The operation of semiconductor lasers is based on light amplification in the active region of a semiconductor material. Most semiconductor lasers use radiative quantum transitions between two allowed energy bands of a semiconductor (interband transitions): the conduction band and the valence band. The active region of lasers based on interband transitions is characterised by a thermodynamically non-equilibrium distribution of carriers, which in the conduction band has the shape typical of a degenerate n-type semiconductor, and in the valence band, that of a p-type semiconductor. Such distribution of charge carriers is called “double-degenerate” (see Fig.).

The optical gain in a semiconductor active medium can be very high (up to 10⁴ cm^-1); so the size of the active element and the cavity length in a semiconductor laser are extremely small (cavity length ~ 50 um to 1 mm, and in vertical cavity lasers it equals fractions of a micron). In addition to such compactness, a semiconductor laser is characterised by low inertia (~ 10^-9 s), high efficiency (up to 50%), the possibility of spectral tuning and a wide choice of substances for generation over a broad spectral range from λ = 0.3 microns to 30 microns.

The most important method for pumping the semiconductor lasers is injection through p-n-junctions or heterojunctions, which allows the direct conversion of electrical energy into coherent radiation. Semiconductor lasers pumped using this method are called “injection lasers” or “laser diodes” (LD).

In injection lasers (LD), double-degenerate state in the active region is created by a high forward current flow through the semiconductor diode induced by injection of excess carriers in the layer adjacent to the junction. Semiconductor lasers based on heterostructures (heterolasers) are the most widely used due to their moderate threshold current density at room temperature. The most effective heterolasers contain two heterojunctions: one p-n junction injecting electrons (emitter) and one p-p junction limiting diffusive spreading of carriers from the active layer; the active region lies between them.