Gallium nitride (GaN) is a III–V -V compound semiconductor built from with a direct band gap of about 3.4 eV, a strolectronvolts at room temperature, crystallising in the wurtzite structure and remaining chemical bond and a polar wurtzite latticlly stable and mechanically hard up to high temperature. Those three attributesproperties, together explain why the material has become twith a breakdown field roughly ten times that of silicon, make it the basis of two very different technologies. The wide gap connects it to the visible spectrum: alloying GaN with indium and aluminium tunes the emitted photon energy across the ultraviolet, bluboth 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 green, which is how blusilicon, and the lattice and white light-emitting diodes, violet laser diodes and deep-ultraviolet sources are made. The bond strthermal mismatch with those substrates leaves threading dislocation densities far above those of mature semiconductors ^[1]. Achievingth connects it to power: the critical electric field of GaN is roughly ten times that of silicon, so devices can block high voltage in a thi 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 layer and switch quickly. And the polar lattice connects it to radio frequency: at an AlGaN/GaN interfacep-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 discontinuity of rection, the spontaneous and piezoelectric polarization creates a dense sheet of electrons, thesation of an aluminium gallium nitride barrier induces a two-dimensional electron gas, at its interface without any intentional doping — a channel, and that gas carries both high the current density and high of a high-electron velocity. This entry follows the material from the first vapour-grown-mobility transistor crystals^[4]. of 1969 tThrough the buffer layer and acceptor activation breakthroughs that made devices possible, sets out the physical properties that distinguish GaN from its competitors, and reviews how the same crystal supports light emitters, radio-frequency amplifiers and power switches. Practical limits are discussed alongside performance: the absence of a cheap nae remaining obstacles are the cost and the size of native substrate, the difficulty of p-type doping, threading dislocations inherited from heteroepitaxy, the droop of light-emitting-diode efficiency at operating current, trapping effects such as current collapse, and the thermal impedance of epitaxial layers growns, and the trapping and degradation that limit the reliability of power transistors under switching on silicon^[5].