The physical mechanism for creation of intraband population inversion between levels of quantum dots under injection of electron-hole pairs of suggested. The method is based on employment of generation of interband radiation providing fast depopulation of quantum dot ground level. Spontaneous far-IR radiation from diode laser structures with InGaAs/AlGaAs quantum dots connected with intraband hole and/or electron transitions between levels of size quantization in quantum dots was found and investigated for the first time. Spontaneous far-IR radiation is observed only under simultaneous generation of stimulated near-IR radiation connected with interband carrier transitions. Far- IR emission is observed also from laser structures with InGaAs/GaAs quantum wells. Intensity of this radiation is about of order less then intensity of radiation from structures with quantum dots. Qualitative explanations of phenomena observed are proposed.