The semiphenomenological transport equations that are commonly used in the theory of semiconductors have been rewritten in variables appropriate to the discussion of an illuminated isothermal semiconductor in which the band gap is a linear function of one of the position coordinates. It has been found necessary to neglect those terms in the transport equations that arise from variations in mobility and effective mass. The first section considers electrical conduction in a uniformly doped specimen under weak illumination; deviations from Ohm's law were found to be small. In the second section, the potential distribution in a highly illuminated sample of such material, both with and without a junction present, is found. The major contribution to the total potential is the same as that found earlier by Tauc. These results are then used to determine the best geometry for a solar‐energy converter in which such materials are used, and to calculate the efficiency of this converter as a function of illumination. The greatest efficiency possible was found to be 43%.