An asymptotic procedure is proposed for solving the kinetic equations of carriers and phonons (treated as a participating species), in the presence of traps for electrons. Fluid-dynamical equations are constructed, which are related to extended thermodynamical ones, for the chemical potentials of carriers, temperature and drift velocity. The generation-recombination rate is compared with the Shockley-Read-Hall model. In the drift-diffusion approximation, the constitutive laws are derived and the Onsager relation is recovered.