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chips. According to Soraa’s physical models, at around 90%, the extraction efficiency of specially “roughened surface” volumetric chips is well beyond the thin-film limit currently in the range of 82 to 84%. What’s more, the devices exhibit excellent power density uniformity, with no current crowding even when driven at ten times the current density of traditional LEDs. This means that GaN-on-GaN LED dies about 15 to 25 times smaller than typical power LED dies can be driven at high power and yet retain a high external quantum efficiency. Soraa stress-tested its LEDs for several thousands of hours at junction temperatures up to 140ºC without noticeable performance shifts. Using the TM-21 convention, the company predicts a lifetime of over 30,000 hours for its products under standard conditions. Using proprietary Si-based wafer level packaging, Soraa packed up to 36 dies under a mix of red, green and blue phosphors to produce high brightness compact LEDs. When asked if Soraa could be cost competitive despite its reliance on expensive GaN substrates, David replied with a resounding yes! “The substrate is expensive, but we can drive the LEDs from 15 to 25 times harder, so we get much more light output while using much less substrate”. “The first generation of LED technology (on foreign substrates) is maturing slowly, but GaN-on-GaN LEDs offer a breakthrough in output power per wafer, and we are only at the beginning” he added. Then back on the white light quality issue, David only had praise for the new devices, with a measured CRI of 95. Because Soraa’s chips emit violet light to pump a proprietary mix of phosphors, the company is able to generate cyan light, as opposed to the cyan gap of common blue-pumped LEDs. “But the CRI is only part of the story when it comes to quality of light” emphasized David, “It is only a metric for so-called colour fidelity, and a debatable one according to ongoing academic research”. For better whiteness rendering, a lot of everyday objects (including white shirts, paper, plastic, and even your teeth) contain optical brightening agents that absorb UV and violet light and emit blue, making white look brighter. By emitting violet light, Soraa’s LEDs are able to properly activate the whitening agents and render white objects, which regular white LEDs can’t do. David would not be too specific about the GaN substrates the company uses, except that they are commercially available Hydride Vapor Phase Epitaxy (HVPE) GaN substrates. But he acknowledged that the company is also doing research on the growth of bulk GaN, experimenting with bulk GaN wafers (so fare too small and expensive to be commercially viable). As for the future of GaN-on-GaN LEDs, David is very optimistic. “Currently, the LED market is still vertical, manufacturers want to do it all by themselves. But substrate price erosion A slide comparing the light spectrums of different white lights. alone will drive further adoption of this technology”. “Then if in a few years we can make cheap GaN substrates, the benefits will be so compelling for the whole LED industry, that you will ask, why bother do anything else?” Helping GaN-on-GaN LEDs take the lighting world by storm as David would bet, Soraa is commercializing a slick MR16 Lamp which received the 2013 Red Dot Award for all its innovative features. Later after his keynote, David was kind enough to demonstrate the lamp against the nearest competing device, and the colour rendering difference was striking. Exit the yellowish tones! Welcome the bright whites. David is cautious to express the luminous output of Soraa’s products as the centre beam candlepower (CBCP), arguing that communicating in lumens can be deceiving, because the lumens need to be in the centre beam (not outside a specific beam angle) to be considered as useful. The company says it achieves a high CBCP by combining a high lumens output from a small source size together with good optical design. Here again, centre beam candlepower comparisons should only be made among lamps designed with the same beam angle. With this in mind, David gave us some elements of comparison. “For instance, you will find that for a 3000K 24 degree lamp, Philips’ state of the art MR16 lamp outputs just about 2000 cd of CBCP at 10 watts (40watt equivalent, CRI80) while a Soraa product with a CRI of 95 can deliver a CBCP of 2750 cd (11.5watts, 65watt equivalent)” David noted in complementary email exchange. Soraa’s single volumetric GaN-on-GaN LED (top), and a fully packaged LED assembly (bottom). www.electronics-eetimes.com Electronic Engineering Times Europe January 2014 17


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