COMMENTS ON THE THEORY OF AMORPHOUS SEMICONDUCTORS.

The author presents comments on the nature of the intrinsic band tails in pure amorphous silicon, the Lloyd model, the influence of shallow impurity levels on band tails, the effect of disorder on deep levels, the question of whether excitons exist in amorphous semiconductors, the conductivity problem, the mobility edge problem, and optical absorption. New results are presented concerning the Lloyd model, shallow and deep impurity or defect levels, and excitons. In particular, it is concluded that excitons continue to exist as bound states only when the exciton lies below a Lifshitz limit; otherwise they survive only as resonances.