The threshold condition for laser action is usually achieved when the population difference between the initial and final energy levels of the laser transition reaches a critical value, determined by the equality between optical gain and losses. But the threshold condition can also be achieved by increasing the oscillator strength of the laser transition itself, while the population difference is held constant. This forms the basis of a new class of semiconductor lasers, a notable feature of which is broad wavelength tunability (on application of an electric field) in the technologically important mid-infrared region of the spectrum.