<b>Electronic Processes in Non-Crystalline</b>

TEN YEARS ago our theoretical understanding of electrons in non-crystalline materials was rudimentary. The classification of materials into metals, semiconductors, and insulators was based on band theory, and band theory starts from the assumption that the material is crystalline. According to band theory, an insulator is a material with an energy gap between the conduction and valence bands, and a transparent insulator is one in which the gap is greater than the quantum energy of visible light. Ordinary soda glass is an insulator and transparent; a gap seemed to exist, but we did not know how to describe the gap. Even now we do not know how to calculate it, but the concepts that we have to use are fairly clear.

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4. 4. Proceedings of the 4th International Conference on Amorphous and Liquid Semiconductors, held (1971) in Ann Arbor, Michigan, eds. M. H. Cohen and G. Lucovsky. (ref: (1972) /. non-cryst. Solids 8-10.)
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6. 6. Proceedings of the 6th International Conference on Amorphous and Liquid Semiconductors, held (1975) in Leningrad, U.S.S.R., ed. B. T. Kolomiets, Academy of Sciences of U.S.S.R., Nauka, 1976. (refs: (1976) Leningrad, Electronic Phenomena in Non-Crystalline Semiconductors; (1976) Leningrad, Structure and Properties of Non-Crystalline Semiconductors.)