The high-brightness LED market has undergone another year of 50% growth. All of the major industry players are expanding capacity, and epi wafer and chip production capacity is growing rapidly in Taiwan, Korea, and China. In addition, new strides are being made in everything from research funding initiatives to development of noninvasive methods to observe the aging of components within in-service LED illumination systems.
During an industry panel discussion at the annual Strategies in Light Conference (San Mateo, CA) in February, Jason Posselt, director of product marketing at Lumileds (San Jose, CA) described his company's development of a supporting infrastructure to help bridge the gap from just selling a bag of LEDs to actually providing a lighting system (see figure). Posselt's talk concluded the panel discussion with an upbeat look at Lumileds' pioneering efforts in the high-brightness market, new technology and product developments, and the fact that excitement over the potential of high-brightness LEDs has spread well beyond chipmakers to establish itself firmly in general lighting and business communities. The upbeat attitude was also characteristic of morale throughout the conference, which exceeded all four previous years in size with 22 exhibitors and more than 300 attendees.
Leading off the same industry panel on high-power LEDs, Norbert Hiller, vice president and general manager of Cree (Durham, NC) said the Cree lighting business will focus on high-power, packaged LEDs separate from that company's established chip business. Cree has traditionally focused on LED chips that have progressively grown by approximately an order of magnitude in both power output and form factor since their introduction of a blue GaN LED chip in the mid-1990s. Established illumination applications include mobile phone and automotive interior displays.
Development into more demanding applications for LED lighting, from architectural and automotive exteriors to general illumination, will require increases in luminous efficiency, current density, and wafer diameter analogous to historical efficiency improvements in the semiconductor industry, as well as the further development of optically transmissive, heat-dissipating, and durable packaging technology. Hiller defined the transition point from the historical standard LEDs to power LEDs for lighting applications at 100,000 lumens per wafer. The company's initial lighting product, a 13-lumen white LED lamp (also available in blue, green, and red) went into preproduction sampling in the first quarter of this year with full production targeted for this summer.
Following Hiller's talk, Hiroki Oguro, who heads the power-LED development effort at Nichia (Tokushima, Japan), made a lasting retinal impression on the dark-adapted audience by shining a 1000-lumen LED lamp prototype into our eyes. "We can produce such high-efficiency LED lamps as strong light sources, but we must first discuss the design requirements for specific applications," Oguro said.
He also said that new markets are needed for LEDs to reach new business targets. Sales revenues for LEDs passed through an embryonic growth period in the 1990s, fueled primarily by display applications. The growth has continued exponentially into what Oguro described as a first-step growth period in the early part of this decade, fueled largely by digital still and video cameras, cellular phones, personal digital assistants, and automotive applications. Continued exponential growth in the second half of this decade will require new markets, which Oguro speculated might come from environmental or medical needs. At Photonics West (San Jose, CA) the previous week, Oguro also described a 400-mW, 365-nm ultraviolet LED introduced by Nichia, as well as a three-phase development program for new phosphors to improve color-rendering-index performance, Nichia's luminous efficacy roadmap, and packaging trends to meet diverse applications.
Ruediger Mueller, president and CEO of Osram Opto Semiconductors (Regensburg, Germany), followed Oguro's talk with an emphasis on thin-film chip structures and innovative packaging designs. While a number of strategies for increasing the extraction efficiency of light from LED chips make use of the chip sides, thin film technology increases light extraction efficiency through just one surface, which can make a big difference in emitted light as chip area is increased. "Light emitted per square millimeter is crucial in many LED applications," he said. "This is where thin film has its strength."
A microprism structure and buried metal reflector in the LED structure provides a pure surface emitter with a perfect Lambertian profile, Mueller said. He described a red emitting (618-nm) aluminum indium gallium phosphide (AlInGaP) chip with an extraction efficiency on the order of 75% and yielding 130 lumens/mm2 that is currently in production in sizes up to 1 mm2. Indium gallium nitride (InGaN) thin-film chips with luminous flux on the order of 2 lumens are ramping up for production and available in prototype, in blue (470 nm) and green (527 nm) respectively.
As did other speakers, Mueller emphasized the importance of packaging in prolonging chip life by reducing temperature. He also raised the important communication issue that even though the chip manufacturer handles thermal resistance up to the solder point, the customer must handle thermal resistance from the solder point to the application. In a presentation given after the panel session, Jean-Paul Freyssinier, a design specialist at the Rensselaer Polytechnic Institute Lighting Research Center (Troy, NY), described research that may eventually enable users to monitor junction temperature remotely without disassembling installed lighting systems.
Optoelectronic applications account for about 7% of sales revenue at Toyoda Gosei (Aichi, Japan), and despite the company's primary focus on automotive applications, only about 10% of its optoelectronics goes into automobiles. The lion's share (70%) goes into consumer applications such as mobile phones and personal digital assistants, with another 8% going into signage, according to Ryoichi Tohmon, who followed Mueller on the panel. Tohmon described the company's focus on obtaining white light with good color rendering (mimicking sunlight) by using a short wavelength LED with red, green, and blue phosphors. He also described a packaging design currently in prototype to maximize light coming from the top of the LED and minimize light emitting from the side.
The emphasis on lumens per square millimeter was new, said Steve Paolini, director of business development at Lumileds. "At previous conferences the need to decrease lumens per watt and lumens per dollar has been discussed," he said. "But this is the first time I've heard lumens per square millimeter discussed here, even though it is something that we (at Lumileds) have been looking at internally."
There are applications, such as projectors and automotive headlights, he said, in which lumens per square millimeter are important, and in which lumens per watt and lumens per dollar are less of an issue.