Light-emitting diodes are made of III.-IV. group compounds, such as GaAs (gallium arsenide), GaP (gallium phosphide), GaAsP (gallium arsenide phosphate) and other semiconductors, and their core is a PN junction. Therefore, it has the I-N characteristics of the general P-N junction, that is, forward conduction, reverse cut-off, and breakdown characteristics. In addition, under certain conditions, it also has luminescent properties. At the forward voltage, electrons are injected into the P region from the N region, and holes are injected into the N region from the P region. A minority of carriers (few tons) entering the opposing region partially recombines with the majority of carriers (polytons) and emits light. Assuming that luminescence occurs in the P region, the injected electrons are directly recombined with valence band holes to emit luminescence, or they are first captured by the luminescence center and then recombined with the holes. In addition to this luminescent recombination, some electrons are captured by a non-luminescent center (this center is near the middle of the conduction band and the mediator band), and then recombined with holes, each time releasing little energy and cannot form visible light. The greater the proportion of luminescent recombination to the non-luminescent recombination, the higher the optical quantum efficiency. Since the recombination emits light in the minority diffusion region, light is only generated within a few μm of the * near PN junction surface.