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 (NH4OH) treatment. By performing the gate recess process, the threshold voltage ( ${V}_{\text {T}}$ ) of the ISFET increased from −3.33 to −0.31 V and the maximum conductance ( ${G}_{\text {M}}$ ) of the ISFET increased from 0.8 to 2 mS, with the current sensitivity of the pH sensor improving from 52.25 to $78.86~\mu \text{A}$ /pH. Further, after performing the ammonium hydroxide treatment, the ${V}_{\text {T}}$ of the ISFET increased from −0.33 to −0.14 V, with the current sensitivity of the pH sensor improving from 78.86 to $84.39~\mu \text{A}$ /pH. To characterize the surface conditions the X-ray photoelectron spectroscopy (XPS) was deployed. The results indicated that many nitrogen vacancies ( ${V}_{\text {N}}$ ) were introduced during the recess process, leading to a negative ${V}_{\text {T}}$ shift and a smaller potential sensitivity ( ${S}_{V}$ ), which can be improved by ammonium hydroxide treatment.