Applications of ultra-bright LEDs

I. Introduction

     LED has a development history of nearly 30 years. In the 1970s, the earliest GaP and GaAsP homogeneous red, yellow, and green LEDs with low luminous efficiency began to be used in indicator lights, digital and text displays. Since then, LEDs have begun to enter a variety of application fields, including aerospace, aircraft, automobiles, industrial applications, communications, consumer products, etc., covering all sectors of 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, and compatibility with TTL and CMOS digital circuits. .

     In the past ten years, high brightness and full color have been the forefront topics in research on LED materials and device process technology. Ultra-high brightness (UHB) refers to LEDs whose luminous intensity reaches or exceeds 100mcd, also known as candela (cd)-level LEDs. The development of high-brightness A1GaInP and InGaN LEDs has progressed very rapidly, and has now reached performance levels that are impossible to achieve with conventional materials GaA1As, GaAsP, and GaP. In 1991, Toshiba Corporation of Japan and HP Company of the United States developed InGaAlP620nm orange ultra-high-brightness LEDs. In 1992, InGaAlP590nm yellow ultra-high-brightness LEDs were put into practical use. In the same year, the InGaA1P573nm yellow-green ultra-high brightness LED developed by Toshiba has a normal light intensity of 2cd. In 1994, Japan's Nichia Corporation developed an InGaN 450nm blue (green) ultra-high brightness LED. So far, the three primary colors of red, green, blue and orange and yellow LEDs required for color display have reached candela-level luminous intensity, achieving ultra-high brightness and full color, making the outdoor full-color luminescent tube Manifestation becomes reality.

    The development of LED in my country started in the 1970s and formed an industry in the 1980s. There are about 100 companies in the country, 95% of which are engaged in back-end packaging production, and almost all the required die are imported from overseas. Through several "Five-Year Plans" of technological transformation, technological research, and the introduction of foreign advanced equipment and some key technologies, my country'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 capabilities for GaAs and GaP single crystals, epitaxial wafers, and chips. The Puliang LED chip production line of Xinlei Optoelectronics Company, established by Nanchang 746 Factory, produced 700 million die in 1998 and more than 1 billion in 1999. Hebei Huiyou Power Electronics Company and Hebei Lide Electronics Company, affiliated to the 13th Research Institute of Electronics of the Ministry of Information Industry, have respectively built InGaA1P ultra-high brightness LED epitaxial wafer and chip production lines. By the end of 1999, they had reached an annual output of 10,000 epitaxial wafers and 1 chip. The production capacity is 100 million units, thus changing the situation in which all ultra-high brightness LED epitaxial wafers and chips in my country are imported from overseas.

     This article will briefly introduce the application of ultra-high brightness InGaA1PLED and InGaNLED in automobile indicators, traffic lights, large-screen displays, liquid crystal display (LCD) backlighting, etc.

2. Structure and performance of ultra-high brightness LEDs

     Ultra-high brightness red A1GaAs LED 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 greater than red GaAsPGaPLED. Ultra-high brightness InGaAlPLED provides the same colors 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 lumen efficiency of InGaAlPLED absorbing substrate (AS) is 101m/W, and the transparent substrate (TS) is 201m/W. In the wavelength range of 590nm to 626nm, the lumen efficiency is 10 times to 20 times higher than that of GaAsPGaPLED; In the wavelength range of 560nm to 570nm, it is 2 to 4 times higher than GaAsPGaPLED. Ultra-high brightness InGaN LED provides blue light and green light. Its wavelength range is: blue is 450nm~480nm, blue-green is 500nm, and green is 520nm; its lumen efficiency is 3m/W~151m/W. The current lumen efficiency of ultra-high brightness LEDs has exceeded that of incandescent lamps with filters, and they can replace incandescent lamps with a power of less than 1W. The LED array can replace incandescent lamps with a power of less than 150W. For many applications, incandescent lamps use filters to get red, orange, green and blue, and the same colors can be achieved with ultra-high-brightness LEDs. In recent years, ultra-high-brightness LEDs made of AlGaInP and InGaN materials combine multiple (red, blue, green) ultra-high-brightness LED chips together. Various colors can be obtained without filters, including red, orange, and yellow. , green, and blue. Currently, their luminous efficiency has exceeded that of incandescent lamps and is close to that of forward fluorescent lamps. The luminous brightness is higher than 1000mcd, which can meet the needs of outdoor all-weather, full-color display. The large LED color screen can express the sky and ocean and realize three-dimensional animation. A new generation of red, green and blue ultra-high brightness LEDs achieve unprecedented performance.

