Replace the traditional high-brightness LED lighting technology to be overcome key

Replace the traditional high-brightness LED lighting technology to be overcome key
ITRI ​​Taiwan Nano Electro-Optical Center Yinshang Bin Opto Semiconductors
With the LED luminous efficiency continues to improve, no doubt for the light-emitting diode light source one of the most attention in recent years. On the one hand by virtue of its light, thin, short, small features, the other by means of its encapsulation type of impact, and seismic and special-shaped light-emitting light-emitting diodes to the general people indeed a very different light options. However, now see the LED heat dissipation problems, including difficulties in development, as well as special light-emitting diode light-shaped utilization. How to overcome the test of various manufacturers R & D capabilities.
LEDs have a great feature that has a low current, low voltage-driven power-down feature, and this lack of features in the world and national energy concept for the upgrading of green at the same time, in particular, to attract everyone's attention. In addition to the current governments committed to the development of new energy, the efficiency of existing electrical equipment and environmental studies also raise considerable effort bet. In the R & D to reduce industrial electricity consumption at the same time, the current penetration rate of about 80% of household appliance power consumption gradually be taken seriously. In the light emitting area, the Commission estimates to be able to view the results, the use of a high current efficiency of fluorescent light (66 ~ 75lm / W) to replace the traditional use of 60W incandescent bulbs, the annual lighting hours of 3,500 hours of down calculation, the annual electricity savings of approximately about 689 million degrees (about 88 600 kW).
The fluorescent Although there is currently a high luminous efficiency, lower manufacturing costs, etc., but because fluorescent lamps contain mercury, and materials used to encapsulate the fluorescent UV-absorbing glass Youyi be mainly glass fragile features with easy recycling of mercury waste, will cause serious environmental pollution. Therefore, the EU has explicitly disabled in 2007 these mercury products, and therefore the development of new light source development has become a goal of governments, and LED (light emitting diode), that is, we usually say the light-emitting diode, is current national development priorities in the lighting.
Light emitting diode theory
Its so-called light-emitting diode structure is basically the traditional pn diode, but its main function is not used for the rectifier, but with positive bias in the use of current through the pn junction, when after, prompting some of the electron-hole junction combined with the shine, the light-emitting properties can refer to Figure 1.
The light-emitting diodes in addition to the wavelength of light emitted by the diode depends on the wavelength of semiconductor materials used, but also depends on the mixing ratio between the different materials. Figure 2 shows the light-emitting materials, the band, the lattice constant and the relationship between light wavelength, we can see that the current red, yellow, green, mainly InGaAlP-based materials, while the blue and green InGaN-based materials is based.
Light-emitting diodes of process technology
For the semiconductor light-emitting diodes, the lattice matching is a major issue, because for most of the group III-V semiconductors, and not just for the substrate (substrate) can carry the top of the epitaxial layer, while the growth of Lei crystal layer of the lattice size must be matched with the substrate lattice, they are not due to stress factors leading to lattice defects, making the component is defective absorption of photons emitted, and significantly reduce the component of the luminous efficiency. The first isomer of III-V semiconductor epitaxial (heteroepitaxy) is the use of GaAs as a substrate, and on which the epitaxial growth of GaAlAs layers, because the two materials are very similar to the lattice, so the epitaxial layer and substrate between the stress is extremely small, so the process does not happen too much R & D problems. But then gradually developed epitaxial growth, such as GaAs1-xPx on the GaAs substrate, or GaAsxP1-x grown on GaP substrates have stress problems. Therefore, optical materials, often by adjusting two yuan, three or even four yuan materials ratio, so by the party in addition to different sizes to match the proportion of multi-atomic lattice structure of the substrate, but also because the adjustment of semiconductors size of the bandgap, and adjust the wavelength of light-emitting components, only this method to adjust the parameters in the epitaxial also more complicated, and therefore can be seen, the semiconductor light-emitting epitaxial technology component technology can be called the core.
In epitaxy method to enhance the same time, the structure of epitaxial continued in improvements. The first structure is of course the traditional p-? N junction light-emitting diodes, but the luminous efficiency and can not be significantly improved, so the use of a single heterojunction (Single? Heterojunction, SH) structure of the method began to be used in the epitaxial of the process, you can increase the minority carrier injection diode (minority carrier injection) efficiency, so luminous efficiency has significantly improved. More developed after double-heterojunction (Double Heterojunction, DH) structure, which is higher than the bandgap of the material on both sides of the middle person, which can be very effective in the carrier injection into the bilateral middle layer and the carrier will complete these trapped in this context, and produce very high photoelectric conversion efficiency. The latest way is to use the quantum structure of epitaxial layers which, when double-heterojunction structure of the middle layer thickness gradually reduced to the number of 10 angstroms (A), the electron or hole that generate quantum effects, which can significantly enhance the optical conversion of the results.
In this proposed technique is mainly for epitaxial III-V-wavelength light-emitting material concentrated in the red, Huang Guangbo segment series of GaAs material. This series of light-emitting diodes of the older, earlier also get a better result. But if they want to get full-color semiconductor light source, in any case must develop out of the blue, green band of the semiconductor light-emitting diodes, and GaN light-emitting diodes are series such demand, in recent years has been significant progress.
