I. Introduction
LED has been in development for nearly 30 years. In the 1970s, the earliest GaP, GaAsP homogeneous red, yellow, and green low-luminance LEDs began to be used for indicator, digital, and text displays. Since then, LEDs have entered a variety of applications, including aerospace, aircraft, automotive, industrial applications, communications, consumer products, etc., throughout the national economy and thousands of households. By 1996, LED sales worldwide had reached billions of dollars. Although LEDs have been limited by color and luminous efficiency for many years, GaP and GaAsPLEDs have been favored by users because of their long life, high reliability, low operating current, compatibility with TTL and CMOS digital circuits. .
In the last decade, high brightness and full colorization have been the frontier topics in the research of LED materials and device technology. Ultra High Brightness (UHB) refers to LEDs with luminous intensities up to or exceeding 100mcd, also known as Candela (cd) grade LEDs. The development of high-brightness A1GaInP and InGaN LEDs has progressed very rapidly, and has reached the performance level that conventional materials GaA1As, GaAsP, and GaP cannot achieve. In 1991, Toshiba Corporation of Japan and HP of the United States developed InGaAlP620nm orange ultra-high brightness LED. In 1992, InGaAlP590nm yellow ultra-high brightness LED became practical. In the same year, the normal light intensity of the InGaA1P573nm yellow-green ultra-high brightness LED developed by Toshiba was 2 cd. In 1994, Japan Nichia Corporation developed InGaN 450nm blue (green) color ultra-high brightness LED. At this point, the three primary colors of red, green, blue, and orange and yellow LEDs required for color display have reached the candela-level luminous intensity, achieving ultra-high brightness and full color, and making the outdoor full color of the luminous tube. The display becomes a reality.
China's development of LED started in the 1970s and formed an industry in the 1980s. There are more than 100 enterprises in the country, and 95% of the manufacturers are engaged in post-package production. The required dies are almost all imported from overseas. Through several "five-year plan" technical transformation, technical research, the introduction of foreign advanced equipment and some key technologies, China's LED production technology has taken a step forward. Some manufacturers in Beijing, Changchun, Nanchang, Shanghai, Shandong, Hebei and other places now have mass production capacity of GaAs and GaP single crystals, epitaxial wafers and chips. The Puliang LED chip production line of Xinlei Optoelectronics Co., Ltd., which was established by Nanchang 746 Factory, produced 700 million cores in 1998 and more than 1 billion in 1999. Hebei Huiyou Power Electronics Co., Ltd. and Hebei Lide Electronics Co., Ltd., affiliated to the 13th Research Institute of the Ministry of Information Industry, have completed the InGaA1P ultra-high brightness LED epitaxial wafer and chip production line respectively. By the end of 1999, the company has reached an annual output of 10,000 pieces of epitaxial wafers and chips. The production capacity of 100 million has changed the situation that China's ultra-high brightness LED epitaxial wafers and chips are all imported from overseas.
This article will give a brief introduction to the application of ultra-high brightness InGaA1PLED and InGaNLED in automotive indicator lights, traffic lights, large-screen displays, and liquid crystal display (LCD) backlighting.
Second, the structure and performance of ultra-high brightness LED
Ultra-high brightness red A1GaAsLED has higher luminous efficiency than GaAsP-GaPLED. The lumen efficiency of transparent substrate (TS) A1GaAsLED (640nm) is close to 10lm/W, which is 10 times larger than red GaAsP-GaPLED. Ultra-high brightness InGaAlPLED provides the same color as GaAsP-GaPLED, including green-yellow (560nm), light green-yellow (570nm), yellow (585nm), light yellow (590nm), orange (605nm), light red (625nm), deep red (640nm). The InGaAlPLED absorber substrate (AS) has a lumen efficiency of 101m/W, a transparent substrate (TS) of 201m/W, and a lumen efficiency of 10 to 20 times higher than the GaAsP-GaPLED in the wavelength range of 590 nm to 626 nm. The wavelength range of 560 nm to 570 nm is 2 to 4 times higher than that of GaAsP-GaPLED. Ultra-high-brightness InGaN LEDs provide blue and green light with wavelengths ranging from 450nm to 480nm for blue, 500nm for blue-green, and 520nm for green, and lumens efficiency of 3m/W to 151m/W. The current lumen efficiency of ultra-high-brightness LEDs has surpassed that of incandescent lamps with filters, and can replace the powerful ED arrays with a power of less than 1W to replace incandescent lamps with a power of less than 150W. For many applications, incandescent lamps use filters to get red, orange, green, and blue, while ultra-high brightness LEDs give the same color. In recent years, ultra-high-brightness LEDs made of AlGaInP materials and InGaN materials have combined multiple (red, blue, green) ultra-high-brightness LED chips, and various colors, including red, orange, and yellow, can be obtained without filters. , green, blue, the current luminous efficiency has exceeded the incandescent lamp, the positive fluorescent lamp is close. The brightness of the light has been higher than 1000mcd, which can meet the needs of outdoor all-weather and full-color display. The LED color large screen can express the sky and the ocean to realize three-dimensional animation. A new generation of red, green and blue ultra-high brightness LEDs has achieved unprecedented performance.
