Abstract Solid-state sources covering the wave number range from 5 to 120 cm −1 are considered. Experimental data on the negative effective masses, the inverted cyclotron resonance transitions, as well as the light-to-heavy hole band transition inversion are compared with existing theoretical models. On the basis of this comparison, the phonon-difference process inversion is proposed as a new mechanism of stimulated FIR radiation. Contrary to the previous models which require complicated carrier-energy band structures, like that in p-type Ge, the two-phonon difference radiation is expected to act both in p- and n-type materials. Magnetic fields and cooling down to liquid helium temperatures are not necessary for its operation. Crystalline Si, Ge, A 3B 5, A 2B 6 and other semiconductor compounds are candidates for creating lasers on phonons.