GaN process successfully overcome problems
Used blue, green light-emitting diode materials, mainly early ZnSe and GaN. Because ZnSe have reliability problems, so just let GaN have more room for development. Early studies of GaN is the delay for significant progress, mainly because he could not find matches with the lattice constant of GaN substrate, resulting in epitaxial defect density is too high, and thus luminous efficiency has not improved. Another cause can not get a breakthrough in GaN reason is that some components of P-GaN growth difficult, not only P-GaN doped (doping) is too low, and the hole of the mobility (mobility) is also low. Until 1983, Japan Tian Zhen history (S. Yoshida) and others in Sapphire (Sapphire) substrate on the first high-temperature growth of aluminum nitride (AlN) as a buffer layer of GaN was then grown out to obtain better crystals, After Isamu Akasaki of Nagoya University professor (I. Akasaki), who used MOCVD at low temperature (600 ℃) AlN buffer layer grown first, and get over it grow at high temperatures, such as mirror-like GaN. In 1991 Nichia (Nichia Co.) Researcher Shuji Nakamura (S. Nakamura) using low-temperature growth of non-crystalline GaN buffer layer, and then get the same high-temperature growth of mirror-like GaN, epitaxy at this time part of the problem has been major breakthrough. On the other hand, 1989? Years, Professor Isamu Akasaki electron-beam irradiation of magnesium (Mg)-doped P-GaN, obtained significantly the P-type GaN, after Nichia's Shuji Nakamura, a more direct use of thermal annealing at 700 ℃ to complete P -GaN production, thus plagued the development of two major issues GaN final breakthrough.
In 1993, Nichia use the above two studies, successfully developed may issue a candle (Candela) of GaN blue light emitting diodes, the life of tens of thousands of hours. Then green light emitting diodes, blue, green diode lasers were developed in succession.
Manufacturers strive to improve the efficiency of light-emitting diodes
LED luminous efficiency is generally referred to as the component's external quantum efficiency (external quantum efficiency), it is the component of the internal quantum efficiency (internal quantum efficiency) and the components of the extraction efficiency (extraction efficiency) of the product. The so-called internal quantum efficiency of the component is in fact electro-optical conversion efficiency of the component itself, the main features and components such as components of their own material the band, defects, impurities and components of the composition and structure of epitaxial related. The components of the extraction efficiency refers to the photons generated inside the component, the component itself, after absorption, refraction, reflection after the fact in the external components can be measured to the number of photons. Therefore, the relevant factors in the extraction efficiency of absorption of the component materials themselves, component geometry, components and packaging materials and components of the refractive index difference between the structure of the scattering characteristics. The efficiency of the product of these two is that the components of the glow, which is the component of the external quantum efficiency. Focused on enhancing the development of early components of its internal quantum efficiency, the method is mainly used to improve the quality of epitaxial and epitaxial structure changes, so easy to convert electrical energy into heat energy, thereby indirectly increasing the LED luminous efficiency, and receive about 70% theoretical internal quantum efficiency. But such internal quantum efficiency of almost close to the theoretical limit, in such circumstances, to enhance the components alone is not possible to improve the internal quantum efficiency of the total amount of light component, which is the external quantum efficiency of 2 to 3 times the current , thus enhancing the efficiency of the component out will become an important issue. Components currently used to enhance the extraction efficiency of the method, mainly consists of the following ways:
Grain structure appearance change-TIP
The production of traditional grains light-emitting diodes as a standard rectangular appearance.Because the general semiconductor packaging materials, refractive index and large differences in epoxy resin, leaving the critical angle of total reflection of the interface is small, while the four rectangular cross-section parallel to each other, leaving the interface in the semiconductor photon probability smaller, so that photons can only be total internal reflection until it is absorbed and exhausted, to turn into heat in the form of light, resulting in poor light is more effective.Therefore, the change in shape is a valid upgrade LED luminous efficiency of the method. The development of HP's TIP (Truncated? Inverted? Pyramid) type grain structure, four-section will no longer be parallel to each other, and the light can be very effective out to be cited, the external quantum efficiency is increased dramatically to 55%, light efficiency up to 100 lm / W, is the first to achieve this goal light-emitting diode (Figure 3).
However, HP's TIPLED only easy to process in the quaternary red light-emitting diodes, for the use of extremely high hardness of sapphire (Sapphire) substrate of GaN light-emitting diode, a series of considerable difficulty. In early 2001, Cree's concept of the same structure (Figure 4), buttressed by its advantage of SiC substrate is also successfully GaN / SiC light-emitting diodes made of the same with the slope of the LED, and external quantum efficiency increased dramatically to 32%; Sapphire, however, much more expensive than the SiC substrate, so far in this technology, there is no further progress.
Surface roughness (surfaceroughness) technology
By the component's internal and external geometry of the coarse, destroy the total reflection of light within the component, the component resorted to enhance efficiency. This method was first proposed by the Nichia, the crude method is essentially the geometry of the component to form the concave and convex shape of the rules, and this rule in accordance with the structure of the distribution of the location were divided into two forms , a bump in the shape of the component within the set, another way is to create rules in the top of the bump component shape, and set in the back of the reflector assembly. The use of traditional processes can GaN system compound semiconductor layer in the interface settings convex shape, so the first approach has a higher practicality. The use of current 405nm UV wavelength components, obtained 43% external quantum efficiency of 60% removal efficiency for the world's highest external quantum efficiency and extraction efficiency.
