Model for amorphous semiconductors predicts energy band gaps

How do amorphous semiconductors get their energy band gaps? Experimental results over the past few years have shown that amorphous silicon and germanium films have gaps between valence and conduction bands that are similar to the gaps that exist when these substances are crystalline. Efforts to justify these gaps theoretically bog down because of the lack of Bragg planes in amorphous substances; energy band gaps in crystals arise because of scattering by the planes, and most solid‐state theorists have assumed that some kind of periodicity is needed to explain gaps.