High-Brightness LEDs Shine In Novel Lighting Applications

High-brightness LEDs continue to make inroads into lighting applications that were traditionally dominated by incandescents and other light sources. Their use in traffic signals, automotive brake lights, architectural lighting, and full-color displays keeps growing. Designers also are finding new ways to apply these versatile semiconductor components as they improve their performance.

LEDs promise significant reductions in power consumption compared to incandescents, as well as longer life and greater reliability. Moreover, improvements in LED semiconductors and packaging are increasing their light output, making them brighter and reducing their cost versus the replaced sources. Consequently, LEDs are becoming viable in more applications.

For example, improvements in LED technology will soon make it possible for LEDs to supplant cold-cathode fluorescent lamps (CCFLs) as backlighting sources in notebook computers. Backlighting is becoming a very important application for high-brightness (HB) LEDs due to the availability of brighter white LEDs. As handheld devices move to full-color LCD displays, LEDs are being used to provide the white sources of backlighting required by color displays in cell phones, PDAs, digital cameras, and camcorders.

Beyond backlighting, though, are innumerable other lighting applications for LEDs, if their performance can be improved and their cost lowered sufficiently. As the global lighting market accounted for $12 billion in sales last year, LED developers have ample motivation to push their technology further.

But members of the LED industry aren't alone in this quest to advance the technology. An effort is now under way to establish a government and industry partnership to speed LED and organic LED-based (OLED) lighting development. Collectively, they're called solid-state lighting.

Last July, legislation to fund the Next-Generation Lighting Initiative (NGLI) was introduced into Congress as part of the Senate Energy Bill S.1766, Sec.1213. This act would create a 10-year program through which the Department of Energy and an industry-led consortium would jointly conduct the R&D needed to make solid-state lighting a primary general-lighting source. The Optoelectronics Industry Development Association (OIDA) is facilitating the industry consortium.

Solid-state lighting adoption is expected to dramatically affect energy consumption and pollution. In the U.S., it could reduce the demand for electrical energy by 10% by 2020. The new light sources would consist of flat arrays of LEDs or laminates of OLEDs, assembled in any pattern or form, then mounted to floors, walls, ceilings, or even furniture.

But before such futuristic lamps can be manufactured, the LED industry must further improve the performance of HB devices. That means higher values for power conversion efficiency (lm/W), operating life, color rendering index, and light output per device. At the same time, it's critical that cost, measured in dollars per thousand-lumen, be reduced dramatically.

One of the NGLI activities is developing a roadmap of technical performance goals for each of these parameters (see the table). Along the way, the roadmap establishes the criteria necessary for white LEDs to replace incandescent lamps, and later fluorescent lamps.^{1, 2} The table's data is only tentative. Final numbers are expected later this spring.

Parameters like those shown in the table are useful tools for describing the state-of-the-art for solid-state lighting and drawing comparisons among existing lighting sources. For instance, a comparison of the lumens per watt efficiencies of red and green LEDs with their incandescent and fluorescent counterparts, which require filtering, shows higher efficiencies for the LEDs. Viewing these same sources in terms of cost measured in dollars per lumen may also give LEDs the edge, when life-cost factors, like lamp replacement and energy savings, are considered. This is why LEDs are now commonplace in traffic signals and other long-life applications.

For white LEDs, the situation isn't that advanced yet, particularly when using them in general lighting applications. In terms of efficiency, white LEDs are already competitive with incandescents in terms of lumens per watt. Still, the LEDs' cost per lumen is much higher. Efficiency comparisons with fluorescents are even less favorable.

But numbers only tell part of the story. Because LEDs are so different from incandescents and fluorescents, they can't be viewed simply on a lamp versus lamp basis. As Makarand Chipalkatti, director of lamp module development at Osram Opto-Semiconductors, explains, when changing to solid-state lighting, "You're not replacing the lamp, you're replacing the lighting system." For example, before viewing different light sources in terms of an illumination specification like candellas/m², designers must consider how much light generated by the incandescent or fluorescent is actually blocked by the fixture.

Beyond those comparisons, solid-state lighting has profound implications for both lighting and architectural design. Solid-state lights will offer other advantages besides reducing power consumption and extending lamp life ("cost of use" factors). It will be possible to dim them without affecting their color, efficiency, or operating life, and to create electronically controlled full-color light sources.

Furthermore, the low profile of tile-shaped LED light sources could call into question the need for drop ceilings that currently hide fluorescent lighting fixtures. Although not the only elements overhead, lighting fixtures may be the critical ones. So consider that in a multistory building, replacing overhead fixtures and their associated drop ceilings could add up. Space savings may be equivalent to an additional story.

