Key points are not available for this paper at this time.
The intensity of optical absorption close to the edge in semiconductors is examined using band theory together with the effective-mass approximation for the excitons. Direct transitions which occur when the band extrema on either side of the forbidden gap are at the same K, give a line spectrum and a continuous absorption of characteristically different form and intensity, according as transitions between band states at the extrema are allowed or forbidden. If the extrema are at different K values, indirect transitions involving phonons occur, giving absorption proportional to () ^1{2} for each exciton band, and to () ^2 for the continuum. The experimental results on Cu₂O and Ge are in good qualitative agreement with direct forbidden and indirect transitions, respectively.
R. J. Elliott (Sun,) studied this question.