We consider effects of diffusion in the photogenerated carrier system in highly photoexcited polar semiconductors. We develop a quantum quasihydrodynamic description of the system based on the nonequilibrium statistical operator formalism. We derive a generalized Fick's diffusion equation for the charge density of the carriers, with the ambipolar diffusion coefficient obtained at the microscopic level and depending on the evolving macroscopic (nonequilibriium thermodynamic) state of the sample. A detailed numerical calculation for the case of GaAs is done, obtaining good agreement with experimental data.48151087310884