Even now, LEDs are enabling the development of lighting designs that were previously impractical. A striking example of such LED lighting illuminates the text frieze that encircles the interior of the Jefferson Memorial in Washington, D.C. (Fig. 1). In this application, a series of 17,000 surface-mount LEDs are assembled on 17-in. linear strips, mounted on a high ledge just below the frieze. The entire 250-ft. long fixture mixes white and yellow LED strips to produce a hue of light that nicely matches the marble wall. Before the LED lighting was installed, the frieze re-mained unlit because the ledge holding the LED strips was too shallow to sit a conventional light fixture on.

How Bright Is High Brightness? Though not strictly de-fined, a few characteristics separate HB LEDs from their jellybean counterparts em-ployed everywhere as equipment indicators. Robert Steele, director of optoelectronics at consulting firm Strategies Unlimited, Mountain View, Calif., cites three factors by which HB LEDs may be identified.

Material is one factor. The current materials of choice are aluminum indium gallium phosphide (AlInGaP) for red, yellow, and green lamps; and gallium nitride (GaN) for greens and blues. White LEDs are typically blue LEDs coated with a phosphor that causes them to emit a whitish light.

Luminous intensity is another yardstick. As a rule of thumb, an LED that produces hundreds of millicandellas (mcd) in the standard 5-mm (T1-3/4) package is an HB LED.

Yet a third means of distinguishing HB LEDs is that they're viewable in sunlight. This makes them usable in outdoor signals or displays.

Improvements in chip packaging and chemistry are pushing the performance of both AlInGaP and GaN-based LEDs. A look at some recent HB LED developments indicates the current state-of-the-art and where performance is heading in the near future.

One well known HB LED is the Luxeon chip from Lumi-Leds Lighting. Measuring 1 mm², it's considerably larger than the 8-mil die that typically appears in the popular 5-mm package. As a result, the Luxeon LED can operate at much higher drive currents, in the 350- to 700-mA range. Presently, the Luxeon series includes an AlInGaP red/
orange device that delivers 50 lm/W. That value should rise to 58 lm/W this year.

However, more dramatic improvement is expected from the Luxeon InGaN devices. The InGaN green should go from its current 32 to 60+ lm/W. The blue InGaN chip will increase its output from 8 to 14 lm/W.

Beyond boosting the efficiency with which the chip produces light, vendors are improving chip packaging so that more of the chip's light output can be extracted from the package. A typical 5-mm LED will produce only a few lumens per package in blue or white, whereas much higher levels will be possible with the packaged Luxeon device. This year, blues should increase from 5 to 20 lm/package; greens from 25 to 100 lm/package; and whites from approximately 18 to almost 50 lm/package.

In terms of performance at the chip level, large-area power chips are boosting light output to new highs. For example, Cree recently introduced a 900- by 900-µm die that generates 150 mW of blue light, making perhaps the brightest blue chip on the market. However, the device's 350-mA drive current demands special packaging to handle its high power (heat) dissipation.

Thermal management is generally a concern for all LEDs driven at high currents. In a typical LED, the heat that builds up in the chip gets transferred to the LED's copper leadframe. Unfortunately, there's a temperature coefficient of expansion (TCE) mismatch between the chip and the leadframe. That mismatch is one of the threats to LED reliability. Electrostatic discharge (ESD) is another.

Semiconductor manufacturer California Micro Devices has found a way to address both of these problems for InGaN LEDs fabricated on sapphire substrates. The company produces a silicon carrier or submount that serves as an interposer between the InGaN chip and the leadframe. It alleviates the TCE mismatch while providing ESD protection via integrated zener diodes.

There are optical benefits too. The submount contains solder bumps, enabling flip-chip assembly of the LED chip to the submount, which is then wirebonded to the leadframe. This approach eliminates shadows caused by wirebonding directly to the LED. In addition, the submount reflects much of the LED's light that's normally lost.

White LEDs Improve: Enhancements in luminous efficiency aren't the only factors driving LED development. For white LEDs, vendors also are striving to improve the quality of light, or color rendering.

Light sources are graded on a scale between 1 and 100 for their color rendering. Incandescents are considered perfect in this regard and are ranked at the top of the color-rendering scale with a score of 100. Fluorescents are rated in the low to high 80s, while white LED scores range from 60 to 70 for inexpensive LEDs, to the low 70s for better-quality LEDs.

Until LEDs provide better color rendering, their use in general lighting applications may remain limited. So, vendors are working to improve their white LEDs by using better phosphors.

