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A surface passivation technique of high electron mobility transistor (HEMT) devices is reported. The passivated HEMT device has a much higher RF performance of FT and Fmax than a non-passivated one. The AlGaN/GaN HEMT has a short gate length of 0.15 μm using an E-beam lithography system and very low ohmic contact resistance of 1.3 × 10-6 Ωcm-2 using a rapid thermal processing alloy system. In addition, to protect the active layer from the surface trap, an SiO2 thin film passivation process is applied by the plasma-enhanced chemical vapour deposition system. The fabricated AlGaN/GaN HEMT exhibits a maximum drain current of 900 mA/mm and a maximum transconductance of 320 mS/mm. In particular, this device produces excellent RF performance of small-signal characteristics, such as a current gain cut-off frequency (fT) of 55 GHz and maximum oscillation frequencies (fmax) of 130 GHz.