A simple one‐speed solution to the Boltzmann equation is used to evaluate the mobility and diffusion coefficient for carriers in forward‐biased semiconductor barriers. The analysis shows that although the average kinetic energy of carriers remains near thermal equilibrium, the mobility and diffusion coefficient are strongly reduced by the built‐in field. Conventional macroscopic transport equations, which treat the carrier mobility and diffusion coefficient as single valued functions of the kinetic energy will improperly treat transport in forward‐biased barriers. The results are important for the careful analysis of metal–semiconductor and heterojunction diodes. ©1995 American Institute of Physics.