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Backlights, Airports And Vehicles Boost LED Market

As chip manufacturers continue to improve the performance and reduce the cost of their products, new applications in areas such as airport lighting and car headlamps continue to emerge. Tim Whitaker reports from the Intertech LEDs conference.
The LED market continues to expand, driven not only by existing applications such as signage and backlights for mobile appliances, but also by the emergence of new applications. Examples such as car headlamps, airport lighting and dental curing tools were among the many subjects covered at the recent LEDs 2003 meeting, which also included details of the progress being made by leading device manufacturers.

Backlighting drives market growth

According to Jagdish Rebello of market research company iSuppli, the total LED chip market will reach $3.4 billion in 2003, compared with $3 billion in 2002. Rebello also said that the compound annual growth rate (CAGR) will be 12% between 2002 and 2007, and the market will exceed $5 billion by the end of that period. The high-brightness (HB) segment (iSuppli defines HB-LEDs as having an output of more than 500 mcd) will be about 45% of the total market in 2003, and about 57% in 2007.

In a number of applications, long lifetime is a key factor in the adoption of LEDs. However, while there may be a high initial demand for LEDs, their long lifetime means that LED-based products have a long replacement cycle, which can slow market growth. Therefore, says Rebello, it is imperative for LEDs to continue to find new applications in order to stimulate further growth in the overall market.

Figure 1 shows the 2002 LED market by application. Backlighting and signage comprised 17% and 10.3% of the total LED market respectively, and these segments, which mainly use HB-LEDs, offer the highest growth potential. The largest segments for standard-brightness LEDs were for status indicators and alphanumeric displays (37.5% of the total market) and IR transmitters (21.3%).

Rebello says that LEDs are particularly well suited for backlighting small LCD panels in cell phones, PDAs, digital cameras and camcorders. The market for larger panels - tablet PCs, laptop screens - is several years out, with the barrier being uniform illumination across large areas. The human eye requires a color uniformity of at least 95%.

iSuppli estimates that the backlighting market for LEDs will grow at a CAGR of 18% from around $500 million in 2002 to more than $1.1 billion in 2007. The market should be around $700 million in 2003. In handsets, the average number of LEDs per display is decreasing as LED performance improves; however, some phones now have more than one screen. The introduction of higher-quality cameras in phones might reduce the demand for stand-alone digital cameras, which would in turn reduce the demand for LED backlights in the latter application.

Osram s thin-film technologyNumerous techniques have been developed to enhance the extraction efficiency of LEDs, and several of these rely in part on the emission from the sidewalls of window layers and transparent substrates. In some cases, notably Lumileds TIP-LED and Cree s XBright device, the chips are shaped. Sidewall emission becomes less effective when the chip area is increased, so that in larger chips the extraction efficiency and hence the luminance (which correlates to brightness) is reduced. Osram Opto Semiconductors has developed a new approach that does not rely on sidewall emission, and allows the luminance to be scaled with chip area.

Osram s "Thin Film" technology involves bonding the LED structure to a metallized carrier substrate, after which the original epitaxial growth substrate is removed by laser lift-off (Compound Semiconductor June 2003). The company has manufactured prototype 5 mm radial blue (460 nm) LEDs with an output of 16 mW at 20 mA.

At the conference, Osram s Klaus Streubel described the fabrication of AlGaInP Thin Film devices, a process carried out using 4 inch wafers. After growth, microprisms are etched into the epitaxial layer structure, and the surface is coated with isolation and metallization layers. This structure is bonded to a metallized carrier before removal of the original GaAs substrate. In the final device structure, shown in figure 2a, a p-contact is formed at the top of each microprism. Light emission takes place from the active layer within each microprism, but does not take place in the area directly underneath the n-contact bond pad. Furthermore, the geometry of the microprisms causes light to be efficiently reflected towards the top surface. As shown in figure 2b, each device contains a number of microprisms.

According to Streubel, 618 nm Thin Film devices have demonstrated an extremely high luminous efficacy at 20 mA of 96 lm/W, with a wall-plug efficiency of 33%. At 10 mA, the efficacy was slightly higher at 98 lm/W. The 300 x 300 µm2 devices had an output of 12 lm at 70 mA. Streubel also reported yellow LEDs (595 nm) with an efficacy of 49 lm/W at 20 mA and an output of 7 lm at 70 mA.

In comparison, Lumileds has reported an efficacy of 100 lm/W for its 605 nm TIP-LED chip, which has a much larger size of 1 x 1 mm2.

The above results have been generated with Osram s third-generation Thin Film devices, while first-generation devices have been commercially available for about a year. At a current of 600 mA, large first-generation devices measuring 1 x 1 mm2 generated an output of 44 lm at 615 nm.
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