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A direct band gap has its conduction band minimum and valence band maximum at the same crystal momentum. Near-edge absorption or emission can therefore conserve momentum with the electron and photon alone, since optical photon momentum is small on the Brillouin-zone scale. An indirect band gap has its extrema at different wavevectors and normally needs a phonon or another momentum-transfer process in addition to the photon.

Direct-gap materials are usually preferred for efficient light emission, because the radiative transition avoids the extra phonon-assisted process. Their stronger near-edge absorption also permits thinner photovoltaic absorbers. Indirect-gap materials can nevertheless make excellent solar cells: sufficiently thick optical paths, light trapping, long carrier lifetimes and suitable material quality can compensate for weaker absorption. The direct/indirect distinction alone does not rank every device.

This momentum distinction and the difference between bound exciton lines and the free-pair continuum are treated together in the primary effective-mass absorption theory.^[ref]

Roger James Elliott calculated how excitons alter optical absorption near a semiconductor band gap. Treating the electron-hole attraction with an effective mass model produces bound-state absorption lines as well as a modified continuum. The 1957 theory distinguishes direct and phonon-assisted indirect transitions, and shows how optical selection rules change the edge's shape and strength. It connects microscopic bound states with features visible in a measured absorption spectrum.^[ref]