We performed the (NH₄)₂S surface treatments before Al₂O₃ deposition to improve the Al₂O₃/_III-Nitride interface quality in Al₂O₃/AlGaN/GaN metal-oxide-semiconductor heterostructure field-effect transistors (MOSHFETs). Interface state density at the Al₂O₃/GaN interface was decreased by the (NH₄)₂S treatment. The hysteresis width in I_D-V_GS and g_m-V_GS characteristics of the Al₂O₃/AlGaN MOSHFETs with the (NH₄)₂S treatment was smaller than that without the (NH₄)₂S treatment. In addition, transconductance (g_m) decrease at a large gate voltage was relaxed by the (NH₄)₂S treatment. We also performed ultraviolet (UV) illumination during the (NH₄)₂S treatment for further improvement of the Al₂O₃/_III-Nitride interface quality. Interface state density of the Al₂O₃/GaN MOS diodes with the UV illumination was smaller than that without the UV illumination.