We calculate the electronic structure of geometrically ideal interfaces between column III metals and silicon. For a single neutral adatom the associated metallic Fermi energy must lie between the energy (0,+), for the removal of an electron, and the energy (0,−), for adding an electron, but is otherwise unrestricted. This circumstance also holds for many uncoupled adatoms and will produce pinning if the (0,+) and (0,−) levels are in the gap and the semiconductor Fermi energy is outside this range. At still larger coverages the occupied (0,+) levels and the (0,−) levels shift and broaden in energy until the gap between them disappears, pinningn‐type andp‐type semiconductors at the same energy. We find that the (0,+) level for a single aluminum adatom is in the silicon gap and that the (0,−) level is in the conduction band. In addition, the (0,+) level moves towards the valence band maximum as the metal coverage is increased.