Chip adhesive technology (waferbonding)
Because the light produced by light-emitting diodes after repeated total reflection, most of the semiconductor material itself have been absorbed and packaging materials. Therefore, if the use will absorb light GaAs? As AlGaInP? LED substrate, the light-emitting diodes will make changes within the large absorption loss, and significantly reduce the component of light extraction efficiency. In order to reduce the light emitted by the LED substrate absorption, HP first transparent substrate made of paste technology. The so-called transparent substrate adhesive technology mainly to the first light-emitting diode die pressure in high temperature environment, and the transparent substrate pasted GaP, GaAs and then later removed, so you can improve the rate of twice the light out.
Paste the chip technology is mainly applied on the quad LED components, but recently have begun to apply this technology in GaN? LED on. Osram Opto Semiconductors in February 2003, also published a new research-ThinGaN, can be blue LED light extraction efficiency to 75%, than the traditional raised 3 times.
Flip-chip packaging technology (Flipchip)
For sapphire substrates (sapphire substrate) of GaN series material is concerned, because the P pole and N pole electrode assembly must be done in the same side, so if the package using traditional methods, accounting for most components of the angle of light on Fang Faguang block electrode surface because the light will be lost considerable amount of light. The so-called Flip? Chip structure that is inverted the traditional components, and production in p-type electrode above the high reflectivity of the reflection layer to the top of the original components emitted from the light from the light emitting angle of the other export components, sapphire substrate side edge by taking the light (Figure 5). Because this method reduces the optical loss in the electrode side, close to the traditional package may have about twice the amount of light output. On the other hand, because the flip-chip structure can bump or directly by the electrode structure with the package structure in direct contact with the heat, and greatly enhance the cooling effect of the component, the component to further enhance the amount of light.
Stage to become the focus of white LED
Beginning in the efficiency of LED light colors while significantly improved, the high brightness LED lighting used in the possibility of higher and higher. And this is how to consider the application of developed white light emitting diodes. Currently dubbed the use of white light emitting diode into three main methods were as follows:
Single crystal blue LED and yellow phosphor
Nichia blue light-emitting semiconductor companies have been successfully developed, the product is developed with the white light emitting diodes. In fact, Nichia's white LED semiconductor material itself is not directly a white light, blue light-emitting diodes to stimulate it by coating on the top of the yellow YAG phosphor, the phosphor is excited with yellow light generated from the original use to stimulate complementary and blue produce white light. Nichia is currently commercially available products but the use of 460nm InGaN blue semiconductor excitation YAG phosphor, and 555nm to produce the yellow, and has been fully commercialized, and also in the development of several other high-brightness LED maker Lumileds Lighting , Cree, Toyoda Gosei (Toyoda Gosei) in the LED market, constantly competing. With Blu-ray luminescence efficiency of grain rising and YAG phosphor synthesis technology matures, blue and yellow phosphor grains of white light emitting diode package is currently more mature white light emitting diode technology.
Single crystal type UVLED + RGB phosphor
Although the use of blue with yellow YAG phosphor grain white LED packaging technology is more mature technology, but using this method out of the white light emitting diode package has several serious problems delay can not be solved. The first is uniformity, because the Blu-ray excitation yellow phosphor grains actually involved in white color, so Blu-ray crystal wavelength offset, intensity changes and the phosphor coating thickness will affect the change White uniformity. Most visible example is the use of letters in this way into the white light emitting diodes, the central part of the look more blue (or white), and the next area look more yellow (phosphor coating thick), each pieces of white LED color more different.
On the other hand, the development of this technology, Nichia has most of the relevant process technology in Blu-ray crystal and yellow YAG? Phosphor white LED-related patents, while Nichia patent is for the market to change the attitude of oligopoly, So for the use of blue with yellow phosphor grains produced white light emitting diode manufacturers are the victims. The use of blue yellow phosphor grains coupled with white light emitting diode technology, more white high color temperature, color rendering is low and so on. Therefore, developing a better and there is no patent issues is the light-emitting diode technology vendors a major issue.
UV? LED coupled with three-color (R, G, B) phosphor development provides another direction direction. The method is mainly used in fact not involved with a white UV LED excitation of red, green and blue phosphors, issued by the three tri-color phosphor dubbed white shade. This method is not practical because the UV LED in white color, so the UV? LED wavelength and intensity fluctuations for the case with a white light is not particularly sensitive. And by the choice of colors and the ratio of phosphor, modulation of acceptable color rendering and color temperature of white light. In the patent, the use of UV LED + RGB phosphor-related R & D is still considerable space to play. But such technology, while there are various advantages, but there are still considerable technical difficulties, these difficulties include the choice of wavelength UV light with the phosphor (phosphor conversion efficiency of the best excitation wavelength), UV? LED and the difficulty of making anti- UV packaging materials development, etc., are to be the R & D unit eleven to resolve.
Polymorphism RGBLED
Will issue a red, blue, and green grain, packaged together directly, through red, green, white and blue color dubbed direct way, can be made of white light emitting diodes. Using three-color white LED die directly into this package is made of white light for the way first, through the phosphor without the advantage of the conversion of grain by three-color direct dubbed white, except to avoid fluorescence powder conversion loss and get better luminous efficiency, but also can be controlled by separate three-color light-emitting diode light intensity to reach full-color color effect (variable color temperature), and by the wavelength and intensity of grain selection get better color rendering. However, the drawback of mixing problems, this light source in front of the user can easily be observed throughout many different colors, and see the color in the shelter behind the shadow. In addition, because the use of the three grains are heat, heat dissipation is the other kind of package types of 3-fold increase in its use difficult. Currently the use of multi-crystal RGB? LED package types of white light emitting diodes will receive 25 ~ 30lm / W efficiency. Mainly used in less serious heat problems billboards outdoor display, outdoor landscape lights, wall lights can change color wash. On the other hand, if can be controlled by electronic circuit design, the use of multi-crystal RGB LED package types of light-emitting diode, a good chance to replace the current use of the LCD backlight CCFL backlight module one of the main light source.
