When light falls on a semiconductor, electrons are excited from the 'valence band' - where they are tightly bound to their parent atom - into the 'conduction band', where they can move and contribute to current flow. Each electron leaves a 'positive hole' in the valence band, and the electron stays close to this hole in a bound system called an exciton. The exciton can move through the semiconductor, but it is not influenced by electric fields because it has no net charge.

But an exciton can have an overall negative charge if it attracts an extra electron to form a 'trion'. To encourage trions to form, Shields and colleagues added extra electrons, in the form of silicon 'donor' atoms, to a gallium arsenide structure. When they applied a voltage across the structure, they found that the trions drifted several micrometres towards the positive terminal. This overturns the long-held idea that trions are held still - or 'pinned' - by the attraction of positive ions in the semiconductors.

'Previous research had suggested that trions are localised by the potential of donor ions', Shields told PhysicsWeb, 'although the results of some experiments could be best explained by a model in which the excitons are free'.

Electrons emit their excess energy as light when they recombine with positive holes, and this means that excitons are the source of light in semiconductors. 'We have shown that charged excitons can be controlled using applied voltages', says Shields. 'Since these excitons produce light, this gives us another handle to control optoelectronic devices such as light-emitting diodes.'