Cheap, super-efficient LED lights on the horizon

29 January 2009 by Colin Barras

Incandescent tungsten-filament light bulbs face a global switch-off as governments push for energy efficient fluorescent lamps to become the standard. But the light could soon go out on those lamps too, now that UK materials scientists have discovered a cheaper way to produce LED bulbs, which are three times as efficient as fluorescent lamps.

Although the ultimate dominance of LED lights has long been predicted, the expense of the super-efficient technology has made the timescale uncertain. The researchers now say LED bulbs based on their new process could be commercially available within five years.

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.

Cracking up

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.

Early switch-over

A 15-centimetre silicon wafer costs just $15 and can accommodate 150,000 LEDs making the cost per unit tiny. That levels the playing field with CFLs, which many people only ever saw as a stopgap solution to the lighting problem.

Humphreys reckons that the UK government encouraged consumers to drop tungsten bulbs too soon. "We should have stayed with tungsten for another five years and then switched to LEDs," he says.

Humphreys's team was funded by the UK Engineering and Physical Sciences Research Council. The UK government's Technology Strategy Board will now provide the funding to turn the new technology into a commercial process.