It is pointed out that a model which agrees well with the observed properties of semiconducting glasses is an attractive Hubbard model of localized states. Such a model has no gap for two-electron excitations but an energy gap for one-electron ones. The suggested physical model for a two-electron excitation is a new covalent bond in the structure, which is severely localized. It is also proposed that the one-electron excitation spectrum is wholly, or almost wholly, extended, and all observed gaps are identical with the mobility gap.

• Received 25 November 1974

DOI:https://doi.org/10.1103/PhysRevLett.34.953