The analysis of existing theoretical models for the description of relaxation processes in semiconductors and semiconductor barrier structures has been carried out. The criteria allowing the use of transport models of nonequilibrium charge carriers in a relaxation or recombination semiconductors have been formulated. It is shown that a space charge region (SCR) of semiconductor barrier structure under reverse bias can be considered as a relaxation semiconductor in which the relaxation time of charge carriers is determined by the span time while the base maintains the properties of a recombination semiconductor where the electric field is absent.