Resorting to a theory of responses to thermal and mechanical perturbations, based on statistical irreversible thermodynamics for systems arbitrarily away from equilibrium, we obtain the diffusion and mobility coefficients in a highly photoexcited plasma in semiconductors in the presence of an electric field. They are dependent on the evolution of the nonequilibrium thermodynamic state of the system. From these transport coefficients we derived a generalized Einstein relation for ultrafast transient regimes and for non-Ohmic conditions. In all cases this generalized Einstein law acquires values that are field dependent and larger than those in its original form only valid in steady-state conditions and the limit of weak fields. Numerical results appropriate for the case of a GaAs sample are presented. © 1995 The American Physical Society.52191393613945