1. Led luminescence mechanism: the terminal voltage of pn junction constitutes a potential barrier. When the voltage of positive bias voltage is added, the majority of the carriers in the p-zone and n regions are diffused to each other. Because the electron mobility rate is much larger than the hole migration rate, a large number of electrons will be distributed to the p area, which constitutes the injection of the minority carrier in p area. These electrons are compounded with the hole in the valence band, and the energy is released in the form of light energy. This is the principle of pn junction luminescence.
2. Led luminescence efficiency: generally referred to as the external quantum efficiency of the component, the product of the internal quantum efficiency of the component and the extraction efficiency of the components. So-called internal quantum efficiency of the component, in fact, the component itself electro-optic conversion efficiency, mainly with the characteristics of the component itself (such as a band of component materials, defects and impurities), base composition and crystal structure of components, etc. And the extraction efficiency of the component refers to the number of photons produced within the component, which can be measured in the outside of the component after the absorption, refraction and reflection of the component itself. Therefore, the factors about the extraction efficiency include the absorption of the components, the geometry of the components, the difference in refractive index of the components and the packaging materials, and the scattering characteristics of the component structure. The product of the internal quantum efficiency of the component and the extraction efficiency of the component is the luminous effect of the entire component, which is the external quantum efficiency of the component. Early component development focused on improving its internal quantum efficiency, by improving the barrier is the main method of crystal quality and change of crystal structure, that is not easy to convert electrical energy into heat energy, thereby indirectly improve the luminous efficiency of leds, which can obtain 70% internal quantum efficiency, the theory of but the internal quantum efficiency is almost close to the theoretical limit. In such a situation, it is impossible to improve the total amount of light by improving the internal quantum efficiency of the components, thus improving the extraction efficiency of components becomes an important research topic. The current method is mainly: the change of grain shape - tip structure, surface roughing technology.
3, led electrical characteristics: current control device, UI curve of load characteristic similar pn junction, is extremely small changes to the voltage will cause great changes of the forward current (index level), reverse leakage current is very small, has a reverse breakdown voltage. In actual use, should choose. Led forward voltage decreases with temperature and has negative temperature coefficient. Leds consume power, and some convert to light energy, which is what we need. The rest is converted to thermal energy, making the knot warm up. The heat emitted can be expressed as.
4, led optical properties: led provides a full width at half maximum monochromatic light, as a result of the semiconductor energy gap decreases with temperature rise, therefore its emission peak wavelength with the temperature rising and growth, namely the spectrum redshift, temperature coefficient is + 2 ~ 3 a /. The led luminance l is approximately proportional to the positive current: k is the proportional coefficient. The current increases, and the luminescence brightness is also approximately increased. In addition, the luminance brightness is also related to ambient temperature. When the ambient temperature is high, the compound efficiency decreases and the luminescence intensity decreases.
5. Led thermal characteristics: small electric current, led temperature rise is not obvious. If the ambient temperature is high, the main wavelength of the led will be redshifted, and the brightness will decrease, and the uniformity and consistency of the leds will be reduced. In particular, the temperature rise of the large display is more significant to the reliability and stability of the led. So cooling design is critical.
6. Led life: the long working time of led will cause aging, especially for high-power led, the problem of light failure is more serious. In measuring the led life, only to the damage of the lamp for led the end of life is not enough, should be specified by the percentage of light attenuation to the led the life of the led, such as 35%, so it makes more sense.
7. High power led packaging: mainly consider heat dissipation and light emission. In heat dissipation, the copper base heat liner is connected to the aluminum base radiator, which is welded as a connection between the grain and the heat liner. The heat dissipation effect is better and the cost performance is higher. In the light aspect, the chip inversion technology is adopted, and the reflection surface of the base and side is reflected to reflect the wasted light energy, which can get more and more of the light.
8, white led: natural white led spectrum there are three main types: the first is a more mature and has been commercialized blue chip + yellow phosphor for white light, the lowest cost of white light, but blu-ray grain luminescence wavelength offset, the change of intensity and phosphor coating thickness change affects the uniformity of white light, and the spectrum is a narrow strip, color is not complete, color temperature on the high side, low color, soft light to the eyes. The most suitable for human eyes is the sunlight, the continuous spectrum of incandescent lamp is the best, the color temperature is 2500k, the color index is 100. So the white light needs to be improved, such as the addition of glowing processes to improve the spectrum, making it continuous and wide enough. The second is ultraviolet light red, blue, green or purple light chip + tricolor phosphors for white light, luminous principle similar to the fluorescent lamp, the method better, color and uv - led don't participate in white color, so the uv - led wavelength and intensity fluctuations to match the white light is not particularly sensitive, and can be made of the selection and proportion of various phosphor modulation out acceptable color temperature and color of white light. However, the efficiency of the effective conversion efficiency of phosphor powder is low, especially the efficiency of red phosphor powder. This kind of phosphor luminescence stability difference, light failure, the choice of ultraviolet wavelength of phosphor powder, uv - led production difficulty and the development of anti-uv packaging materials also need to overcome difficulties. And the third is to use tricolor principle by RGB three high brightness led mixed into white light, the advantage of this method is not required to pass through conversion of phosphor and directly with the white light, in addition to avoid phosphor conversion loss and get better luminous efficiency, more can separate control red, green and blue leds luminous intensity, to achieve full color effect (color temperature), and can be obtained by led wavelength and intensity of choice a better play of colour. However, the problem of this method is that the conversion efficiency of green light is low, the confusion is difficult, and the driving circuit is complicated. In addition, because these three kinds of light colors are heat sources, the heat dissipation problem is three times as many as other forms of packaging, increasing the difficulty in use. Polarized led and three-wavelength all-colored white leds will be the future development direction.
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