Electronic properties, related to the correlation between quantum states and Fermi level (E_F) at the semiconductor boundaries, are discussed as one of the essential issues for understanding nanometer-scale physics and its technological application. To understand the mechanism of surface E_F stabilization, the origin of two-dimensional electrons in the native electron accumulation layer was explored using a low-temperature scanning tunneling microscope in ultra-high vacuum at the clean (111)A surface of the undoped InAs thin film grown by molecular beam epitaxy. According to direct comparison of densities between surface defects and accumulated electrons, it was found that the electron accumulation layer was introduced by the two-dimensionally distributed native donor point defects composed of In adatoms and In antisites at the density of 10¹² cm⁻². The mechanism of the surface E_F pinning was explained using a model based on the shallow defect band formation due to highly formed surface donors. It was shown that this surface band was composed of the hydrogenic donor bound states and delocalized due to many-body effects induced by the Mott transition in correlated donor-electron system at the clean surface, and was not composed of the atomic orbitals of point defects or the dangling bonds of surface atoms.