The injected minority carriers in a p-n junction can recombine both radiatively and nonradiatively. The recombination radiation can interact with valence electrons and be absorbed or interact with electrons in the conduction band to stimulate an identical photon. When the injected carrier concentration becomes large enough, the stimulated emission can exceed the absorption so that optical gain occurs. To obtain oscillation, it is necessary to have positive feedback of a portion of the radiation. This feedback is generally provided by a pair of cleaved facets that are perpendicular to the waveguide axis. Oscillation occurs when the single-pass gain between the cleaved mirrors equals the total loss over this same distance. These concepts are applied to the commonly encountered double heterostructure lasers of GaAs-AlxGa1-xAs to illustrate the basic properties of diode lasers.
H. C. Casey, H. C. Casey,
"Introduction To Diode Lasers", Proc. SPIE 0239, Guided Wave Optical and Surface Acoustic Wave Devices: Systems and Applications, (25 February 1981); doi: 10.1117/12.959180; https://doi.org/10.1117/12.959180