Researchers at the University of Cambridge may have found the mechanism that makes green LEDs less efficient that other colours, but can’t reveal details.

“We have a mechanism in mind, but want to check it before talking about it,” Professor Colin Humphries told Electronics Weekly. “We have found the green situation more complicated than other people realise.”

Despite relatively modest funding, compared with the huge government LED research budgets in the US, Japan and China, the Cambridge department of materials science and metallurgy has world class LED technology.

“We have grown deep green [555nm] LEDs with an internal quantum efficiency of 10 per cent,” said Humphries, “We don’t know anyone else who is better.”

The green LEDs under investigation are made from indium gallium nitride - InGaN, the same basic material that is used in blue and white LEDs.

“Green emission seems to be different blue. You would think it would be the same,” is all Humphries would reveal about the Cambridge theory.

Humphries team, which also includes the department’s electron microscopy facility, is no stranger to theories that go against conventional wisdom.

Last year Cambridge became suspicious that the accepted location of emission centres in InGaN LEDs, around indium clusters, was wrong.

Instead Humphries postulated that indium clusters do not exist in LEDs until they are irradiated by the electron microscope used to examine emission, and that emission is actually centred around atomic layer steps in quantum well thickness.

“In September there was a special session on this [at an academic LED conference]. Most of the world now agrees with us, although it is still hotly debated,” he said.

The DTI is funding white LED research in a consortium that includes: the Cambridge group, looking into the semiconductors; the University of Surrey for the phosphors; Semelab which want to apply them in head-up displays; and Cumbria-based LED maker Forge Europa. “We are making good progress here,” said Humphries.

Can this team and the rest of the UK produce LED intellectual property that will compete with the likes of the US, Japan and China?

“Certainly, the UK can play a significant part,” said Humphries - pointing out that Cambridge’s Thomas Swan Scientific Equipment already produces many of the MOCVD (metal organic chemical vapour deposition) reaction chambers used to make InGaN LEDs around the world.

Cambridge University