White LEDs: The backlight of choice for small liquid-crystal displays
From mobile phones to laptop computers, white LEDs provide a cost-effective and energy-efficient solution for backlighting full-color LCDs
In the past five years, white light-emitting diodes (LEDs) have become the preferred choice for backlighting small, full-color liquid-crystal displays (LCDs) in portable electronic products, including mobile phones, PDAs, digital still cameras, video cameras, MP3 players, and handheld video game players. White LEDs have some obvious advantages compared to the traditional LCD backlight technology (cold cathode fluorescent lamps, or CCFLs) for these applications, namely high efficiency, compactness, low voltage, and availability in surface-mounted device (SMD) packages that can be reflow soldered on the PCB that contains other electronic components.
White LEDs came into widespread use around 2000 when full-color displays began to be seen in mobile phones. The first widespread adoption of full-color mobile phone displays began in Japan, then rapidly spread to South Korea and other Asian countries, then to Europe and North America. Whereas in 2000, just 7% of mobile phones had full-color displays, in 2006 more than 90% of mobile phones sold worldwide will have full-color displays, almost all of them using LCDs backlit with white LEDs.
Prior to 2000, LEDs had been used as backlights for monochrome displays, but these LEDs were of a much older generation than the high-performance white LEDs that were developed in the late 1990s. These older-generation LEDs were developed in the early 1970s using doped gallium phosphide (GaP) as the light-emitting material. Emitting in the yellow-green region of the spectrum, these devices were low in efficiency but also very cheap.
LEDs that emit white light are produced using a blue LED chip combined with a phosphor such as Ce:YAG. The blue emission (at approximately 460 nm) excites the phosphor, causing it to emit in the yellow region of the spectrum. However, not all of the blue light is adsorbed by the phosphor, and the combination of blue plus white emission appears white. This white spectrum is not of high quality as a light source in terms of its ability to render true colors, but it is adequate as a backlight for small displays. White LEDs were first developed by Nichia Corp. of Japan (www.nichia.co.jp), and are now produced by a wide variety of LED manufacturers. However, Nichia, along with its licensing partner Citizen Electronics (www.c-e.co.jp), still holds a dominant position in the small LCD backlight market due to its commanding technology position, as well as its strong IP position in Ce:YAG phosphors and white LED packaging.
The white LEDs used to backlight small full-color LCDs are side-emitting SMDs, also termed side view SMDs. The performance of the white LEDs used for backlighting has improved dramatically since their widespread use began in 2000, and continues to improve. In 2004, the highest-performing white side view LEDs offered by Nichia were in the range of 720 to 1,000 millicandela (designated the “T bin” by Nichia). However, in 2006, the performance of Nichia’s highest bin (V1) was 1,440 to 1,720 millicandela, nearly twice the performance of just two years ago. The need for brighter LED backlights is driven by the evolution of mobile phone displays to higher information content (including video), higher pixel counts (up to one-quarter VGA, or 240×320), and larger sizes.
As performance has continued to advance, the dimensions of the SMDs have continued to shrink in response to the never ending push for decreasing handset thickness on the part of mobile phone manufacturers. The smallest white side view SMD available in 2006 measures just 3.8 (L) × 1.2 (W) × 0.6 (H) mm, and SMDs with a thickness of just 0.4 to 0.5 mm are beginning to be offered to the market.
With the successful adoption of white LEDs as backlights for mobile phone displays, it is not surprising that these devices have also become widely adopted in other portable electronic products. Other portable products in which white LEDs are widely used as backlights include:
• Digital still cameras
• Video cameras
• PDAs
• MP3 players
• Handheld video game players
Although the total unit sales of such products are much smaller than those for mobile phones, the overall growth is projected to be much higher in percentage terms. In 2005, approximately 680 mobile phone handsets with full-color displays were sold worldwide. The total sales of all the products listed above were just 147 million units (including only devices with full-color displays). However, while mobile phone sales are expected to experience slower growth in the next five years, the unit sales of the products listed above are forecasted to more than double during that period.
