We report on metal-oxide-semiconductor (MOS) AlGaN/GaN heterostructure field effect transistors (HFETs) based on InGaN/GaN multiple quantum well (MQW) structure using Ta2O5 dielectric deposited by electron beam evaporation (EBE) simultaneously for surface passivation and as a gate insulator. The device features a 5-pair MQW layer inserted into the AlGaN/GaN two-channel HFET structure. It results in a raised potential barrier, which leads to better carrier confinement and effective access to the InGaN layer. However, it revealed a pronounced leakage current which may be generated from the bottom Si-doped GaN and/or the sidewall leakage paths due to the exposure of channels after mesa etching. Both passivated MQW-HFET and MOS MQW-HFET present enhanced dc- and pulsed-mode performance compared to unpassivated one. In terms of transfer characteristics, MOS MQW-HFET exhibits the larger and broader main peak yet smaller satellite peak relative to passivated MQW-HFET. The reduced gate and mesa-to-mesa leakage current indicates the successful passivation effect from EBE-Ta2O5 dielectric.