Gallium nitride is a III-V compound semiconductor with a direct band gap of about 3.4 electronvolts at room temperature, crystallising in the wurtzite structure and remaining chemically stable and mechanically hard up to high temperature. Those properties, together with a breakdown field roughly ten times that of silicon, make it the basis of both short-wavelength light emitters and high-frequency power devices. It is not usually grown as a bulk crystal but epitaxially on foreign substrates, among them sapphire, silicon carbide and silicon, and the lattice and thermal mismatch with those substrates leaves threading dislocation densities far above those of mature semiconductors ^[1]. Achieving p-type conductivity proved the decisive obstacle: magnesium acceptors are passivated by hydrogen during growth, and low-energy electron beam irradiation or thermal annealing is needed to activate them ^[2]. Once p-type and n-type layers could be combined, bright blue indium gallium nitride light-emitting diodes followed and enabled solid-state white lighting ^[3]. In the electronic direction, the spontaneous and piezoelectric polarisation of an aluminium gallium nitride barrier induces a two-dimensional electron gas at its interface without any doping, and that gas carries the current of a high-electron-mobility transistor ^[4]. The remaining obstacles are the cost and the size of native substrates, and the trapping and degradation that limit the reliability of power transistors under switching ^[5].