In the paper, a possibility of increasing the terahertz (THz) radiation intensity under optical interband pumping in the epitaxial GaAs layer doped with shallow donors has been studied. An increase in the intensity of THz radiation was achieved by implementation of conditions for stimulated interband radiation in the near-IR range, which depopulated intensively the donor ground state. The photoluminescence spectra of the samples were measured by Fourier spectrometer. Photoluminescence spectra were recorded in the near-IR and THz ranges in the sub- and post-threshold working conditions of radiation generation in the near-IR range. In the THz spectra, a change in behavior of the dependence of the radiation intensity on pumping was observed. The change was due to a decrease in the radiative lifetime of electrons at the impurity level.