7 February 2012

Plessey acquires Cambridge GaN-on-Si HB-LED spin-off CamGaN

Plessey Semiconductors Ltd has acquired CamGaN Ltd, a University of Cambridge spin-off formed in 2010 to commercialize proprietary technology for growing gallium nitride (GaN)-based high-brightness (HB) LEDs on large-area silicon substrates.

Picture: An LED emitting light on a 6-inch silicon wafer.

The acquisition will enable Plessey to exploit synergies with its 6-inch silicon processing facility in Plymouth, UK to produce HB-LEDs based on CamGaN’s proprietary 6-inch GaN-on-Si technology. The new technique will be commercialized by a new arm of Plessey called Plessey Lighting, which will initially make LEDs on silicon for external manufacturers, but in time hopes to develop its own light bulbs in-house.

The technology was co-invented by professor Sir Colin Humphreys CBE, director of research at the University of Cambridge’s Department of Materials Science and Metallurgy (and director of the Cambridge Centre for Gallium Nitride). It is based on more than 10 years of basic and applied research on GaN involving $16m in funding (including funding from the UK’s Engineering and Physical Sciences Research Council).

“If we had stopped at the research stage, our work would probably have been picked up and commercialized overseas,” says Humphreys. “This way, we can create more jobs in a low-employment part of the country and potentially turn Britain into a major centre for better, greener lighting,” he adds. “Plessey is committed to exploiting GaN-on-Si technology in the UK,” Humphreys comments.

“Not only could his research result in a highly marketable, low-carbon alternative to the everyday light bulb, but he has worked closely with industry to ensure that the commercialization process also happens in the UK,” comments David Willetts (the UK Government’s Minister for Universities and Science) on Humphreys’ work.

The technology enables the growth of thin HB-LED structures on standard, readily available, silicon substrates. Whereas existing silicon carbide (SiC) and sapphire substrates are expensive and difficult to scale up, GaN-on-Si offers cost reductions of about 80% for LEDs due to the advantages of using a proven semiconductor process, e.g. by reducing scrap rates, minimizing batch process time, and enabling the use of automated semiconductor processing equipment. These cost reductions can be achieved while enabling luminous efficiency in excess of 150 lumens per watt - a combination that should allow Plessey to offer the most cost-effective solutions in the HB-LED industry, the firm reckons.

“To date, the biggest technological challenge preventing the commercialization of HB LEDs grown on large-area silicon substrates has been the large lattice mismatch between GaN and silicon,” says Plessey’s chief engineer Dr John Ellis. The thermal expansion rate of GaN is very different to that of silicon. So, when the two substances cool down from the growth temperature of 1000°C, the material tends to crack. Humphreys’ team introduced layers to the process that put the GaN in a state of compression before it heats up. “This compression balances the tension when it cools down, which allows the material to relax,” says Humphreys. “As a result, we can grow LED structures which are totally crack-free.”

The new process can grow LEDs on 6-inch silicon. Each wafer costs about £20 (cheaper than 2-inch sapphire wafers) and ten times more LEDs can be grown on a 6-inch wafer. Combined with the intrinsic cost savings of using Plessey's existing automated 6 inch processing equipment, the new technology can put the resultant HB-LED lighting products at the forefront of the industry, reckons Ellis.

“We’ve got higher efficiency for growing gallium nitride on silicon than anyone else we know,” believes Humphreys. “LED light bulbs currently cost as much as £40, but we expect to be able to reduce that cost by a factor of five by growing on silicon,” he adds. “Mass manufacturing may reduce the cost further.”

Plessey’s first samples of a blue LED are characterized by peak emission at a wavelength of 460nm, but the technology also extends to other emission wavelengths such as cyan and green. Being able to achieve such high brightness at the blue end of the spectrum enables phosphors to be used to produce white light with a balanced spectrum of light emission that is better for the eye, Plessey notes. The firm plans that by late fourth-quarter 2012 it will have reached white output efficacy of 150 lumens/watt (comparable to state-of-the-art conventional LEDs based on sapphire or SiC).

“HB-LED lighting represents the future of domestic, architectural, medical and automotive lighting,” says Plessey's managing director Michael LeGoff. “Achieving the goals of high efficiency and brightness is key to the rapid deployment of energy-saving solid-state lighting.”

Because tungsten filament light bulbs lose much of their energy as heat, just 5% of the energy they consume is used as light. Fluorescent tubes are 20-25% efficient. LEDs are currently about 30% efficient and Humphreys hopes to raise that to 60%. In the UK alone, the researchers estimate that the nationwide use of LEDs would save 15% of the electricity generated by power stations, resulting in a similar reduction in CO2 emissions. Also, a US Department of Energy report in 2010 estimated that savings of $20bn per year would result if LED light bulbs became widespread in the USA. Researchers estimate that a worldwide switch to LEDs would enable the closure of 560 power plants and result in annual CO2 savings equivalent to the output of all the cars on the planet. LEDs are also being designed to give off a more natural quality of light (better for people’s health).

Plessey’s broader work in electronics also means it aims to develop LED-based ‘smart lighting’ products (incorporating existing sensing and control technologies including its EPIC sensor), enabling intelligent energy management, remote control, controlled dimming and automated response to ambient daylight conditions.

“The plans for its commercial development within Plessey have tremendous potential,” Humphreys reckons. “The opportunities to develop affordable smart-lighting products for domestic, medical, automotive and other applications provide a real opportunity to have a positive impact on people's lives.”

Tags: Plessey GaN-on-Si HB-LEDs

Visit: www.plesseysemiconductors.com

Visit: http://camgan.com

Visit: www.gan.msm.cam.ac.uk

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