Upon room‐temperature deposition of minute amounts of Cs on InAs(110) surfaces, one induces probably the highest Fermi‐level pinning position (≊0.6 eV) for a semiconductor above the conduction‐band minimum. Synchrotron‐radiation core‐level photoemission spectroscopy was used to follow the Fermi‐level movement from the shift of the In 4dand As 3dcore levels as a function of Cs coverages at room temperature. Already at very low coverages the Fermi level reaches an extremely high maximum above the conduction‐band minimum. The maximum of the Fermi‐level position correlates fairly well with the ionization energy of the individual atoms, as expected in the framework of the theory of donor‐type surface states induced by metallic adatoms. We thus consider that individual Cs adatoms produce donor‐type surface states placed at ≊0.6 eV above the conduction‐band minimum. This induces a very strong downward band bending which suggests the existence of a two‐dimensional electron gas at the open, nearly clean InAs(110) surface.