Light-emitting diode (LED) technology is on a tear. Higher brightness levels, higher efficiencies, longer lifetimes, and decreasing costs have spun out from a barrage of advances in heat dissipation, packaging, and processing.
LEDs made from indium-gallium-nitride (InGaN), aluminum-indium-gallium-phosphide (AlInGaP), gallium-nitride (GaN), silicon-carbide (SiC), and yttrium-aluminum-garnet (YAG) processes now come in red, amber, red-orange, blue, cyan, green, and white, often at economically viable prices. They come in various form factors, including discrete devices, neon-look light pipes or light guides, and reflector systems, spreading their appeal to interior and exterior lighting.
LED efficiency has made dramatic gains (Fig. 1). These improvements result from better light generation within the chip and better means of extracting that light from the chip and its package.
Applications for LEDs? They're simply all over the place these days. Just a sampling of their whereabouts finds them in displays and indicators for automotive and aircraft dashboards, cell phones, flat-panel-display backlighting, traffic signals, architectural and outdoor stadium lighting, and even Christmas tree lighting.
Deepening LED market penetration is evident. For example, some estimates say that one-third of all U.S. traffic signals use LEDs. Digital signage applications are rampant among retail businesses. The advent of a full-color range of high-power LEDs has paved the way for more advanced architectural designs and stage and studio lighting.
Market growth is particularly strong for high-brightness LEDs. According to Strategies Unlimited, their compound annual growth rate (CAGR) is 47% since 1995, which in dollars totals out to $2.7 billion today.
The biggest advantages of LEDs over other lighting approaches is their relatively small size and great flexibility. Compact, digitally addressable LED arrays can be produced using programmable controls.
LED light bars from companies such as Para Light Corp. exemplify LED lighting in small enclosed areas, spotlight projectors, and even architectural applications. One of the company's light-bar modules consists of 64 individual 150-mA, high-power LEDs with a total flux output of 200 to 400 lumens in amber, red, green, cyan, and blue, as well as red-green-blue (RGB) versions with color-changing capability.
Oki Electric uses LED printheads for electrophotographic printing. While conventional lasers use elaborate combinations of rotating mirrors and lenses that must remain in alignment throughout the printing process, LED printheads have no moving parts (Fig. 2).
Lighting Science's patented Optimized Digital Lighting (ODL) technology generates less heat than other light sources while delivering 30% more light output. The company claims it cuts energy consumption by 90% with a useful life that's 25 times longer than existing incandescent lamps.
DOMINATION BY LCD BACKLIGHTING
LEDs rule in the world of liquid-crystal-display (LCD) flat-panel backlighting. With an LED backlight, an LCD can have about 30% more color range versus other types of backlighting, matching the capability of film-based still and movie cameras.
Samsung has shown a variety of LED backlit LCDs with more than the usual RGB pixels. Properly applied, these pixels can produce four to six primary colors with wider ranges of color and brightness than other LED backlighting techniques.
Agilent Technologies recently described an illumination and color-management system for backlighting LCD TVs that utilize LEDs. Developed through a joint venture with Philips Electronics, the system increases the brilliance of colors in an LCD screen by some 25% over current technologies.
Digital signage represents another quickly expanding application area for LEDs, but it may be a more expensive proposition. Depending on the configuration, a red LED sign can use 20% to 60% less power than a neon light for the same light output for both outdoor and indoor signage applications.
Color Kinetics pioneered LED-driven intelligent digital signage applications with its iColor Flex SL, iColor Tile FX, and iColor Cove products. These technologies illuminate the high-profile Washington Mutual beanstalk sign in New York City's Times Square. The structure's 80-ft. tall 3D sign has a fiberglass exterior that shimmers in varying shades of green.
Market-research firm Intertech expects the value of LEDs used in digital signage applications to hit $7 billion by 2007. Key areas include electronic billboards and promotional signs.
WHITE LEDs FOR GENERAL USE
LEDs are generally monochromatic. More recently, LED manufacturers have produced "cool" white lighting indirectly, using ultraviolet LEDs to excite phosphors that emit a white light. This spawned one of today's hottest application areas for illuminating homes, offices, and industrial plants. Strategies Unlimited expects this market to jump from $1.84 billion in 2002 to $4.7 billion in 2007.
Generating a white light from an LED involves several mixing RGB color emissions, phosphor emissions, or a combination of both. More is needed for higher white LED efficiencies, which continue to improve. They're more efficient than incandescent and halogen lamps but still much less efficient than CFLs. Although their costs have dropped recently, they're at least five times more expensive than CFLs.
At General Electric's Global Research Center, scientists are investigating promising methods of increasing LED efficiencies and decreasing their costs to make them more competitive for illumination applications. They've discovered that 405-nm emitters with a vertical structure and grown on free-standing GaN substrates outperform lateral devices grown on either GaN or sapphire substrates.
Another effort to make white LEDs more cost-effective with other illumination sources involves scientists at the University of California at Los Angeles (UCLA). They've demonstrated a novel approach to LED fabrication using a polymer wet-coating process that has the potential for large-scale LED manufacturing at low cost. Researchers are using what they've coined as "twistacene," derived from "twisted acene," a chain of benzene rings that are usually flat.
One challenging technical hurdle in the LED universe concerns thermal management. Though LEDs don't radiate as much heat as other lighting sources, they still need an appropriate heatsink so that light output and lifespan don't decrease. A high-brightness LED with a 25-lumen output typically consumes more than 1 W. Much of this is in the form of heat, which must be removed from the LED chip's junction to maintain its high-efficiency advantage.
