The problem of the creation of a sub-Poissonian radiation source on the basis of a semiconductor laser with an external cavity is considered, which operates, from the viewpoint of quantum optics, in the single-mode regime. The conditions of the generation of sub-Poissonian radiation, under which the whole set of sub-threshold modes, including the nearest modes of the external (long) cavity, is completely suppressed at room temperature, were experimentally revealed and theoretically studied. Single-frequency low-noise lasing is of interest not only for the creation of nonclassical light sources, but also for sensitive spectroscopic applications in the classical domain.