Rocks and Rolls

Rolls-Royce is working with engineers at University College London to create diamond transistors that will be able to withstand the intense heat inside the next generation of jet engines.

The university’s school of electronic and electrical engineering is leading the project, which aims to develop bipolar switches that can operate at temperatures above 400°C. The work could also lead to diamond-based electronic systems able to work in other extreme environments, such as nuclear reactors.

Cambridgeshire-based semiconductor specialist CamSemi and Element Six — the division of De Beers that develops industrial diamonds — are also members of the research consortium.

The project is being driven by the requirements of Rolls-Royce, which needs switches that will operate inside its planned ‘more-electric’ engines, scheduled to be ready within 10–12 years. These engines will use more electronics and fewer mechanical or hydraulic systems.

According to UCL’s Dr Richard Jackman, who is leading the research, the thrust towards additional electronics will bring numerous advantages to the aerospace industry.

‘The more-electric engine’s reduced weight will hugely improve efficiency and it has the potential to save time by needing less servicing,’ said Jackman.

However, a more-electric engine would also need advanced electronics that can operate at very high temperatures. Silicon-based electronics are ineffective for this application, because silicon ceases to conduct at the temperatures experienced inside an aero-engine.

Researchers have known for some time that diamond would be the perfect conductor under such conditions, but have been unable to produce sufficient quantities of the material to make it commercially viable.

This has now changed because of recent dramatic improvements in the quality of single crystal diamonds grown by chemical vapour deposition (CVD), which works by passing gas, usually methane, over a heated substrate.

The gas reacts with the substrate and decomposes, gradually leaving a solid layer behind. Advances in CVD technology pioneered by Element Six have allowed the production of diamonds of extremely high quality, superior even to that of mined diamonds.

Before diamonds can be used they must be treated with chemicals to become effective in transistors. In nature, boron — the gas that makes diamonds blue —also creates what is known as a p-type diamond. This can be replicated by adding boron gases in the lab. But to make conductive diamond transistors, n-type diamonds containing phosphorus are needed as well as p-type diamonds.

Jackman’s team has developed a method of producing n-type diamonds by adding phosphorus without distorting the diamond’s lattice too much. This is often a problem because of the size of a phosphorus atom.

‘Although we managed to introduce phosphorus, it is not entirely active without introducing some energy — which, by a stroke of luck, is exactly what we’d get at 400°C,’ he said. ‘We will be able to produce a functioning diamond transistor, which has never been done before.’

The team’s work has already attracted some international attention. France’s nuclear authority The CEA has shown a strong interest in the technology, said Jackman. Diamond devices are robust enough to withstand radiation, and so the research has opened up the possibility of using integrated electronics inside nuclear reactors.

Jackman said: ‘Diamond electronics will never compete with silicon chips in the general consumer market, but could be great for specialist applications. A transistor that can operate at 400°C has so many potential uses, ranging from applications in the automotive to the nuclear industry.’

The team plans to have a prototype transistor ready within three years.