Led is the hottest topic in IT today. As long as the word LED is added, the product will double in value. Looking at the propaganda of the manufacturers, the media's vigorous hype, almost let the LED reach the peak of the peak. However, no one has doubts about the sacred term LED. We have made a small interview with this, hoping to help everyone. LED is long How hot is the LED in reality? It can be described as a mess of fire. Shanghai World Expo The 2008 Beijing Olympic Games and the 2010 Shanghai World Expo will let the world recognize the unlimited business opportunities of LED engineering lighting. In 2009, China's lighting sales exceeded 200 billion yuan. Faced with the good prospects and huge temptations of the semiconductor market, a battle for the high point of semiconductor lighting emerging industries has quietly started in the world. How close is the relationship between LED and us? LEDs are everywhere in our side. In the civilian sector, LEDs are mainly used in the following major areas: LED dot matrix A. Display and communication number display the application of the light source. The application of LED lamps has the advantages of shock resistance, shock resistance, fast light response, power saving and long life. LED tail light LED decoration B. Application in the automotive industry. Automotive lights include dashboards, audio indicators, backlights for switches, reading lights and external brake lights, tail lights, side lights, and headlights. Take advantage of LED long life and fast response. C. The LED backlight is a backlight with high-efficiency side illumination. Mainly used in LED backlight TV, LED light source projector. Take advantage of the wide color gamut, thin, and low power consumption of LEDs. D, LED lighting. The use of LED light source energy saving features. LED "toy" E, decoration, instructions. Mainly used in toys, decorative lights, clothing and so on. The advantages of small size, no pollution, rich color, not easy to break and safety of low pressure use are utilized. LED bulb F, home indoor lighting. Use LED to save power, pollution-free, long life. Hitachi projector Hitachi 78WX Review Quote Evaluation Forum Does the LED have any disadvantages? LEDs have shortcomings. Simply put, the price is too high. The advantages of LED have been hyped by major media. Even the national propaganda is that LED has only advantages and no shortcomings. However, the current LED penetration rate is still not high, the reason is still the price issue. Why is the LED price high? Low-brightness LED products are priced at a lower price. Due to technical and manufacturing processes, high-brightness LEDs are difficult to reduce costs. The shortcomings of LED can be subdivided into low luminous efficiency, high packaging cost, high price of high-performance materials, and ultimately lead to high manufacturing cost of LED, and the market price is naturally high. Doesn't it all mean that LEDs are energy-efficient? Why do LEDs have low luminous efficiency? Energy saving means high light efficiency and low heat. However, the so-called energy-saving LEDs are mainly manifested in low-brightness LED products, and high-brightness LEDs are not energy-saving. Today's application of LED products mainly uses multiple, dot matrix and other methods, and the application of low-brightness LED products. LED dot matrix Since the luminous efficiency of the LED itself is relatively low, about 70% of the input power is converted into heat. This is the trend diagram of the light effect of the lW power LED with the input current. We know that the LED performance gradually decreases with the increase of the current, and the white light drops most obviously. Along with the decline in light efficiency, a large amount of thermal energy is generated. lW power LED light effect with input current trend chart It is said that LED luminous efficiency is low, mainly refers to high-brightness LED. In reality, high-brightness LED is more useful. For example, LED light source projectors require a single high brightness LED product. At this stage, high-brightness, mature LED projector products are few, and the price is not cheap, using a special high-end material, high brightness with high cost, lost practical value in the civilian market. Isn't LED cheap? Low-end, low-brightness LEDs are cheap, but high-brightness, high-light-efficiency LEDs are expensive, and there is almost no price/performance ratio at this stage. To understand LEDs, we must start with its structure. The semiconductor light emitting device includes a semiconductor light emitting diode (LED), a digital tube, a symbol tube, a meter tube, and a dot matrix display (referred to as a matrix tube). In fact, each of the digital tube, the symbol tube, the meter tube, and the matrix tube is a light emitting diode. LED, the English full name Light Emitting Diode, LED, is a