Current saturation has been observed in semiconducting CdS for different values of the conductivity. It is found that a small interval in the carrier condentration exists, above which a stimulated emission of phonons is the main mechanism giving rise to current saturation, and below which sound amplification is mainly due to a classical interaction between acoustical wave and carrier bunching. In the small intermediate range the two mechanisms coexist. It is also found that the stimulated emission process can be superimposed to classical amplification, so that the crystal behaves as a medium with negative acoustical losses.