Most semiconductor devices rely on doping – the introduction of impurity atoms to supply electrons or 'holes' – for their operation. However, as devices become smaller it gets more difficult to control the amount of doping. Now Charles Lieber and colleagues at Harvard University and the Oak Ridge National Laboratory in the US have made a dopant-free nanowire device that can act as a high electron mobility transistor (HEMT).

The device is made by coating a gallium nitride (GaN) nanowire with an extremely thin (1.8 nm) layer of aluminium nitride (AlN), followed by a 10 nm layer of aluminium gallium nitride (Al0.25Ga0.75N). The devices, known as radial nanowire heterostructures, were grown with metal-organic chemical vapour deposition, and electron microscopy measurements confirm that the structures are single crystals that do not contain any dislocations. The nanowires are between 10 and 20 microns long.

The conduction band of GaN lies below that of the outermost AlGaN layer, which means that a high mobility electron gas can form in the core: the intermediate AlN layer reduces scattering and helps confine the electrons in the core. The nanowires can also be used to make n-channel field-effect transistors with impressive performance characteristics, which could be improved further by optimising the thickness and composition of the AlGaN layer.