Technical Insight
Innovations push white LEDs towards new applications
White LED makers are making strides to improve the color consistency, white light quality, lifetime, efficiency and cost of their products, writes Tim Whitaker.
Backlights for color screens in mobile appliances, automotive headlights and general lighting are all at different points on the applications roadmap for white LEDs (figure 1). A number of companies are pushing the technology forward, and several of these spoke at a white LED technology forum during the Strategies in Light meeting in February (Compound Semiconductor April 2003).
There are three basic approaches to making white light using LEDs. One is to combine the output from two or more LED chips; these could be blue and yellow, or more commonly red, green and blue (RGB). The second approach is phosphor conversion, in which a blue LED chip is combined with a phosphor. Some of the blue photons are down-converted by the phosphor to produce a broad emission centered on yellow; this mixes with other blue photons to create white. The third main approach is to use a UV LED as the pump chip for a mixture of phosphors that emit across the visible spectrum.
Gelcore, the joint venture between GE Lighting and Emcore, is firmly in favor of the UV approach. According to Rebecca Bompiedi, VP of technology at Gelcore, UV-phosphor blends can generate white light with excellent color rendering index (CRI) and high efficacy.
"One of the keys to designing UV-based white LEDs is to have the correct combination of the UV chip, the phosphor and the package," said Bompiedi. Requirements include high-power UV chips; packages that are tolerant to UV light and high-power operation; efficient and stable phosphors; robust encapsulants; and a high overall system efficiency.
Gelcore has developed a range of new phosphors to control UV bleed-through, and therefore to prevent the loss of unconverted UV energy. The company has also developed its ThermaLED package, which has a power rating in excess of 1 W and a low junction-to-board thermal impedance of
There are three basic approaches to making white light using LEDs. One is to combine the output from two or more LED chips; these could be blue and yellow, or more commonly red, green and blue (RGB). The second approach is phosphor conversion, in which a blue LED chip is combined with a phosphor. Some of the blue photons are down-converted by the phosphor to produce a broad emission centered on yellow; this mixes with other blue photons to create white. The third main approach is to use a UV LED as the pump chip for a mixture of phosphors that emit across the visible spectrum.
Gelcore, the joint venture between GE Lighting and Emcore, is firmly in favor of the UV approach. According to Rebecca Bompiedi, VP of technology at Gelcore, UV-phosphor blends can generate white light with excellent color rendering index (CRI) and high efficacy.
"One of the keys to designing UV-based white LEDs is to have the correct combination of the UV chip, the phosphor and the package," said Bompiedi. Requirements include high-power UV chips; packages that are tolerant to UV light and high-power operation; efficient and stable phosphors; robust encapsulants; and a high overall system efficiency.
Gelcore has developed a range of new phosphors to control UV bleed-through, and therefore to prevent the loss of unconverted UV energy. The company has also developed its ThermaLED package, which has a power rating in excess of 1 W and a low junction-to-board thermal impedance of