White LED must pass the test of heat
Although the luminous efficiency white light emitting diodes with the gradual improvement of the white light emitting diodes used in the possibility of growing, but obviously, its single white LED drive power was low, and therefore the current package type is unlikely to single white LED lighting to achieve the required number of lumens. For in this issue, the key solution can be broadly divided into two categories, one is more traditional to use the multi-digit LED light source module using the composition, of which every single LED drive power required for the general using the same (about 20 ~ 30mA); several other methods for the current manufacturer of high-brightness light-emitting diodes used method is to use a so-called large grain process, when grain size is no longer the traditional (0.3mm2), and the process for the larger grain size (0.6mm2 ~ 1mm2), and using such a high drive current to drive the light-emitting components (usually 150 ~ 350mA, it is more than can be up to 500mA). But no matter what method is used, because it will be in a very small light-emitting diode package deal with a very high heat, if these components are being trapped heat, in addition to a variety of packaging materials because of the different expansion coefficients between each other and have a product reliability issue, the luminous efficiency of the grain will rise as the temperature has dropped significantly, and caused it to be shortened significantly. So how dispersed components of high fever, light-emitting diode packaging technologies become an important issue.
For the light-emitting diode, the most important is the output of the light flux and shape, so the LED must not shade one side, and need to use the high transparency of epoxy coating material.However, there are almost no thermal conductivity epoxy resin material, so for the current light-emitting diode packaging technology, are the main heat-emitting diode die beneath its metal socket (leadframe) to disperse component the heat issue. On current trends, the metal feet the choice of materials mainly for the high thermal conductivity of the material composition, such as aluminum, copper or ceramic materials, but these materials and the thermal expansion coefficient between the grains varies greatly, hence its direct exposure, probably because of materials at elevated temperatures caused by the stress generated between the reliability problems, so usually between the materials with both conductivity and expansion coefficient of the material as middle of the interval. With the above concepts, Matsushita Electric in 2003, light-emitting diodes made of multiple pieces of metal and metal lines in the composite material made of a multilayer substrate module to form a light source module, high thermal conductivity substrate using light effects, so that the light source output remained stable despite prolonged use (Figure 6). The same idea of ​​using high-heat the substrate, Lumileds its application in a large area of ​​grain products (Figure 7). Lumileds substrate materials used for the high conductivity of copper, and then connect it to the special of the metal circuit board, taking into account the circuit and increase the heat conduction spread of the results.
Apart from Lumileds, but including OsramOptoSemiconductors and Nichia have switched more than 1W launched large grain products (Figure 8, Figure 9). From these high-brightness light-emitting diode manufacturers have been offering large grain, high-power products, it seems that a large grain-related processes, packaging technology seems to have gradually become the mainstream of high-brightness light-emitting diodes. However, large grain-related processes and packaging technology is not only bigger and it will die area, the relevant process and packaging technology for conventional light-emitting diode manufacturers still have a considerable threshold, but if you want to push to the high-brightness light-emitting diode illumination areas, related technology R & D still must pass through the process.
LED best for the future to address key issues
As in recent years, LED luminous efficiency increased gradually, the light-emitting diode light source for the possibility of using higher and higher. However, people only consider the light-emitting diode light to enhance the efficiency, how to make the light-emitting diode characteristics, and its application in the lighting may encounter difficulties, is the major goal of the current lighting plant. Has seen the difficulties of including thermal issues, and special light-emitting diode light-shaped utilization. In the heat, the light-emitting diode light source though claims to be cold, but because there is still a considerable improvement of its electro-optical efficiency of space, that is still a considerable degree of energy converted into light because there is no excess heat caused by these heat concentrated in the grain particle size will result in serious thermal problems. Therefore, a good thermal design and thermal material developed for the current focus. The LED light on the shape of light, the light-emitting diode light source with the traditional characteristics of a completely different light, in addition to itself because of its small grain size, the variety of light-emitting diodes of different package types will result in completely different shaped light-emitting light, thus related to the design of LED lighting applications will no longer be simply put on the light condenser lens or mirror, but must be more careful optical design. In this part, companies and R & D unit has a different direction, but in addition to development of technology, how these technologies for mass production, it is the future of solid-state lighting source can be the key to the mainstream. For more visit: www.bgocled.com

Linksys LED Matrix

tinkeringetc.blogspot.com writes:

My first electronics project of the summer is now finished. Overall, it didn’t take me too long to complete, and I’m very pleased with the results. Watch this video overview and check after the break for all the juicy details.   www.bgocled.com

Lumen Efficiency of 1 W2level High Power White LED

LI Bing2qian
(Dept.of Physics,Foshan University, Foshan 528000,CHN)
Abstract : 　The relationship of lumen efficiency with input power of high power white light2 emitting diodes (LEDs) is studied. The result shows that the lumen efficiency increases with input power in the range of 0～0.11W.At theinputpowerof0.11Wlumenefficiencyis15.6lm/
W. Then the lumen efficiency decreases with the increcse of input power over 0. 11 W ,and the decrease rate is more and more faster followed by the increase of the input power. At the rating power of 1 W , the lumen efficiency is close to 13 lm/ W. The main reasons which induced inefficiency of high power white light2emittingdiodesathighinputpowerare: (1) theincreaseof input power results in higher temperature of LED chip; (2) the increase of the current causes more leakage current.
