In this work we present results concerning the fundamental equations of charge carrier transport in semiconductor structures. We discuss the modeling of the recombination term in the charge transport equations for each type of carriers that respects the charge conservation law. It was obtained that under stationary conditions and equal generation rates of electrons and holes the recombination rates of both kinds of carriers must be matched. Under low excitation conditions (linear regime) the recombination rate can be expressed as a linear combination of the variation of the carrier concentrations deltan and deltap. Explicit calculation of the charge variation has been carried out in the framework of the Shockley-Read-Hall statistics. The study of the space charge in non-equilibrium has been addressed. It was found that the quasi-neutrality condition contradicts the usual assumption deltan=deltap. Finally, boundary conditions for the Poisson equation have been presented