Light emission and absorption in semiconductors can be described by Planck's theory of blackbody radiation. A semiconductor in thermal equilibrium with its surroundings absorbs the same amount of radiation as it emits. But if a voltage is applied across the semiconductor, the concentration of charge carriers grows exponentially and so does the light emission.

Green and colleagues realised that they could further amplify this growth if they could increase the ability of the semiconductor to absorb radiation. To do this, they etched an array of inverted pyramids onto the surface of the silicon LED - the pyramids reflect absorbed light back into the semiconductor. This 'light trapping' technique is commonly used in photovoltaic devices such as solar panels, and produced a factor of ten increase in the light emission of the silicon LED.

The team also reduced the number of 'non-radiative recombination' events in the device - these events occur when electrons and holes recombine but produce heat rather than light. The use of special surface treatment, small contacts and selective doping led to a further tenfold improvement in light emission.

Silicon LEDs currently have efficiencies in the range 0.01 to 0.1%, but the device developed by Green and co-workers exceeds a value of 1% at 300 kelvin. This is similar to the efficiencies of direct bandgap devices - based on materials like gallium arsenide - around a decade ago. "We believe we can reach an efficiency of 5% by improving the rear reflector in the device", says Green. "We will also be developing 'fast modulators' that allow the diode brightness to be changed quickly".

The development of an efficient silicon light emitter has long challenged physicists because silicon has an indirect bandgap, which makes it reluctant to emit light.