In most white LEDs, a yttrium aluminum garnet (YAG) phosphor is used to convert some of the blue chip's light to yellow, which then mixes with the blue to produce a whitish light. GELcore, among other companies, is working on an alternative to YAG that promises more effective generation of high-quality white light. GELcore's phosphor should appear in LED products later this year.

Another technique for generating white light—that can produce very good color rendering—is the combination of discrete red, green, and blue (RGB) LEDs. Although not so popular due to its higher cost and complexity, the same concept is being applied via UV LEDs and multiple phosphors.

A noteworthy example is Toshiba America Electronic Components, which together with Toyoda Gosei Co. Ltd. used such an approach to develop a white LED. Introduced last year, the LED contains a UV InGaN chip that excites RGB phosphors to produce white light. The color of light comes from the mixing ratio of RGB phosphors, so a number of benefits are accrued.

One is a constant color temperature over a wide range of temperatures and forward currents, creating smaller color deviation and shifting than conventional white LEDs. These characteristics are important in the target application where they provide backlighting for automotive panels formerly lit by incandescents.

Moreover, the Toshiba LED is brighter than its equivalent among conventional white LEDs. The output of the Toshiba part is 160 mcd, which the company expects to raise to 200 mcd this year. But the technique for generating white light is interesting for another reason. Varying the mix of RGB phosphors creates new colors, such as pink and violet (Fig. 2).

Other vendors also are developing UV LED chips for applications beyond just white LEDs. In its X-Bright family, Cree has developed a 290- by 290-µm GaN-based LED chip that generates 18 to 20 mW of UV light. (That value reflects packaged performance.) With a drive current of around 20 mA, this chip can be implemented in standard LED packaging.

Another company, Uniroyal, produces a 400-nm InGaN chip that generates 2 to 3 mW of UV output from a die measuring 12 to 13 mil (250 to 300 µm) per side. (This power specification represents the unpackaged die.) One use for this chip is in currency authentification where a fluorescent tag is embedded within the bill. When exposed to the UV light created by the LED, the tag glows visibly. UV LEDs also are finding applications in biomedical equipment with uses in photodynamic therapy and bacteria detection.

Meanwhile, development of InGaNbased blue LEDs continues as a means of making brighter white LEDs. In terms of chip development, Cree's X-Bright family includes a blue device that generates 15 mW. This device is intended for use in standard surface-mount and 5-mm LED packaging.

Moving from the chip to the package level, InGaN blue LEDs also have reached new heights in performance. Consider Nichia's NSCW215/335 ser-ies of 0.8- and 1-mm-high side-view LEDs. With output rated at 600 mcd, these devices have achieved 50% greater light output than the company's existing SMT series.

Side-emitting LEDs are especially suited to the LCD backlights because they improve the optical efficiency of the light guide by eliminating the energy-wasting bends required by conventional top-firing LEDs. With the improved performance of side-view white LEDs, backlight developers now have a greater ability to reduce the number of LEDs in backlight designs

Unlike backlighting small LCD screens, using HB LEDs to illuminate larger displays like those in notebook and desktop monitors isn't common. That's changing, though, as vendors look to increase battery runtime.

LumiLeds has re-ported that display maker FIMI Philips is using its Luxeon series of emitters to backlight a 15-in. medical-grade LCD monitor. LEDs offer several benefits over the CCFLs that they replace, including tunable white points, without reducing grayscale levels. Plus, because the backlight deploys a combination of red, green, and blue LEDs, it provides a wider range of colors while letting users adjust the color in real time. Greater brightness and longer life expectancy (50,000 hours) are other advantages of the LED backlight.

Now, the quest is on to improve the efficiency of HB LEDs to apply them in consumer-grade applications. In a typical 15-in. monitor today, the CCFL draws about 18 W. In the same application, the current generation of Luxeon LEDs would draw about 25 W. But LumiLeds' roadmap says that value should drop to 11 W by next year. The change to LEDs is expected to double notebook runtime. In addition to cutting lamp power, the move to LEDs eliminates the high-voltage inverter, plus the RF protection circuitry and UL approvals accompanying it.

References:

1. "The Solid-State Lighting Industry Initiative: An Industry/DOE Collaborative Effort," by Stephen Johnson, Architectural Lighting Magazine, November/December 2001, http://eetd.lbl.gov/btp/papers/47589.pdf.
2. "The Promise and Challenge of Solid-State Lighting," by Arpad Bergh, George Craford, et al., Physics Today, Dec. 2001, see www.aip.org/pt/vol-54/iss-12/p42.html.