V ), which can be improved by ammonium hydroxide treatment.