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In this paper, we investigated the inversion-layer scattering mechanisms of HfAlO In0.53Ga0.47As nMOSFETs with a plasma-PH3 passivation layer to understand the physical origins of mobility enhancement compared with a nonpassivated device. It has been found in low Eeff that the mobility enhancement caused by the plasma- PH3 passivation is due to the reduction in Coulomb scattering caused by reduction in Dit in the upper half of the band gap, as shown from the plasma-PH3 reaction, which involves P-for-As exchange reaction that reduces the As vacancy sites. Plasma- PH3 passivation also results in reduction of the phonon scattering caused by soft optical phonons in the HfAlO, which has weak temperature dependence. This is due to the thicker passivation layer of the plasma-PH3 -passivated device compared with the interfacial layer present in the nonpassivated device. Plasma- PH3 passivation also helps to reduce the interface dipole scattering caused by fluctuating dipoles at the HfAlO/ In0.53Ga0.47As interface, which may be attributed to the interdiffusion of elements from HfAlO and In0.53Ga0.47As . In addition, it is found that effective channel mobility is decreased as gate length reduces until sub-100 nm, due to increased effects of neutral scattering of charges near the source/drain as well as the effect of ballistic transport, thus possibly degrading mobility with further device scaling.