Herein, we successfully improved the maximum oscillation frequency and maximum stable gain (MSG) across a wide bias range using surface-oxide-controlled (SOC) InGaAs/InAlAs inverted-type metal-oxide-semiconductor HEMTs (inverted MOS-HEMTs) due to a reduction in the gate leakage current and drain conductance. H2O vapor treatment selectively decreased the narrow band gap oxide (InOx) at the surface of the In-based epitaxial layer. Consequently, SOC inverted MOS-HEMTs show a high MSG of over 12 dB across a wide bias range at 100 GHz.