Key words : 　high power white L ED ; semiconductor lighting source ; luminous flux ; lumen efficiency  www.bgocled.com

Replace the traditional high-brightness LED lighting technology to be overcome key

ITRI ​​Taiwan Nano Electro-Optical Center Yinshang Bin Opto Semiconductors
With the LED luminous efficiency continues to improve, no doubt for the light-emitting diode light source one of the most attention in recent years. On the one hand by virtue of its light, thin, short, small features, the other by means of its encapsulation type of impact, and seismic and special-shaped light-emitting light-emitting diodes to the general people indeed a very different light options. However, now see the LED heat dissipation problems, including difficulties in development, as well as special light-emitting diode light-shaped utilization. How to overcome the test of various manufacturers R & D capabilities.
LEDs have a great feature that has a low current, low voltage-driven power-down feature, and this lack of features in the world and national energy concept for the upgrading of green at the same time, in particular, to attract everyone's attention. In addition to the current governments committed to the development of new energy, the efficiency of existing electrical equipment and environmental studies also raise considerable effort bet. In the R & D to reduce industrial electricity consumption at the same time, the current penetration rate of about 80% of household appliance power consumption gradually be taken seriously. In the light emitting area, the Commission estimates to be able to view the results, the use of a high current efficiency of fluorescent light (66 ~ 75lm / W) to replace the traditional use of 60W incandescent bulbs, the annual lighting hours of 3,500 hours of down calculation, the annual electricity savings of approximately about 689 million degrees (about 88 600 kW).
The fluorescent Although there is currently a high luminous efficiency, lower manufacturing costs, etc., but because fluorescent lamps contain mercury, and materials used to encapsulate the fluorescent UV-absorbing glass Youyi be mainly glass fragile features with easy recycling of mercury waste, will cause serious environmental pollution. Therefore, the EU has explicitly disabled in 2007 these mercury products, and therefore the development of new light source development has become a goal of governments, and LED (light emitting diode), that is, we usually say the light-emitting diode, is current national development priorities in the lighting.
Light emitting diode theory
Its so-called light-emitting diode structure is basically the traditional pn diode, but its main function is not used for the rectifier, but with positive bias in the use of current through the pn junction, when after, prompting some of the electron-hole junction combined with the shine, the light-emitting properties can refer to Figure 1.
The light-emitting diodes in addition to the wavelength of light emitted by the diode depends on the wavelength of semiconductor materials used, but also depends on the mixing ratio between the different materials. Figure 2 shows the light-emitting materials, the band, the lattice constant and the relationship between light wavelength, we can see that the current red, yellow, green, mainly InGaAlP-based materials, while the blue and green InGaN-based materials is based.
Light-emitting diodes of process technology
For the semiconductor light-emitting diodes, the lattice matching is a major issue, because for most of the group III-V semiconductors, and not just for the substrate (substrate) can carry the top of the epitaxial layer, while the growth of Lei crystal layer of the lattice size must be matched with the substrate lattice, they are not due to stress factors leading to lattice defects, making the component is defective absorption of photons emitted, and significantly reduce the component of the luminous efficiency. The first isomer of III-V semiconductor epitaxial (heteroepitaxy) is the use of GaAs as a substrate, and on which the epitaxial growth of GaAlAs layers, because the two materials are very similar to the lattice, so the epitaxial layer and substrate between the stress is extremely small, so the process does not happen too much R & D problems. But then gradually developed epitaxial growth, such as GaAs1-xPx on the GaAs substrate, or GaAsxP1-x grown on GaP substrates have stress problems. Therefore, optical materials, often by adjusting two yuan, three or even four yuan materials ratio, so by the party in addition to different sizes to match the proportion of multi-atomic lattice structure of the substrate, but also because the adjustment of semiconductors size of the bandgap, and adjust the wavelength of light-emitting components, only this method to adjust the parameters in the epitaxial also more complicated, and therefore can be seen, the semiconductor light-emitting epitaxial technology component technology can be called the core.
In epitaxy method to enhance the same time, the structure of epitaxial continued in improvements. The first structure is of course the traditional p-? N junction light-emitting diodes, but the luminous efficiency and can not be significantly improved, so the use of a single heterojunction (Single? Heterojunction, SH) structure of the method began to be used in the epitaxial of the process, you can increase the minority carrier injection diode (minority carrier injection) efficiency, so luminous efficiency has significantly improved. More developed after double-heterojunction (Double Heterojunction, DH) structure, which is higher than the bandgap of the material on both sides of the middle person, which can be very effective in the carrier injection into the bilateral middle layer and the carrier will complete these trapped in this context, and produce very high photoelectric conversion efficiency. The latest way is to use the quantum structure of epitaxial layers which, when double-heterojunction structure of the middle layer thickness gradually reduced to the number of 10 angstroms (A), the electron or hole that generate quantum effects, which can significantly enhance the optical conversion of the results.