3. Application of ultra-high brightness LEDs

3.1 Information indicator light

3.1.1 Car signal indication

     Car indicators on the outside of the car are mainly direction lights, tail lights and brake lights; on the inside of the car they are mainly the lighting and display of various instruments. Ultra-high brightness LEDs are used in automotive indicator lights. Compared with traditional incandescent lamps, they have many advantages and have a wide market in the automotive industry. LEDs can withstand strong mechanical shock and vibration. The average working life (MTBF) is several orders of magnitude higher than that of incandescent bulbs and far longer than the working life of the car itself, so LED brake lights can be packaged as a whole without having to consider maintenance. Transparent substrates AlGaAs and AlInGaPLEDs have considerably higher lumen efficiency than incandescent bulbs with filters. In this way, LED brake lights and direction lights can operate at lower drive currents. The typical drive current is only that of incandescent bulbs. 1/4. Lower power can also reduce the size and weight of the car's internal wiring system, while also reducing the internal temperature rise of integrated LED signal lights, allowing the lens and outer cover to use plastics with lower temperature resistance. The response time of LED brake lights is 100ns, which is shorter than that of incandescent lights. This leaves more reaction time for the driver, thereby improving driving safety. The illumination and color of the car's exterior indicator lights are clearly defined. Although the internal lighting display of a car is not controlled by relevant government departments like external signal lights, the manufacturer of the car has requirements for the color and illumination of the LED. GaPLED has long been used in cars, and ultra-high brightness AlGaInP and InGaNLED will increasingly replace incandescent lamps in cars because they can meet the manufacturer's requirements in terms of color and illumination. From a price point of view, although LED lights are still more expensive than incandescent lights, there is no obvious price difference between the two from the perspective of the entire system. With the practical development of ultra-high brightness TSAlGaAs and AlGaInPLED, prices have been continuously decreasing in recent years, and the decrease will be even greater in the future.

3.1.2 Traffic signal instructions

     Replacing incandescent lamps with ultra-high-brightness LEDs for traffic lights, warning lights, and sign lights has now spread all over the world. The market is vast and the demand is growing rapidly. According to statistics from the U.S. Department of Transportation in 1994, there were 260,000 intersections with traffic lights in the United States, and each intersection must have at least 12 red, yellow, and blue-green signal lights. Many intersections also have additional transition signs and crosswalk warning lights across the road. In this way, there may be 20 signal lights at each intersection, and they must light up at the same time. From this it can be deduced that there are approximately 135 million traffic lights in the United States. At present, the use of ultra-high brightness LEDs to replace traditional incandescent lamps has achieved significant results in reducing power losses. Japan consumes about 1 million kilowatts of electricity on traffic lights every year. After replacing incandescent lamps with ultra-high-brightness LEDs, its electricity consumption is only 12% of the original.

     For traffic lights, the competent authorities in each country must formulate corresponding specifications, stipulating the color of the signal, the minimum lighting intensity, the pattern of the spatial distribution of the beam, and the requirements for the installation environment. Although these requirements are written for incandescent lamps, they are basically applicable to the ultra-high brightness LED traffic lights currently used.

     Compared with incandescent lamps, LED traffic lights have a longer working life, generally up to 10 years. Considering the impact of the harsh outdoor environment, the lifespan is expected to be reduced to 5 to 6 years. At present, ultra-high-brightness AlGaInP red, orange, and yellow LEDs have been industrialized and are relatively cheap. If a module composed of red ultra-high-brightness LEDs is used to replace the traditional red incandescent traffic signal lamp head, the sudden failure of the red incandescent lamp can be avoided. The impact is reduced to a minimum. Generally, LED traffic signal modules are composed of several groups of LED single lights connected in series. Taking the 12-inch red LED traffic signal module as an example, there are 3 to 9 groups of LED single lights connected in series, and the number of LED single lights in each group is 70. ~75 (the total number is 210~675 LED single lights). When one LED single light fails, only one group of signals will be affected, and the remaining groups will be reduced to 2/3 (67%) or 8/ 9 (89%) and does not render the entire signal head useless like incandescent lamps.

      The main problem with LED traffic signal modules is that the cost is still relatively high. Take the 12-inch TS-AlGaAs red LED traffic signal module as an example. It was first used in 1994 and its cost was 350$. By 1996, its performance was better. The 12-inch AlGaInPLED traffic signal module costs 200$. It is not expected that the price of InGaN blue-green LED traffic signal modules will be comparable to that of AlGaInP before long. Although the cost of incandescent traffic light heads is low, they consume a lot of power. An incandescent traffic light head with a diameter of 12 inches consumes 150W, and a traffic warning light crossing the road and sidewalk consumes 67W. According to calculations, The annual power consumption of incandescent signal lights at each intersection is 18133kWh, which is equivalent to an annual electricity bill of 1450$; however, LED traffic signal modules are very power-saving, and the power consumption of each 8-inch to 12-inch red LED traffic signal module is They are 15W and 20W respectively. The LED signs at the intersection corners can be displayed with arrow switches, and the power consumption is only 9W. According to calculations, each intersection can save 9916kWh of electricity per year, which is equivalent to saving 793$ in electricity bills per year. Assuming that the average cost of each LED traffic signal module is 200$, the red LED traffic signal module can recover its initial cost after three years by using only the electricity bill it saves, and start to receive continuous economic returns. Therefore, although the cost of using AlGaInPLED traffic information modules is higher, it is still cost-effective in the long run.