solid-state semiconductor device that can directly convert electricity into light. The core of the LED is a semiconductor wafer. One end of the wafer is attached to a bracket, one end is a negative pole, and the other end is connected to the positive pole of the power supply, so that the entire wafer is encapsulated by epoxy resin. a wide variety of LEDs Our common LEDs are straw hat, oval, flat head, and patch. The text symbol of the LED is “VDâ€. The graphic symbol is to add two small outward arrows next to the common diode symbol. The triangle indicates the positive pole and the short bar indicates the negative pole. Light-emitting diodes are also diverse: 1, from the light color is divided into red, green, yellow and other colors of visible light and infrared light; 2, from the shape of a cylindrical, square and various special shapes; 3, from the volume is divided into large, medium and small and many other specifications. LED working principle Light-emitting diodes operate on a forward-biased PN junction diode made of a semiconductor material. The illuminating mechanism is that when a forward current is injected at both ends of the PN junction, the injected unbalanced carriers recombine in the diffusion process, and the emission process mainly corresponds to the spontaneous emission process of the light. According to the position of the light output, the light-emitting diodes can be divided into a surface emitting type and an edge emitting type. Our most commonly used LEDs are InGaAsP/InP double heterojunction LEDs. LED material characteristics analysis The light-emitting diode is made of a III-IV compound such as GaAs (gallium arsenide), GaP (gallium phosphide), GaAsP (phosphorus gallium arsenide) and the like, and its core is a PN junction. Therefore, it has the IN characteristic of a general PN junction, that is, forward conduction, reverse cutoff, and breakdown characteristics. In addition, it has luminescent properties under certain conditions. 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 (small children) entering the other area is combined with the majority carriers (multiple sub-) to emit light. Assuming that luminescence occurs in the P region, the injected electrons directly composite with the valence band holes to emit light, or are first captured by the luminescent center and then condensed with the holes. In addition to this luminescent composite, some electrons are trapped by the non-luminous center (this center is near the middle of the conduction band and the intermediate band), and then recombined with the holes, and the energy released each time is not large, and visible light cannot be formed. The greater the ratio of the composite amount of luminescence to the non-luminous composite amount, the higher the photon efficiency. Since the recombination emits light in the minority carrier diffusion region, light is generated only within a few μm of the near-PN junction surface. Theory and practice have proved that the peak wavelength λ of light is related to the forbidden band width Eg of the semiconductor material in the light-emitting region, ie ≈≈1240/Eg(mm) The unit of Eg in the formula is electron volt (eV). If visible light (wavelength between 380 nm and 780 nm red) is produced, the Eg of the semiconductor material should be between 3.26 and 1.63 eV. Light longer than the wavelength of red light is infrared light. There are now infrared, red, yellow, green and blue light-emitting diodes, but the blue light diodes are expensive and expensive, and are not commonly used. Is it a material problem, is it so difficult to achieve high brightness in LED? LEDs, achieving high brightness do have some technical difficulties. LED has limit parameters: (1) Allowable power consumption Pm: The maximum value of the product of the forward DC voltage applied to the LED and the current flowing through it. Above this value, the LED is hot and damaged. (2) Maximum forward DC current IFm: The maximum forward DC current allowed to be applied. Exceeding this value can damage the diode. (3) Maximum reverse voltage VRm: The maximum reverse voltage allowed to be applied. Above this value, the LED may be damaged by breakdown. (4) Working environment topm: The ambient temperature range in which the LED can work normally. Below or above this temperature range, the LED will not work properly and the efficiency will be greatly reduced. Under the premise of not changing the material, in the limit range of the LED, the means to increase the brightness is to increase the current, and as the current increases, the amount of heat generated by the LED will increase sharply. Friends who have used LED light source portable projectors, or micro-projectors, must have a deep understanding, LED light source projectors, very hot, and generally there will be significant noise. These products, the small body is on the one hand, the key is still caused by its own heat. High-heat micro-injection products If the heat does not dissipate well, the junction temperature of the LED will increase, and the lifetime of the LED will decrease. So the fundamental way to extend the life of LEDs is to improve their heat dissipation. Improve heat dissipation from the LED itself. Schematic diagram of heat dissipation of two types of chip-type LEDs This 3014, by increasing the metal area of ​​the bottom plate, has improved heat dissipation, reducing its thermal resistance to only 51 ° C / W. As long as the heat sink is done well enough to ensure that the pin temperature is below 60 ° C, its life can reach more than 50,000 hours. 