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=== Generation of stimulated emission ===
In the absence of stimulated emission (e.g., lasing) conditions, electrons and holes may coexist in proximity to one another, without recombining, for a certain time, termed the "upper-state lifetime" or "recombination time" (about a nanosecond for typical diode laser materials), before they recombine. A nearby photon with energy equal to the recombination energy can cause recombination by [[stimulated emission]]. This generates another photon of the same frequency, [[polarization (waves)|polarization]], and [[phase (waves)|phase]], travelling in the same direction as the first photon. This means that stimulated emission will cause gain in an optical wave (of the correct wavelength) in the injection region, and the gain increases as the number of electrons and holes injected across the junction increases. The spontaneous and stimulated emission processes are vastly more efficient in [[direct bandgap]] semiconductors than in [[indirect bandgap]] semiconductors; therefore [[silicon]] is not a common material for laser diodes.