Light-emitting diode into a lower energy level and releases energy in the form of a photon. The wavelength of the light emitted, and thus its color, depends on the band gap energy of the materials forming the "p-n junction". In silicon or germanium diodes, the electrons and holes usually recombine by a "non-radiative transition", which produces no optical emission, because these are indirect band gap materials. The materials used for the LED have a direct band gap with energies corresponding to near-infrared, visible, or near-ultraviolet light. LED development began with infrared and red devices made with gallium arsenide. Advances in materials science
Laser diode frequency doubling of near-IR VECSEL emitters to produce blue and green light. External-cavity diode lasers are tunable lasers which use mainly double heterostructures diodes of the AlGaAs type. The first external-cavity diode lasers used intracavity etalons and simple tuning Littrow gratings. Other designs include gratings in grazing-incidence configuration and multiple-prism grating configurations. Laser diodes have the same reliability and failure issues as light emitting diodes. In addition they are subject to "catastrophic optical damage" (COD) when operated at higher power. Many of the advances in reliability of diode lasers in the last 20 years remain proprietary to their developers. The