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We have quantitatively evaluated the interface trap density inside the conduction band (CB) of InxGa1-xAs metal-oxide-semiconductor (MOS) structures and have systematically investigated the impact of the interface traps inside the CB on the inversion-layer mobility in InxGa1-xAs MOS field-effect transistors with various interface structures. Furthermore, we have tried to clarify the physical origin of the interface traps inside the CB. It was found that a large number of interface traps are distributed inside the CB of InxGa1-xAs inducing Fermi level pinning (FLP), the energy level of which is tunable by changing the InxGa1-xAs MOS interface structures. Furthermore, it was clarified that FLP inside the CB degrades the mobility in the high inversion carrier concentration region. We also found from the obtained results and reported theoretical results that a possible physical origin of the interface traps inside the CB is As-As dimers formed at the interfaces.