The chemical potential for electrons in a solid. This is the energy in a solid at which the average number of particles per quantum state is ½‎; i.e. one half of the quantum states are occupied. The Fermi level in conductors lies in the conduction band (see energy bands), in insulators it lies in the valence band, and in semiconductors it falls in the gap between the conduction band and the valence band. At absolute zero all the electrons would occupy energy levels up to the Fermi level and no higher levels would be occupied. It is named after the Italian-born US physicist Enrico Fermi (1901–54).