This paper discusses a way to optimize the In_0.53Ga_0.47As quantum-well MOSFET structures from the prospective of channel mobility. We experimentally demonstrated that the barrier thickness and interfacial defect density are two key factors determining the channel mobility, while the effect of the oxide charge is negligible. The mobility model consisting of phonon scattering and coulomb scattering was applied to fit the experimental data. According to the model, for quantum-well MOSFET structures with in-situ ALD Al₂O₃, the mobility is dominated by coulomb scattering for the thin barrier case (<5 nm) and dominated by phonon scattering for the thick barrier case (>5 nm). At a barrier thickness of 4 nm, compared to a structure with in-situ ALD Al₂O₃ with the mobility of 6807 cm²/Vs, which corresponds to an interfacial charged defect density of 1.1×10¹³ cm^-2, the structure with in-situ CVD Al₂O₃ showed higher mobility (8883 cm²/Vs), which corresponds to a lower charged defect density (5×10¹² cm^-2).