The light-emitting diode lamp market has experienced a resurgence as vendors, many of them based in Japan, have launched a new generation of devices that emit a white light. In addition to backlighting color displays in cellular phones, other applications that show promise for white LEDs include dashboard lighting.
There are two kinds of single-chip white LEDs that are gallium nitride-based diodes with an indium gallium nitride (InGaN) active layer.
One version combines a blue LED with a yellow phosphor that generates white light by mixing blue with the yellow light emitted by the phosphor. The combination of an ultraviolet LED with red, blue and green phosphors generates white light, just as fluorescent lamps do. Ultraviolet light excites the phosphors to generate visible light.
White LEDs currently offer an efficiency of around 20 lumens (lm) per watt, which is better than incandescent light bulbs, but not high enough to compete with fluorescent lights that have an efficiency rating of from 60 to 100 lm/W.
Nichia Corp., which was among the first to market gallium nitride-based blue LEDs in the early 1990s, unveiled a white LED using a blue LED die in 1996. The company has been actively working to boost the efficiency of its InGaN LEDs. The company said it intends to increase the efficiency of these LEDs from about 20 lm/W, achieved in 2001, to more than 60 lm/W by 2004.
With the wide range of patents Nichia holds for InGaN LED technology as well as for the phosphors it uses for white LEDs in-house, that company has held a virtual monopoly on the supply of InGaN white LED chips. Until last year, Nichia had reserved its technologies for internal use.
Earlier this year, Nichia brought to market a surface-mount white LED, dubbed the NSCW215. The NSCW215 is a side-view surface-mount LED that is 1 mm high and emits 600 millicandelas of light at 20 milliamps. It is among the new white LEDs targeting the backlighting of mobile phones and other portable devices. A 0.8-mm-high version is also for sale. Both are now available in volume.
Nichia has also developed high-power InGaN LEDs that can handle from 1 W to 2 W, a power-handling capability that's 10 times higher than current LEDs.
When LEDs encounter large power demands, heat and deterioration of the encapsulating package can result, which has hindered their development. Nichia, however, has resolved the power-handling problem by eliminating the organic material in LED packages and by introducing a heat sink. The newly developed NSHx180F unit has an airtight, entirely inorganic package; no organic plastics or organic resins are used. As a result, the heat resistance of the package is one-tenth that of conventional packages.
With a 500-mA current, the LED emits a 380-nanometer ultraviolet light with an optical output power of a little less than 100 mW. The unit's lifetime is estimated at 100,000 hours. The package is 10 mm in diameter and stands 2.3-mm high.
The second package solution, called the NSCx190D, is a resin package with a lens, but it contains a copper heat sink for heat radiation. With that structure, the thermal resistance measures approximately one-twentieth that of conventional LED packages.
A variety of LEDs are available in white, blue, blue-green and green. With a 350-mA current, the packaged LEDs emit light with a luminous flux of 23 lm (lumen) for white, 7 lm for blue, 28 lm for blue-green and 20 lm for green. The package measures 11.2 wide x 7.2 long x 6 mm high. Its estimated lifetime is 50,000 hours. These high-power LEDs can be mounted with automated assembly equipment. Engineering samples are available.
Nichia is now offering its white LEDs mainly as backlights for mobile phone displays and in car dashboard-illumination applications. Nichia said it expects that its high-power white LEDs will stimulate the market for other lighting applications.
"There are various kinds of lighting equipment and it is difficult to generalize. White LEDs will first find usage in the lighting market in such small-area-lighting applications as reading lights," said Hiroyoshi Ogawa, general manager of the business-planning department in Nichia's Optoelectronics Product Division. "There are still many challenges in terms of cost, performance and other aspects before white LEDs are used widely for lighting equipment. We expect that it will take more than five years for the industry to resolve these issues before white LEDs become general light sources for lighting equipment," he said.
Using LED dies supplied by Nichia, Citizen Electronics has developed white LED packages that the company claims are the thinnest ever, in sizes ranging from 0.55 to 1 mm.
Citizen's side-view CL-430 series with a reflection frame intensifies brightness 2.5 times more than the company's existing products. The package houses one or two LED chips. One-chip lamps have the brightness of 290 to 590 mcd and two-chip lamps shine at 690 to 1,000 mcd. Sampling began in April.
In terms of light-emitting efficiency, technology combining a blue diode and yellow phosphor has the advantage at present, but is weak in color rendering, which is the quality of the white color. "The light is yellowish-white, which is not good quality for lighting," said Osamu Ueda, general manger of the technical department at Yamada Shomei Lighting Co. Ltd. On the other hand, the combination of an ultraviolet LED and red, green and blue phosphors yields white-light-like fluorescent lights.