In this proposed technique is mainly for epitaxial III-V-wavelength light-emitting material concentrated in the red, Huang Guangbo segment series of GaAs material. This series of light-emitting diodes of the older, earlier also get a better result. But if they want to get full-color semiconductor light source, in any case must develop out of the blue, green band of the semiconductor light-emitting diodes, and GaN light-emitting diodes are series such demand, in recent years has been significant progress.
GaN process successfully overcome problems
Used blue, green light-emitting diode materials, mainly early ZnSe and GaN. Because ZnSe have reliability problems, so just let GaN have more room for development. Early studies of GaN is the delay for significant progress, mainly because he could not find matches with the lattice constant of GaN substrate, resulting in epitaxial defect density is too high, and thus luminous efficiency has not improved. Another cause can not get a breakthrough in GaN reason is that some components of P-GaN growth difficult, not only P-GaN doped (doping) is too low, and the hole of the mobility (mobility) is also low. Until 1983, Japan Tian Zhen history (S. Yoshida) and others in Sapphire (Sapphire) substrate on the first high-temperature growth of aluminum nitride (AlN) as a buffer layer of GaN was then grown out to obtain better crystals, After Isamu Akasaki of Nagoya University professor (I. Akasaki), who used MOCVD at low temperature (600 ℃) AlN buffer layer grown first, and get over it grow at high temperatures, such as mirror-like GaN. In 1991 Nichia (Nichia Co.) Researcher Shuji Nakamura (S. Nakamura) using low-temperature growth of non-crystalline GaN buffer layer, and then get the same high-temperature growth of mirror-like GaN, epitaxy at this time part of the problem has been major breakthrough. On the other hand, 1989? Years, Professor Isamu Akasaki electron-beam irradiation of magnesium (Mg)-doped P-GaN, obtained significantly the P-type GaN, after Nichia's Shuji Nakamura, a more direct use of thermal annealing at 700 ℃ to complete P -GaN production, thus plagued the development of two major issues GaN final breakthrough.
In 1993, Nichia use the above two studies, successfully developed may issue a candle (Candela) of GaN blue light emitting diodes, the life of tens of thousands of hours. Then green light emitting diodes, blue, green diode lasers were developed in succession.
Manufacturers strive to improve the efficiency of light-emitting diodes
LED luminous efficiency is generally referred to as the component's external quantum efficiency (external quantum efficiency), it is the component of the internal quantum efficiency (internal quantum efficiency) and the components of the extraction efficiency (extraction efficiency) of the product. The so-called internal quantum efficiency of the component is in fact electro-optical conversion efficiency of the component itself, the main features and components such as components of their own material the band, defects, impurities and components of the composition and structure of epitaxial related. The components of the extraction efficiency refers to the photons generated inside the component, the component itself, after absorption, refraction, reflection after the fact in the external components can be measured to the number of photons. Therefore, the relevant factors in the extraction efficiency of absorption of the component materials themselves, component geometry, components and packaging materials and components of the refractive index difference between the structure of the scattering characteristics. The efficiency of the product of these two is that the components of the glow, which is the component of the external quantum efficiency. Focused on enhancing the development of early components of its internal quantum efficiency, the method is mainly used to improve the quality of epitaxial and epitaxial structure changes, so easy to convert electrical energy into heat energy, thereby indirectly increasing the LED luminous efficiency, and receive about 70% theoretical internal quantum efficiency. But such internal quantum efficiency of almost close to the theoretical limit, in such circumstances, to enhance the components alone is not possible to improve the internal quantum efficiency of the total amount of light component, which is the external quantum efficiency of 2 to 3 times the current , thus enhancing the efficiency of the component out will become an important issue. Components currently used to enhance the extraction efficiency of the method, mainly consists of the following ways:
Grain structure appearance change-TIP
The production of traditional grains light-emitting diodes as a standard rectangular appearance.Because the general semiconductor packaging materials, refractive index and large differences in epoxy resin, leaving the critical angle of total reflection of the interface is small, while the four rectangular cross-section parallel to each other, leaving the interface in the semiconductor photon probability smaller, so that photons can only be total internal reflection until it is absorbed and exhausted, to turn into heat in the form of light, resulting in poor light is more effective.Therefore, the change in shape is a valid upgrade LED luminous efficiency of the method. The development of HP's TIP (Truncated? Inverted? Pyramid) type grain structure, four-section will no longer be parallel to each other, and the light can be very effective out to be cited, the external quantum efficiency is increased dramatically to 55%, light efficiency up to 100 lm / W, is the first to achieve this goal light-emitting diode (Figure 3).
However, HP's TIPLED only easy to process in the quaternary red light-emitting diodes, for the use of extremely high hardness of sapphire (Sapphire) substrate of GaN light-emitting diode, a series of considerable difficulty. In early 2001, Cree's concept of the same structure (Figure 4), buttressed by its advantage of SiC substrate is also successfully GaN / SiC light-emitting diodes made of the same with the slope of the LED, and external quantum efficiency increased dramatically to 32%; Sapphire, however, much more expensive than the SiC substrate, so far in this technology, there is no further progress.
