Recent work has indicated that a significant number of electrons and holes remain in the free-exciton form in silicon at room temperature, a finding which, if supportable by experimental evidence, requires the inclusion of excitons in diode and solar cell theory. Excitons, although neutral, may contribute to device currents by diffusing to the junction region where they may be dissociated by the field. A generalized three-particle theory of transport in semiconductors is presented. The results of application of the theory to silicon devices indicate a decrease in the dark saturation current as well as an increase in light-generated current when excitons are incorporated in the theory so long as exciton diffusion length exceeds that of the minority carriers. The work includes suggestions for experimental methods to confirm exciton involvement and to estimate the value of the exciton-binding parameter from spectral response measurements on solar cells.