In this work we present results concerning the fundamental equations of charge carrier transport in semiconductor structures. We discuss a correct modeling of the recombination terms that do not violate the charge conservation law. We 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 excess concentrations of electrons and holes and only in very precise situations a lifetime can be defined. It has been shown that the bulk space charge established on distances of the order of the Debye's length cannot influence the current-voltage characteristics of a device in linear regime.