1W power LED thermal resistance with environmental temperature trend chart After testing by many professional institutions, with the increase of power, the heat dissipation problem of LEDs has become more and more prominent. A large number of practical applications have shown that the basic reason why LEDs can not increase the input power is because LEDs will release a lot in the work process. The heat causes the junction temperature of the die to rise rapidly and the thermal resistance to become large. The higher the input power, the greater the heating effect. The increase of temperature will lead to device performance change and attenuation, non-radiative compound increase, device leakage current increase, semiconductor material defects increase, metal electrode electromigration, encapsulation epoxy resin yellowing, etc., seriously affecting the photoelectric parameters of LED. Even the power LED is disabled. Therefore, for LED devices, it is increasingly important to reduce the thermal resistance and junction temperature and to study the thermal characteristics of LEDs. Note: For the above materials, refer to the contents of several master's and doctor's thesis. LEDs achieve high brightness, and what are the difficulties? Making high-brightness LEDs, using new materials is an important challenge. LED material development We can check the history of LED materials in the past 20 years: In 1991, Nichia Corporation successfully developed a homojunction GaN-based blue LED with a peak wavelength of 430 nm and a spectral half-width of 55 rim. Its optical output power was 10 times that of the SiC LED on the market at that time, and the external quantum efficiency was about 0.18%. In 1995, Nichia Corporation developed a successful InGaN/AIGaN double heterojunction candle-light ultra-high brightness blue LED with an output power of 1.5mW and an external quantum efficiency of 2.7% at a forward current of 20mA. The half width is 450 nm and 70 rim, respectively. In 1997, Schlotter et al. and Nakamura et al. invented a white LED packaged with a blue-light tube and a yellow phosphor. In 2001, Kafmann et al. used a UV LED to excite a trichromatic phosphor to obtain a white LED. In the past few years, white LEDs have attracted widespread attention from the LED industry and academia. In 2006, Cree announced the launch of a new cool white LED - "XP.G", which has set new records in terms of luminous efficiency and brightness. At a drive current of 350 mA, the luminous flux is 1391 m and the luminous efficiency is 1321 m/W. The brightness and efficacy are 37% and 53% higher than Cree's brightest XR.E LEDs, respectively, and are known as "the industry's brightest and most efficient lighting-grade LEDt211". In 2007, Nichia Corporation released its new LED. The experimental product has a luminous flux of up to 1451m and a luminous efficiency of about 1341m/W with a forward current of 350mA. The chip size is lmm2 and the color temperature is 4988K. In the case of Ir=20 mA, the luminous efficiency is as high as 169 lm/W). In the past two years, Nissan's GaN-based LEDs, whether blue, violet, ultraviolet or white, are the highest in the world, and the external quantum efficiency of 460nm blue LEDs can reach 34.9%. In 2007, Cree Company of the United States grew a double heterojunction on a SiC substrate, and the fabricated device was also excellent. The SiC substrate can be used to fabricate the metal electrode of the Gabl-based LED at the bottom of the substrate, and the current can pass the low-resistance conduction. The vertical flow of the substrate also laid the foundation for the development of other optoelectronic devices. This company grows GaN-based LEDs on SiC. Whether it is small-size chips, blue LEDs and violet LEDs, or large-size blue LEDs and violet LEDs are among the top international standards. High-end LED flashlight After reading it, we will find that in recent years, Gree and Japan Nichia have continued to conduct in-depth research and achieved certain results. However, LED materials and processes have no technical breakthroughs. Although there are many organizations studying, only a few companies have succeeded, and the technical