The effect of in situ vacuum anneal (VA) and silane (SiH4) treatment on the electrical characteristics of AlGaN/GaN metal-oxide-semiconductor high-electron mobility transistors was investigated. Native Ga-O bonds on the AlGaN surface can be completely removed by this in situ passivation technique, which was confirmed by x-ray photoelectron spectroscopy. In situ VA and SiH4 passivation also reduced the device gate leakage current. This is attributed to the suppression of trap-assisted tunneling current through the HfAlO gate dielectric. Saturation drain current for devices with in situ VA and SiH4 passivation was also improved, which is due to increased two-dimensional electron gas density. In addition, devices with in situ VA and SiH4 passivation achieved an Ion/Ioff ratio of around 106 and a subthreshold swing of less than 100 mV/decade.