Third, the application of ultra-high brightness LED
3.1 information indicator
3.1.1 Car signal indication
The car lights are mainly the direction lights, tail lights and brake lights on the outside of the car; the interior of the car is mainly the illumination and display of various instruments. Ultra-high-brightness LEDs are used in automotive lights and have many advantages over traditional incandescent lamps, with a broad market in the automotive industry. LEDs are able to withstand strong mechanical shocks and vibrations. The average working life (MTBF) is orders of magnitude higher than that of incandescent bulbs, far higher than the working life of the car itself, so the LED brake lights can be packaged as a whole without having to consider repairs. Transparent substrates AlGaAs and AlInGaPLEDs have a relatively high lumen efficiency compared to incandescent bulbs with filters, so that LED brake lights and directional lights can operate at lower drive currents. Typical drive currents are only incandescent lamps. 1/4. The lower power also reduces the size and weight of the car's internal wiring system, while also reducing the internal temperature rise of the integrated LED beacon, allowing the lens and housing to use low temperature resistant plastic. The response time of the LED brake light is 100 ns, which is shorter than the response time of the incandescent lamp, which leaves the driver with more reaction time, thus improving the safety of the driving. The illuminance and color of the exterior lights of the car are clearly defined. Although the interior lighting display of a car is not controlled by the relevant government departments like an external signal light, the manufacturer of the car has requirements for the color and illumination of the LED. GaPLEDs have long been used in the car, and ultra-high-brightness AlGaInP and InGaN LEDs will replace the incandescent lamps in the car because they meet the manufacturer's requirements in terms of color and illumination. From the price point of view, although LED lights are more expensive than incandescent lamps, the price of the two systems is not significantly different from the overall system. With the development of ultra-high brightness TSAlGaAs and AlGaInPLED, the price has been continuously reduced in recent years, and the reduction will be even greater in the future.
3.1.2 Traffic signal indication
The use of ultra-high-brightness LEDs to replace incandescent lamps, used in traffic lights, warning lights, and sign lights has now spread all over the world, the market is broad, and the demand is growing rapidly. According to the statistics of the US Department of Transportation in 1994, there are 260,000 crossroads installed in the United States, and at least 12 red, yellow, and blue-green signal lights at each intersection. There are also a number of additional transition signs and crosswalk warning lights across the road at many intersections. Thus, each intersection may have 20 lights and be illuminated at the same time. From this, it can be inferred that there are about 135 million traffic lights in the United States. At present, the use of ultra-high brightness LEDs to replace traditional incandescent lamps to reduce power loss has achieved significant results. In Japan, the annual power consumption on traffic lights is about 1 million kilowatts. After replacing incandescent lamps with ultra-high brightness LEDs, the power consumption is only 12%.
For traffic lights, the competent authorities of each country shall formulate corresponding specifications, specifying the color of the signal, the minimum illumination intensity, the pattern of the spatial distribution of the beam, and the requirements for the installation environment. Although these requirements are written in incandescent lamps, they are basically applicable to the currently used ultra-high brightness LED traffic lights.
Compared with incandescent lamps, LED traffic lights have a long working life, generally up to 10 years. Considering the impact of harsh outdoor environment, the life expectancy is reduced to 5 to 6 years. At present, the ultra-high brightness AlGaInP red, orange and yellow LEDs have been industrialized and the price is relatively cheap. If the module consisting of red ultra-high brightness LEDs replaces the traditional red incandescent traffic signal head, the red incandescent lamp can be suddenly disabled to safety. The impact is minimized. The general LED traffic signal module consists of several sets of LED single lamps connected in series. Take the 12-inch red LED traffic signal module as an example. In 3 groups to 9 groups of LED single lamps, the number of LEDs in each group is 70. ~75 (total of 210 to 675 LED single lamps), when one LED single lamp fails, only one set of signals will be affected, and the remaining groups will be reduced to 2/3 (67%) or 8/. 9 (89%) does not invalidate the entire signal head like an incandescent lamp.
The main problem with the LED traffic signal module is that the cost is still higher. Take the 12-inch TS-AlGaAs red LED traffic signal module as an example. It was first used in 1994, and its cost is 350$. By 1996, the performance was better. The 12-inch AlGaInPLED traffic signal module costs $200. It is not expected that the price of the InGaN blue-green LED traffic signal module will be comparable to AlGaInP. The incandescent traffic signal head is low in cost but consumes a lot of electricity. A 12-inch diameter incandescent traffic signal head consumes 150W, and the traffic warning light that crosses the sidewalk is 67W. According to calculations, The incandescent signal at each intersection consumes 18,133 kWh per year, equivalent to an annual electricity bill of 1,450$; however, the LED traffic signal module is very power efficient, and the power consumption of each 8-inch to 12-inch red LED traffic signal module 15W and 20W respectively, the LED signs at the intersection of the intersection can be displayed with arrow switches, and the power consumption is only 9W. According to calculation, each intersection can save 9916kWh per year, which is equivalent to saving 793$ per year. According to the average cost of each LED traffic signal module of 200$, the red LED traffic signal module only uses its saved electricity costs, and after three years, it can recover the initial cost cost and start to receive economic returns. Therefore, the current AlGaInPLED traffic information module is used, although the cost is higher, but in the long run, it is still cost-effective.