Toyoda Gosei Co. Ltd., another company that has been supplying InGaN LEDs since the mid-1990s, uses sapphire substrates, as does Nichia. The company has teamed with Toshiba Corp. to introduce white LEDs using an ultraviolet LED die. The alliance combines Toyoda Gosei's 380-nm ultraviolet GaN-based LED and Toshiba's red-green-blue fluorescent technology.
The white LED comes in a 3.2 x 2.8-mm SMT package. With a 20-mA current, it has a luminous intensity of 100 mcd. Its efficiency is 4.5 to 5 lm/W. The white LED is available from Toyoda Gosei as the TG White, and from Toshiba as the TLWA1100.
The two companies have also developed a higher-efficiency version, which is available as a sample. The LED is currently being marketed mainly for car-mount illumination applications. A Toshiba spokeswoman said, "White LEDs will begin replacing conventional-lighting equipment gradually around 2006. Some vendors have already introduced LED lights, but they have not made full use of the merit of LEDs. Some drastic change, a big impact like the replacement of candles by light bulbs, is necessary for white LEDs to penetrate the lighting market. For this, there are challenges that we need to work on, such as developing new shapes of lights and drastically reduce power consumption."
Toyoda Gosei is also offering a white LED, the TG-White Hi, that employs a blue diode and yellow phosphor. Volume production of that type began in April.
Toyoda has established a production capacity of 90 to 95 million units per month for InGaN LEDs. But the percentage of white LEDs is not yet large, according to a Toyoda spokesman. "There is a big potential demand, but Nichia has a large share at present," he said.
Meanwhile, Matsushita Electric Industrial Co. Ltd. is a customer of Toyoda Gosei's blue-LED chips. Using those chips, Matsushita has been manufacturing white LEDs since 1999. Its LNJ020W9FRA and LNJ015W8BRAZ products are available in volume.
Matsushita has developed a 0.75-mm side-view LED in-house, which is already used in the company's cellular phones for NTT Docomo. Matsushita has also designed a surface-mount LED chip, which the company claimed can deliver various degrees of "white" depending on customers' needs.
While Nichia and Toyoda Gosei use sapphire substrates, Osram Opto Semiconductors and Cree Inc. have developed GaN-based opto devices using silicon carbide substrates. Cree is supplying blue and ultraviolet LED chips to manufacturers like Fairchild Semiconductor, Sharp, LEDtronics and Kingbright. Those manufacturers package the LED chips into lamps or surface-mount LED devices.
Fairchild Semiconductor Corp. is offering low-current LEDs. "The white LED market will show healthy growth over the next few years in the consumer electronics and communications markets," said Holton Lee, marketer from the optoelectronics product group, "and explosive growth when it can penetrate the illumination market."
Osram Opto Semiconductors offers a wide range of white LEDs, which consist of the combination of a blue LED chip and yellow phosphor. The TOPLED LW E67C is currently its highest-efficiency product at 12 lm/W. The company distributes its products through Infinion's distribution channels. "The development of next-generation white LEDs is in progress, probably faster than most people expect," said Osamu Ueda, general manager at Yamada Shomei Lighting.
Yamada Shomei, a light-equipment manufacturer in Japan, has developed various shapes of white LED light modules. By arranging them, Yamada aims to build the LEDs into any shape required for the home or for the office. "If white LEDs are used for lighting equipment, current production capacity is far short of the expected demand level. One building . . . needs about 10 million LEDs. Unless white LEDs are produced widely in the world, the supply won't catch up to the demand," Ueda said.
"The price will be the last issue to be overcome for lighting use," said Jordon P. Papanier, marketing manager of LEDtronics Inc. "We do offer LED lamps for general-task lighting, small-area lighting, but the cost will still be 10 to 30 times that of an incandescent lamp," he said.
Taking a different approach from current white-LED methods, a government-supported project for next-generation lighting is in progress at the Solid State Lightening and Display Center at the University of California, Santa Barbara. The center aims to develop white light-emitting devices with the efficiency of 200 lumens/watt by 2007.
Cecol Inc., a subsidiary of Citizen Electronics
Fairchild Semiconductor Corp.
Matsushita, Panasonic Industrial Co.
Nichia America Corp.
Osram Opto Semiconductor, distributed by Infineon Technologies Corp.
Yamada Shomei Lighting