Implementing a low-temperature cofired ceramic-on-metal (LTCC-M) substrate to remove heat, Lamina Ceramics came up with a white-light LED engine that's 14 times brighter than any previously demonstrated LED array. It consists of 1120 LEDs with a 5500K correlated color temperature (CCT), a color rendering index (CRI) of 80, and an output of 28,000 lumens. The 5-in.2 Aterion White unit is powered by 1400 W and requires an external heatsink (Fig. 3).
Lamina's patented LTCC-M technology uses LED dies bonded to a copper/molybdenum substrate. The substrate features a thermal conductivity of approximately 170 W/mK and a thermal coefficient of expansion of about 6 ppm/K, a figure that closely matches the LED semiconductor materials' thermal coefficient of expansion.
Good heat management can be seen in Lumileds' Luxeon family of white LEDs. They can withstand maximum junction temperatures of 185°C without the need for an external heatsink. Instead, the heatsink is a metal slug that's part of the chip's package (Fig. 4). The LEDs also feature a reduced thermal resistance of 9°C/W, which means more light output and less heat dissipation.
Also taking that route, Osram Optoelectronics uses a metal copper slug beneath the die in its line of Golden Dragon LEDs mounted on an aluminum heatsink (Fig. 5). The company's Dragontape LED system can assemble very flat LED modules that have a flexible tape with an adhesive backing, simplifying the illumination module's installation.
A TRANSPARENT COMPOUND
The conventional method of attaching the LED die to a package's heatsink is to use resin epoxies. Deviating from that trend, Hong Kong's Cotco buys its dies and then processes and packages them. The LED supplier developed a patented transparent compound that will eutectically bond the die to a leadframe atop a pc board.
"We developed proprietary and simplified packaging and surface-mount process technologies that give us low-cost and high-reliability advantages," says Sidney Chu, Cotco's general manager for Business Development. "Our process does not involve any soldering, hence there's no need to withstand the typical high temperatures involved in surface-mount processes that degrade conventional epoxies."
He cites just one example of the reliability issue: Cotco has supplied General Motors with high-brightness LEDs for brakelights on many of GM's cars and claims zero defects over the last three years. A 3-W line soon will complement its 1-W Dorado LED line, with a 5-W version now under development.
The list of companies producing high-luminosity white LEDs now includes Cree with its Xthin and XLamp, Nichia with its Jupiter and Rigel, Sharp with its GM5WA0626A, Toshiba with its TL10W02-D, and product offerings from Rohm and Toyoda Gosei. Also, Osram licensed its technology to Taiwan's Harvatek.
The average life of new white LEDs runs 50,000 hours when operated properly. Comparative average lifetimes for other illumination technologies, such as incandescent lamps, halogen bulbs, and CFLs, pale by comparison with average lifetimes of 750, 3000, and 10,000 hours, respectively. Even for other colors like red and yellow, studies have shown lifetimes of 100,000 hours at 50% of output. On average, an LED produces 40 to 50 lumens/W compared with 10 to 15 lumens/W for incandescents. However, CFLs can produce 80 to 100 lumens/W.
Key to the LED's longer life is its solid-state nature. It doesn't suffer from the filament breakage and electrode decay endured by other illuminating sources. Moreover, an LED element is much smaller and takes up a lot less space. LEDs also are more energy-efficient than other light sources—about 80% better than incandescents, which dissipate most of their output as heat. In addition, they're environmentally clean light sources.
The LED's popularity as an illumination source for consumer and industrial applications hasn't gone unnoticed by the U.S. government. The Department of Energy (DoE) is working with industrial, academic, and government laboratory partners to accelerate advances in solid-state lighting technology.
According to the DoE, solid-state lighting promises to fundamentally alter lighting in the future. DoE estimates say the widescale use of LEDs for illumination can cut the nation's electric energy consumption in half for lighting. Currently, 25% of our electrical energy is spent on lighting. Beyond the consumption aspect, the quality of building environments should see improvement.
Government efforts involve both LED and organic LED (OLED) technologies (see "OLED Technology Marches On," at www.elecdesign.com, ED Online 10246). Areas of investigation include better processes and materials for making LEDs, boosting production yields, increasing LED efficiencies, and improving LED packaging. Results show up to 76% energy-conversion efficiencies, a record high.
White LEDs have already infiltrated portable handheld consumer electronics. The aforementioned Luxeon line from Lumileds, aimed at the booming market of cell phones with cameras, produces outputs of 40 and 80 lumens at 1 A—that's far higher than the 6- to 8-lumen outputs now presently available.
FAR-REACHING MARKET POTENTIAL
There's no doubt that LED technology will continue to improve in higher efficiencies, high light outputs, and lower costs. Reaching these goals is just a matter of time.
Needless to say, the market potential for LEDs is vast. Nearly every sports stadium and airport facility worldwide uses some form of LED lighting for messages and video images. The world's largest LED building lighting scheme illuminates the Public Institute for Social Security building in Kuwait. It uses nearly a quarter of a million LEDs in 800 fittings distributed around the edges of the building.
"Impressive" is an understatement for the LED lighting of Kawasaki's La Cittadella, in Japan. This retail and cinema complex contains a 130-ft. high tower illuminated by a variety of intelligent LED lighting effects, including rainbow patterns and gradual color shifts, supplied by Color Kinetics. It even has a fountain that's constantly illuminated by changing colored LEDs (Fig. 6).
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