3.2 large screen display
The large screen display is another huge market for ultra-high brightness LED applications, including monochrome, two-color and full-color displays for graphics, text, and numbers. Conventional large-screen active displays generally use incandescent lamps, optical fibers, cathode ray tubes, etc.; passive displays generally use a flop method. LED displays have been limited by the performance and color of the LED itself. Today, ultra-high-brightness AlGaInP, TS-AlGaAs, and InGaN LEDs are available in bright red, yellow, green, and blue colors to meet the full-color large-screen display requirements. The LED display can be assembled into various structures according to the pixel size. The small pixel diameter is generally less than 5mm. Each pixel of the monochrome display uses a T-1 (3/4) LED lamp, and each pixel of the two-color display is two-color. T-1 (3/4) LED lights, full color display requires 3 T-1 red, green, blue lights, or a multi-chip T-1 (3/4) LED light as a pixel . Large pixels are made up by combining many T-1 (3/4) red, green, and blue LED lights. InGaN (480nm) blue, InGaN (515nm) green and AsAlGaAs (637nm) red LED lamps are used as the three primary colors of LED display, which can provide realistic full-color performance, and have a large color range, including blue-green, green-red, etc. It is basically in line with the TV color range stipulated by the International Television System Committee (NTSC).
3.3 Liquid crystal display (LCD) backlight
At least 10% of the liquid crystal display uses active light as the backlight, the light source makes the LCD display easy to read in a dark environment, and the full-color LCD display also requires a light source. The light sources required for LCD backlights are mainly incandescent bulbs, electroluminescence, cold cathode fluorescent, LED, etc. Among them, LEDs are the most competitive in LCD backlights, and the new ultra-high brightness AlGaInP, AlGaAs, InGaN LEDs can provide high efficiency. Luminous and a wide range of colors.
LEDs are used in LCD backlights in three main ways. (1) The simplest is to mount the LED lamp directly behind the LCD scattering film. Many packaged LED lamps can be used, which should have a very wide beam angle to make the axial light uniform. It is also possible to use an unpackaged die, generally using a GaPLED, whereas with AlGaInP and TS-AlGaAsLED, it can operate at a small current to reduce power consumption. (2) Another way is the edge light LCD backlight, using a transparent or translucent rectangular plastic block as the light guide body, which is directly mounted behind the LCD scattering film, and the back surface of the plastic block is coated with white reflective material. The LED light is incident from one side of the plastic block, and the other side is made of a white reflective material. (3) The light emitted from the LED is introduced into the fiber bundle, and the diffusing film of the fiber bundle forms a flat sheet behind, and the light can be taken out from the sheet in different ways as the backlight of the LCD. Liquid crystal displays using LEDs as backlights can be used in mobile phones and notebook computers. With the widespread use of small liquid crystal displays in power-saving communication products, there will be greater demand for ultra-high brightness LEDs.
3.4 solid lighting
The practical and commercialization of full-color ultra-high-brightness LEDs has brought a new revolution in lighting technology. Solid-state lamps made of multiple ultra-high-brightness red, blue and green LEDs can not only continuously emit wavelengths. A variety of shades of light, but also can emit white light with a brightness of tens of candles to one hundred candles, becoming an illumination source. Recently, Nichia Corporation of Japan used its InGaN blue LED and fluorescent technology to introduce a white light solid-state light-emitting device with a color temperature of 6500K and an efficiency of 7.5 lumens per watt. For incandescent lamps and LED solid-state lamps with the same luminous brightness, the latter consumes only 10% to 20% of the former, incandescent lamps generally do not exceed 2000 hours, and LED lamps have a lifetime of tens of thousands of hours. . Such a solid light source with small size, light weight, good directionality, energy saving, long life and resistance to various harsh conditions will certainly have an impact on the traditional light source market. Although the cost of this new type of illuminated solid-state light source is still high, it can be applied to special occasions such as mining, diving, rescue, and military equipment lighting. In the long run, if the production scale of ultra-high brightness LEDs is further expanded and the cost is further reduced, the advantages of energy saving and long life are enough to make up for the disadvantage of high price. Ultra-high brightness LEDs will likely become a new and competitive electric light source.
Fourth, the conclusion
The advent and industrialization of ultra-high-brightness LEDs not only expands the original application field, but also has a market with great potential. In the next few years, InGaN LED will be similar to AlGaInPLED with the improvement of scale production technology and the reduction of product cost. By then, China's LED industry will have a certain scale and the ability to participate in international competition, ultra-high brightness LED will have a big development in China.
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