Making cheaper LED bulbs

UK materials scientists have discovered a cheaper way to produce LED bulbs, which are three times as efficient as fluorescent lamps. Gallium nitride (GaN) LEDs have many advantages over compact fluorescent lamps (CFLs) and incandescent bulbs. They switch on instantly, with no gradual warm-up, and can burn for an average of 100,000 hours before they need replacing – 10 times as long as fluorescent lamps and some 130 times as long as an incandescent bulb. CFLs also contain small levels of mercury, which makes environmentally-friendly disposal of spent bulbs difficult.

The cost of production has kept the LEDs far from homes and offices, however. Gallium nitride cannot be grown on silicon like other solid-state electronic components because it shrinks at twice the rate of silicon as it cools. Crystals of GaN must be grown at 1000°C, so by the time a new LED made on silicon has cooled, it has already cracked, rendering the devices unusable.

One solution is to grow the LEDs on sapphire, which shrinks and cools at much the same rate as GaN. But the expense is too great to be commercially competitive.

Now Colin Humphreys’s team at the University of Cambridge has discovered a simple solution to the shrinkage problem.

They included layers of aluminium gallium nitride in their LED design. These layers shrink at a much slower rate during cooling and help to counteract the fast-shrinkage of pure gallium nitride. These LEDs can be grown on silicon as so many other electronics components are. “They still work well as LEDs even with those extra layers inside,” says Humphreys.

– from newscientist. 29 Jan 2009