editorialview by Dr Richard Stevenson, Editor Which way forward?
The revolution in solid-state lighting is no longer a question of if, but when. So the big question now must be this: What class of emitter is going to drive the revolution?
I’ve just arrived back from the big nitride show of the year, the International Conference on Nitride Semiconductors (ICNS), and one of the most important points that I took away from that 900-strong meeting was the wide variety of solid-state emitters that have a strong chance of being used for lighting.
Today, everyone agrees that the cost-per-lumen is too high for mass adoption of solid-state technology, and the devices of tomorrow are going to have to do much better. LEDs are already moving in this direction, with Cree, for example, making gains of about 40 percent per year, based on the lumen-per-watt metric.
Another option for cutting costs is to switch the substrate to silicon: It is cheaper, available in a far larger diameter, and allows processing of the wafers in under-utilised silicon fabs. The problem with GaN-on-silicon LEDs has been a lag in performance over the incumbents, but that gap is closing fast.
Editor Dr Richard Stevenson
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According to speakers at ICNS, devices made in the labs of Osram and Samsung are now both within touching distance of their sapphire-based cousins.
It may be that the best way forward is to ditch the blue-LED- and-phosphor-combo, and move to colour mixing of red, green and blue chips. This promises higher efficiencies and colour tunability, but it is held back by the poor efficiency of green LEDs. Maybe this could change, however, following development of nitride-based yellow LEDs at Samsung that feature a very thin AlGaN layer between the wells and barriers. This modification to the active region helps to propel efficiency to almost 20 percent.
By far the most radical vision for the future is being put forward by Sandia National Labs: Use lasers, not LEDs, for lighting. The thinking behind this is that lasers perform better than LEDs at very high current densities, because it is only the latter that suffers from droop – a decline in efficiency as the current is cranked up.
German automaker BMW is already developing laser-based headlights, and it will be interesting to see if any firms in the lighting business start to follow suit in order to trim chip costs. I, for one, bet that the evolution of the solid-state lighting industry is going to take a few twists and turns this decade.
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October 2013
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