A new mobile appliance market that began to emerge in 2005 was the use of white side view LEDs to backlight larger displays in notebook computers. Several notebook PC manufacturers introduced models using white LED backlit displays in 2005: Sony, Fujitsu, and Toshiba. LCD sizes ranged from 7.2 to 11.1 inches (diagonal, wide), and the number of LEDs used in the backlights ranges from 20 to 40.
All three models are very lightweight (three pounds or less). The advantages that are being promoted for the LED backlights include a very thin profile for the display (because the side-mounted LEDs are much thinner than the CCFL and its associated light guide) and longer battery life due to the higher efficiency of the LEDs. The latter feature is particularly attractive because battery life continues to be a problem plaguing laptop users, and those laptop makers that have products with longer battery life prominently promote this feature in their sales literature.
The LCD subsystem accounts for 30% to 40% of laptop power consumption on average, and the backlight accounts for 65% to 75% of the display power. Thus the backlight accounts for 25% to 30% of the total power consumption of the laptop, and a reduction in its power could contribute significantly toward longer battery life.
Only recently have white LEDs become efficient enough to compete head-to-head with CCFL backlights for laptop display applications. The best white side view LEDs available in 2006 have luminous efficacies of approximately 60 lm/W. This is not quite as high as a CCFL, which is approximately 70 lm/W. However, when light coupling loss and driver efficiency are taken into account, the overall LED performance is actually better and will improve in the future.
To address the potential for reducing power consumption in laptop computers, 40 PC industry companies, spearheaded by Intel, have formed a group known as the “Mobile PC Extended Battery Life Working Group.” One of the focus areas of the group is the use of LEDs to replace CCFL backlights. The group will be working with laptop PC manufacturers, LCD suppliers, LED suppliers, light guide suppliers, and others to adopt a strategy to accelerate the adoption of LED backlights into mobile PCs.
Although to date only LCDs up to 11 inches have been backlit with LEDs, several Taiwan manufacturers have announced that larger laptop PC displays (12 inches and larger) will become available in 2006. Taiwan is the center of laptop PC manufacturing, as well as the location of the top LCD panel suppliers and many component manufacturers in the LCD supply chain (e.g., backlights, light guides, etc.). Therefore, if these companies become committed to using LEDs as backlights in laptop displays, it is very likely that the market will develop sooner rather than later. For laptop computer display backlights, the cost difference between CCFL and LED is only $1 to $2, and that difference will shrink as LED prices continue to decline. This is a very small differential for a $2,000 laptop.
Another emerging application area is backlighting small to mid-size LCDs used in portable DVD players and automotive navigation and entertainment systems. These displays are typically in the range of 6- to 8-inch diagonal (they are normally referred to as “7-inch displays”). A number of prototypes have been developed and most small LCD panel manufacturers in Taiwan have conducted evaluations and have announced significant production ramp-up in the second half of this year. These manufacturers have started to place orders with Taiwanese white side view LED packagers and chip makers for this application.
Just as was the case for mobile phone and other small mobile appliance displays, white LEDs have become a cost-effective solution for displacing CCFLs in 7-inch displays, and the energy efficiency is comparable. Major LCD panel makers in Taiwan indicate that a large percentage of 7-inch displays will be backlit by LEDs in the next several years. With a market size of 27 million displays in 2006 and growing, this application offers a significant new market opportunity for white side view LEDs.
While full-color LCD screens backlit with white LEDs continue to dominate the portable electronic product market, there is much discussion centered on a potential competing technology known as organic LEDs, or OLEDs. Full-color displays can be made by patterning red, green, and blue OLEDs to form pixels that can be individually addressed, much like the individual cells in an LCD. Because the OLEDs are emissive, no backlight is needed. Even though a great deal of basic research and product development is being carried out on OLEDs around the world, the market to date has been limited primarily to small single-color and multicolor (rather than full-color) displays.
Some early attempts to incorporate OLED full-color displays into digital still cameras and mobile phones were not successful. However, they are now in widespread use in the secondary displays in clamshell-type mobile phones, in both multicolor and full-color versions. As the technology continues to improve, production processes become mature, and costs come down, OLED displays will increasingly penetrate the small display market, challenging the dominance of LED backlit LCDs.
Robert V. Steele is director of optoelectronics at Strategies Unlimited (www.strategies-u.com), a research unit of PennWell Corp., located in Mountain View, CA.
