We present a systematic study of the current–voltage characteristics and electroluminescenceof gallium nitride (GaN) nanowire on silicon (Si) substrate heterostructures where bothsemiconductors are n-type. A novel feature of this device is that by reversing thepolarity of the applied voltage the luminescence can be selectively obtained fromeither the nanowire or the substrate. For one polarity of the applied voltage,ultraviolet (and visible) light is generated in the GaN nanowire, while for theopposite polarity infrared light is emitted from the Si substrate. We propose amodel, which explains the key features of the data, based on electron tunnellingfrom the valence band of one semiconductor into the conduction band of theother semiconductor. For example, for one polarity of the applied voltage, given asufficient potential energy difference between the two semiconductors, electrons cantunnel from the valence band of GaN into the Si conduction band. This processresults in the creation of holes in GaN, which can recombine with conduction bandelectrons generating GaN band-to-band luminescence. A similar process appliesunder the opposite polarity for Si light emission. This device structure affords anadditional experimental handle to the study of electroluminescence in single nanowiresand, furthermore, could be used as a novel approach to two-colour light-emittingdevices.