Surface roughness (surfaceroughness) technology
By the component's internal and external geometry of the coarse, destroy the total reflection of light within the component, the component resorted to enhance efficiency. This method was first proposed by the Nichia, the crude method is essentially the geometry of the component to form the concave and convex shape of the rules, and this rule in accordance with the structure of the distribution of the location were divided into two forms , a bump in the shape of the component within the set, another way is to create rules in the top of the bump component shape, and set in the back of the reflector assembly. The use of traditional processes can GaN system compound semiconductor layer in the interface settings convex shape, so the first approach has a higher practicality. The use of current 405nm UV wavelength components, obtained 43% external quantum efficiency of 60% removal efficiency for the world's highest external quantum efficiency and extraction efficiency.
Chip adhesive technology (waferbonding)
Because the light produced by light-emitting diodes after repeated total reflection, most of the semiconductor material itself have been absorbed and packaging materials. Therefore, if the use will absorb light GaAs? As AlGaInP? LED substrate, the light-emitting diodes will make changes within the large absorption loss, and significantly reduce the component of light extraction efficiency. In order to reduce the light emitted by the LED substrate absorption, HP first transparent substrate made of paste technology. The so-called transparent substrate adhesive technology mainly to the first light-emitting diode die pressure in high temperature environment, and the transparent substrate pasted GaP, GaAs and then later removed, so you can improve the rate of twice the light out.
Paste the chip technology is mainly applied on the quad LED components, but recently have begun to apply this technology in GaN? LED on. Osram Opto Semiconductors in February 2003, also published a new research-ThinGaN, can be blue LED light extraction efficiency to 75%, than the traditional raised 3 times.
Flip-chip packaging technology (Flipchip)
For sapphire substrates (sapphire substrate) of GaN series material is concerned, because the P pole and N pole electrode assembly must be done in the same side, so if the package using traditional methods, accounting for most components of the angle of light on Fang Faguang block electrode surface because the light will be lost considerable amount of light. The so-called Flip? Chip structure that is inverted the traditional components, and production in p-type electrode above the high reflectivity of the reflection layer to the top of the original components emitted from the light from the light emitting angle of the other export components, sapphire substrate side edge by taking the light (Figure 5). Because this method reduces the optical loss in the electrode side, close to the traditional package may have about twice the amount of light output. On the other hand, because the flip-chip structure can bump or directly by the electrode structure with the package structure in direct contact with the heat, and greatly enhance the cooling effect of the component, the component to further enhance the amount of light.
Stage to become the focus of white LED
Beginning in the efficiency of LED light colors while significantly improved, the high brightness LED lighting used in the possibility of higher and higher. And this is how to consider the application of developed white light emitting diodes. Currently dubbed the use of white light emitting diode into three main methods were as follows:
Single crystal blue LED and yellow phosphor
Nichia blue light-emitting semiconductor companies have been successfully developed, the product is developed with the white light emitting diodes. In fact, Nichia's white LED semiconductor material itself is not directly a white light, blue light-emitting diodes to stimulate it by coating on the top of the yellow YAG phosphor, the phosphor is excited with yellow light generated from the original use to stimulate complementary and blue produce white light. Nichia is currently commercially available products but the use of 460nm InGaN blue semiconductor excitation YAG phosphor, and 555nm to produce the yellow, and has been fully commercialized, and also in the development of several other high-brightness LED maker Lumileds Lighting , Cree, Toyoda Gosei (Toyoda Gosei) in the LED market, constantly competing. With Blu-ray luminescence efficiency of grain rising and YAG phosphor synthesis technology matures, blue and yellow phosphor grains of white light emitting diode package is currently more mature white light emitting diode technology.
Single crystal type UVLED + RGB phosphor
Although the use of blue with yellow YAG phosphor grain white LED packaging technology is more mature technology, but using this method out of the white light emitting diode package has several serious problems delay can not be solved. The first is uniformity, because the Blu-ray excitation yellow phosphor grains actually involved in white color, so Blu-ray crystal wavelength offset, intensity changes and the phosphor coating thickness will affect the change White uniformity. Most visible example is the use of letters in this way into the white light emitting diodes, the central part of the look more blue (or white), and the next area look more yellow (phosphor coating thick), each pieces of white LED color more different.
On the other hand, the development of this technology, Nichia has most of the relevant process technology in Blu-ray crystal and yellow YAG? Phosphor white LED-related patents, while Nichia patent is for the market to change the attitude of oligopoly, So for the use of blue with yellow phosphor grains produced white light emitting diode manufacturers are the victims. The use of blue yellow phosphor grains coupled with white light emitting diode technology, more white high color temperature, color rendering is low and so on. Therefore, developing a better and there is no patent issues is the light-emitting diode technology vendors a major issue.
UV? LED coupled with three-color (R, G, B) phosphor development provides another direction direction. The method is mainly used in fact not involved with a white UV LED excitation of red, green and blue phosphors, issued by the three tri-color phosphor dubbed white shade. This method is not practical because the UV LED in white color, so the UV? LED wavelength and intensity fluctuations for the case with a white light is not particularly sensitive. And by the choice of colors and the ratio of phosphor, modulation of acceptable color rendering and color temperature of white light. In the patent, the use of UV LED + RGB phosphor-related R & D is still considerable space to play. But such technology, while there are various advantages, but there are still considerable technical difficulties, these difficulties include the choice of wavelength UV light with the phosphor (phosphor conversion efficiency of the best excitation wavelength), UV? LED and the difficulty of making anti- UV packaging materials development, etc., are to be the R & D unit eleven to resolve.