3.2 Large screen display

     Large-screen display is another huge market for ultra-high-brightness LED applications, including single-color, dual-color and full-color display of graphics, text, and numbers. Traditional large-screen active displays generally use incandescent lamps, optical fibers, cathode ray tubes, etc.; passive displays generally use the flop method. LED displays have always been limited by the performance and color of the LED itself. Today, ultra-high brightness AlGaInP, TS-AlGaAs, and InGaN LED can provide bright red, yellow, green, and blue colors, which can fully meet the requirements of full-color large-screen display. LED display screens can be assembled into various structures according to pixel size. The diameter of small pixels is generally less than 5mm. Each pixel of single-color display uses a T-1 (3/4) LED light. Each pixel of dual-color display is dual-color. T-1 (3/4) LED lights, full-color display requires three T-1 red, green, and blue lights, or a multi-chip T-1 (3/4) LED light as a pixel . Large pixels are formed by combining many T-1 (3/4) red, green, and blue LED lights. Using InGaN (480nm) blue, InGaN (515nm) green and AsAlGaAs (637nm) red LED lights as the three primary colors of LED display can provide realistic full-color performance and a large color range, including blue-green, green-red, etc. , basically consistent with the TV color range specified by the International Television Systems Committee (NTSC).

3.3 Backlighting of liquid crystal display (LCD)

     At least 10% of liquid crystal displays use active light as backlighting. The light source can make the LCD display easy to read in a dark environment. Full-color LCD display also requires a light source. The light sources required for LCD backlighting mainly include incandescent bulbs, electroluminescence, cold cathode fluorescence, LED, etc. Among them, LED is the most competitive in LCD backlighting. The new ultra-high brightness AlGaInP, AlGaAs, and InGaNLED can provide high-efficiency Glowing and wide range of colors.

      There are three main ways in which LEDs are used for LCD backlighting. (1) The simplest is to install the LED light directly behind the LCD scattering film. Many packaged LED lights are available. They should have a very wide beam angle to make the axial light uniformity better. Unpackaged dies can also be used, generally using GaPLEDs. However, AlGaInP and TS-AlGaAsLEDs can operate at low currents and reduce power consumption. (2) Another way is edge light LCD backlighting, using a transparent or translucent rectangular plastic block as the light guide, installing it directly behind the LCD scattering film, and coating the back surface of the plastic block with white reflective material , LED light is injected from one side of the plastic block, and the other sides are made of white reflective material. (3) The light emitted by the LED is introduced into the optical fiber bundle. The back of the scattering film of the optical fiber bundle forms a flat sheet. Different methods can be used to extract the light from the sheet as backlighting for the LCD. LCD displays that use LED as backlight can be used in mobile phones and laptop computers. With the widespread use of small LCD displays in power-saving communication products, there will be greater demand for ultra-high brightness LEDs.

3.4 Solid lighting lamp

     The practicality and commercialization of full-color ultra-high-brightness LEDs has brought a new revolution in lighting technology. Solid-state lighting lamps made of multiple ultra-high-brightness red, blue, and green LEDs can not only emit continuously adjustable wavelengths Various colors of light, and can also emit white with brightness ranging from dozens to a hundred candles, becoming a lighting source. Recently, Japan's Nichia Corporation has used its InGaN blue LED and fluorescent technology to launch a white solid light-emitting device with a color temperature of 6500K and an efficiency of 7.5 lumens per watt. For incandescent lamps and LED solid lighting lamps with the same luminous brightness, the power consumption of the latter only accounts for 10% to 20% of the former. The lifespan of incandescent lamps generally does not exceed 2,000 hours, while the lifespan of LED lamps is as long as tens of thousands of hours. . This kind of solid light source, which is small in size, light in weight, good in directionality, energy-saving, long in life and resistant to various harsh conditions, will definitely have an impact on the traditional light source market. Although the cost of this new lighting solid light source is still high, it can be used in some 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, its energy saving and long life advantages will be enough to make up for the disadvantage of high prices. Ultra-high brightness LED will likely become a very competitive new electric light source.

4. Conclusion

     The advent and industrialization of ultra-high brightness LEDs not only expand the original application fields, but also create a market with huge potential. In the next few years, with the improvement of scale production technology and the reduction of product costs, the price of InGaNLED will be similar to that of AlGaInPLED. By then, my country's LED industry will have a certain scale and the ability to participate in international competition, and ultra-high-brightness LEDs will have great development in our country.