LEDs are usually made by growing a thin layer of gallium nitride on top of a sapphire, silicon carbine, or silicon substrate. Soraa takes a different approach. It uses gallium nitride for the substrate. This reduces a mismatch in the crystal structure between the two layers, which causes the performance of LEDs to diminish as current densities increase. By reducing such mismatches, or "dislocations," by a factor of 1,000, Soraa officials say they can push 10 times more current through a given area of active layer material. The increase in current density results in a tenfold increase in LED brightness.
Soraa's LEDs could prove to be expensive, however. Colin Humphreys, director of research at the University of Cambridge's Department of Materials Science and Metallurgy, says growing gallium nitride in bulk is difficult and time-consuming.
"I pay about $30 for a six-inch-diameter silicon wafer or a two-inch-diameter sapphire wafer, and about $500 for a one-inch-diameter gallium-nitride wafer," he says. Humphreys founded CamGaN, a startup developing LEDs that use gallium nitride on a silicon substrate. The company was recently acquired by Plessey Semiconductors. "[Soraa] may well be able to produce gallium-nitride wafers more cheaply, but surely not at the same price as for silicon and sapphire."
Kim says Soraa has a novel method of fabrication that significantly reduces the cost of gallium-nitride production. He adds that the cost of the substrate is only a "single digit percent" of the total cost of the company's bulbs. Soraa has received more than $100 million in startup funding, and plans to ship its first bulbs for commercial applications by the end of this quarter.
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