LED is a forward biased PN junction diode made of semiconductor materials. Its luminescence mechanism is that when the positive current is injected at both ends of the PN junction, the injected non-equilibrium carriers (electron-hole pairs) emit light in the process of diffusion, and this emission process mainly corresponds to the spontaneous emission process of light. According to the position of light output, led can be divided into surface emitting type and side emitting type. The most commonly used LED is InGaAsP/InP dual heterojunction LED.
The principle of light emitting diode can also be explained by the band structure of PN junction. The materials used to make semiconductor light-emitting diodes are heavily doped. In the state of thermal equilibrium, there are many electrons with high mobility in the N-region and many holes with low mobility in the P-region. Due to the restriction of barrier layer of PN junction, natural recombination of them cannot occur under normal conditions, and when a forward voltage is applied to the PN junction, the electrons in the conduction band of the channel region can escape the barrier of the PN junction and enter the side of the P region.
Therefore, near the PN junction, slightly to the side of the P region, when the electrons in the high energy state meet with the holes, luminous recombination is generated. The light emitted by this luminous compound belongs to spontaneous radiation, and the wavelength of the radiation light is determined by the forbidden band width Eg of the material, i.e λ＝1.24μm?;eV/Eg -led has the remarkable advantages of high reliability, long continuous working time at room temperature, good linearity of optical power and current, etc., and its price is very cheap because the technology has been developed relatively mature. Therefore, in the design of some simple optical fiber sensor, if the LED can be competent, the use of it as a light source can greatly reduce the cost of the whole sensor. However, LED's luminescence mechanism determines that it has many shortcomings, such as small output power, large emission Angle, broad spectral line and low response speed. Therefore, in some sensor designs that need high power, fast modulation rate and good monochrome light source, we have to choose other light sources with higher performance at the cost of increasing cost.
Light emitting diodes made of different materials emit different wavelengths of light due to their different bandgap widths. In addition, some materials are different in composition and doping. For example, some materials have very complex energy band structure, and the corresponding indirect transition radiation, so there are a variety of light-emitting diodes.
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