difficulty of LED can be imagined. Is the LED structure not very simple? Why also said that the packaging technology is costly? Theoretically simple things are often the hardest to break through in technology. The existing LED package has defects, and it is also a key factor affecting the selling price. In some application fields, it even determines the product cost. Various LEDs Since the LED itself does not have the protection against moisture, oxidation, short circuit, and protection of the chip, it needs to be subjected to different types of assembly and wire bonding, and the like. At present, the main LEDs are classified into the following categories according to their different package structures and processes: The first type, LED light [LED LEMP] type. The LED chip is first fixed on the bracket with the pin, and then the gel is packaged, and the pin of the LED lamp is inserted and soldered on the circuit board of the preset circuit to complete the final LED light source. Second, the surface-attached LED [SMD LED]. The chip is first fixed on a small substrate, and then the wire is punched, then the gel is packaged, and finally the packaged LED is soldered on the printed circuit board, and finally completed. Third, flip-chip LEDs. After the chip is completed, the chip is overlaid on the flip-chip adapter board, and soldered by high-frequency method using gold balls, silver balls, solder balls, etc., and then LAMP or SMD is used for colloidal packaging, and finally the finished product is finished. Solder on the printed circuit board and then finalize. Fourth, another flip chip LED. Using gold balls, silver balls, solder balls, etc., the chips are soldered to the flip-chip adapter plate in a high-frequency manner and then sealed into LAMP or SMD, high-power LEDs, and further soldered on the printed circuit board for final production. Into the light source structure. The fifth, CHIP ON BORD. The chip is fixed on the printed circuit board, and then the wire is struck, and then the gel is packaged until it is finally completed. The common disadvantages of the above five methods are that there are many processes, the packaging is cumbersome, the equipment is expensive, and the heat dissipation effect of the LED after packaging is not good; and the finished product, the LED illumination will be reduced due to the deterioration of the packaging material due to heat and other various reasons. Among them, the processes and technologies related to high-frequency machines are mainly carried out abroad or in Taiwan Province. The high frequency and high frequency used by the high frequency machine are two different concepts. The high frequency refers to the electromagnetic wave with a frequency greater than 100Khz, and the ultrasonic wave refers to the acoustic wave with a frequency exceeding 20 kHz. The high-frequency welding principle and welding principle are different from the ultrasonic wave. High-frequency electromagnetic waves use high-frequency electromagnetic fields to make the internal molecules collide with each other to produce high temperature to achieve welding and welding. Ultrasonic waves generate a large amount of heat by using the principle of frictional heat generation. To achieve the purpose of welding and welding. LED's current packaging process materials, adhesive (epoxy or silicone), brackets, chips, gold (aluminum) wire have the following shortcomings and problems to be overcome: 1, high cost. 2, poor heat dissipation. 3. The LED life is reduced due to poor heat dissipation. 4, the sealant not only causes heat accumulation and yellowing of the glue to cause light decay. 5, due to the yield and stress of the wiring and bracket sealing, the stability and reliability are reduced. 6, LED in the process, can only be completed within 280 ° C, 5 seconds. 7, not resistant to thermal shock. The brightness of the LED should be broken again. It must further improve the quantum efficiency inside and outside the epitaxial chip, and reduce the attenuation of the brightness after packaging. The epitaxial and grain process needs to be strengthened. Is it difficult to improve the price of LED? In summary, we can conclude that many of the advantages of LEDs that are now circulating exist in reality, but are limited to low-brightness LED products. Low-brightness LEDs must rely on quantity to win, and high-brightness LEDs are the future development trend. Now, the advantage of low-brightness LEDs is strongly advocated. We can think that the biggest drawback of LED is that the price is too high, and the high-brightness LED is still on the level of pricelessness. Tricolor LED According to Haitz's law, the price of LEDs will be reduced tenfold every ten years. If there are no new alternatives, we still have hope and see the day when high-brightness LEDs prosper.
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