Polymorphism RGBLED
Will issue a red, blue, and green grain, packaged together directly, through red, green, white and blue color dubbed direct way, can be made of white light emitting diodes. Using three-color white LED die directly into this package is made of white light for the way first, through the phosphor without the advantage of the conversion of grain by three-color direct dubbed white, except to avoid fluorescence powder conversion loss and get better luminous efficiency, but also can be controlled by separate three-color light-emitting diode light intensity to reach full-color color effect (variable color temperature), and by the wavelength and intensity of grain selection get better color rendering. However, the drawback of mixing problems, this light source in front of the user can easily be observed throughout many different colors, and see the color in the shelter behind the shadow. In addition, because the use of the three grains are heat, heat dissipation is the other kind of package types of 3-fold increase in its use difficult. Currently the use of multi-crystal RGB? LED package types of white light emitting diodes will receive 25 ~ 30lm / W efficiency. Mainly used in less serious heat problems billboards outdoor display, outdoor landscape lights, wall lights can change color wash. On the other hand, if can be controlled by electronic circuit design, the use of multi-crystal RGB LED package types of light-emitting diode, a good chance to replace the current use of the LCD backlight CCFL backlight module one of the main light source.
White LED must pass the test of heat
Although the luminous efficiency white light emitting diodes with the gradual improvement of the white light emitting diodes used in the possibility of growing, but obviously, its single white LED drive power was low, and therefore the current package type is unlikely to single white LED lighting to achieve the required number of lumens. For in this issue, the key solution can be broadly divided into two categories, one is more traditional to use the multi-digit LED light source module using the composition, of which every single LED drive power required for the general using the same (about 20 ~ 30mA); several other methods for the current manufacturer of high-brightness light-emitting diodes used method is to use a so-called large grain process, when grain size is no longer the traditional (0.3mm2), and the process for the larger grain size (0.6mm2 ~ 1mm2), and using such a high drive current to drive the light-emitting components (usually 150 ~ 350mA, it is more than can be up to 500mA). But no matter what method is used, because it will be in a very small light-emitting diode package deal with a very high heat, if these components are being trapped heat, in addition to a variety of packaging materials because of the different expansion coefficients between each other and have a product reliability issue, the luminous efficiency of the grain will rise as the temperature has dropped significantly, and caused it to be shortened significantly. So how dispersed components of high fever, light-emitting diode packaging technologies become an important issue.
For the light-emitting diode, the most important is the output of the light flux and shape, so the LED must not shade one side, and need to use the high transparency of epoxy coating material.However, there are almost no thermal conductivity epoxy resin material, so for the current light-emitting diode packaging technology, are the main heat-emitting diode die beneath its metal socket (leadframe) to disperse component the heat issue. On current trends, the metal feet the choice of materials mainly for the high thermal conductivity of the material composition, such as aluminum, copper or ceramic materials, but these materials and the thermal expansion coefficient between the grains varies greatly, hence its direct exposure, probably because of materials at elevated temperatures caused by the stress generated between the reliability problems, so usually between the materials with both conductivity and expansion coefficient of the material as middle of the interval. With the above concepts, Matsushita Electric in 2003, light-emitting diodes made of multiple pieces of metal and metal lines in the composite material made of a multilayer substrate module to form a light source module, high thermal conductivity substrate using light effects, so that the light source output remained stable despite prolonged use (Figure 6). The same idea of ​​using high-heat the substrate, Lumileds its application in a large area of ​​grain products (Figure 7). Lumileds substrate materials used for the high conductivity of copper, and then connect it to the special of the metal circuit board, taking into account the circuit and increase the heat conduction spread of the results.
Apart from Lumileds, but including OsramOptoSemiconductors and Nichia have switched more than 1W launched large grain products (Figure 8, Figure 9). From these high-brightness light-emitting diode manufacturers have been offering large grain, high-power products, it seems that a large grain-related processes, packaging technology seems to have gradually become the mainstream of high-brightness light-emitting diodes. However, large grain-related processes and packaging technology is not only bigger and it will die area, the relevant process and packaging technology for conventional light-emitting diode manufacturers still have a considerable threshold, but if you want to push to the high-brightness light-emitting diode illumination areas, related technology R & D still must pass through the process.
LED best for the future to address key issues
As in recent years, LED luminous efficiency increased gradually, the light-emitting diode light source for the possibility of using higher and higher. However, people only consider the light-emitting diode light to enhance the efficiency, how to make the light-emitting diode characteristics, and its application in the lighting may encounter difficulties, is the major goal of the current lighting plant. Has seen the difficulties of including thermal issues, and special light-emitting diode light-shaped utilization. In the heat, the light-emitting diode light source though claims to be cold, but because there is still a considerable improvement of its electro-optical efficiency of space, that is still a considerable degree of energy converted into light because there is no excess heat caused by these heat concentrated in the grain particle size will result in serious thermal problems. Therefore, a good thermal design and thermal material developed for the current focus. The LED light on the shape of light, the light-emitting diode light source with the traditional characteristics of a completely different light, in addition to itself because of its small grain size, the variety of light-emitting diodes of different package types will result in completely different shaped light-emitting light, thus related to the design of LED lighting applications will no longer be simply put on the light condenser lens or mirror, but must be more careful optical design. In this part, companies and R & D unit has a different direction, but in addition to development of technology, how these technologies for mass production, it is the future of solid-state lighting source can be the key to the mainstream. For more visit: www.bgocled.com