Introduction to the principle of LED lighting

Light-emitting diodes are made of III-IV compounds, such as GaAs (gallium arsenide), GaP (gallium phosphide), GaAsP (gallium arsenide phosphide) and other semiconductors, and its core is a PN junction. Therefore, it has the I-N characteristics of a general P-N junction, that is, forward conduction, reverse cut-off, and breakdown characteristics. In addition, under certain conditions, it also has luminous properties. Under 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. Part of the minority carriers (few carriers) entering the opposing region recombine with the majority carriers (many carriers) to emit light, as shown in Figure 1.
Assuming that the luminescence occurs in the P region, then the injected electrons directly recombine with the holes in the valence band to emit light, or they are first captured by the luminescent center and then recombine with the holes to emit light. In addition to this luminescent recombination, some electrons are captured by the non-luminescent center (this center is near the middle of the conduction band and the intermediate band), and then recombine with holes, and the energy released each time is not large, so visible light cannot be formed. The greater the ratio of luminescent recombination to non-luminescent recombination, the higher the photon quantum efficiency. Since the recombination emits light in the minority carrier diffusion region, the light is only generated within a few μm near the PN junction.
Theory and practice have proved that the peak wavelength λ of light is related to the band gap Eg of the semiconductor material in the light-emitting region, that is, λ≈1240/Eg (mm) where the unit of Eg is electron volts (eV). If visible light can be produced (purple light with a wavelength of 380nm to red light with a wavelength of 780nm), the Eg of the semiconductor material should be between 3.26 and 1.63eV. Light with a wavelength longer than red light is infrared light. At present, there are infrared, red, yellow, green and blue light-emitting diodes, but the cost and price of blue light-emitting diodes are very high, and they are not widely used.