LED light emission principle

Light-emitting diodes are made of III-IV compounds, such as GaAs (gallium arsenide), GaP (gallium phosphide), GaAsP (phosphorous gallium arsenide) and other semiconductors, the core of which is a PN junction. Therefore, it has the IN characteristics of the general PN junction, ie, forward conduction, reverse termination, 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 part of the minority carriers (low births) entering the opponent's area is combined with majority carriers (multiple sons) to emit light.

Assuming that the light emission occurs in the P region, the injected electrons directly recombine with the valence band holes to emit light, or they are first captured by the light emission center and then recombine with the holes to emit light. In addition to this light-emitting compound, some electrons are captured by the non-emission center (the center is near the middle of the conduction band and the intermediate zone), and then recombine with the hole. The energy released is not large enough to form visible light. The greater the ratio of the amount of emitted light relative to the amount of non-light emitting complex, the higher the light quantum efficiency. Since recombination emits light in the minority scattering region, light is generated only within a few μm of the PN junction surface.

Theoretically and practically, it is proved that the peak wavelength λ of light is related to the bandgap Eg of the semiconductor material in the light emitting region, ie, λ≈1240/Eg (mm). The unit of Eg is electron volts (eV). If it can produce visible light (wavelength from 380nm violet to 780nm red), the Eg of the semiconductor material should be between 3.26 ~ 1.63eV. The light longer than the red wavelength is infrared light. There are now infrared, red, yellow, green and blue light emitting diodes, but the cost and price of the blue diodes are high and they are not widely used.