Gallium nitride (GaN) is a III-V compound –V semiconductor withbuilt from a direct band gap of about 3.4 electronvolts at room temperature, crystallising in the wurtzite structure and remaininV, a strong chemically stable and mechanically hard up to high temperaturl bond and a polar wurtzite lattice. Those properties,three attributes together with a breakdown field roughly ten times that of silicon, make it texplain why the material has become 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 lattictwo 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, blue and green, which is how blue and thermal mismatch with those substrates leaves threading dislocation densities far above those of mature semiconductors ^[1]. Achiwhite light-emitting diodes, violet laser diodes and deep-ultraviolet sources are made. The bond streving 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 themth 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 thin layer and switch ^[2]. Onquice p-tklype and n-type layers could be combined, bright blue indium gallium nitride light-emitting diodes followed and enabled solid-state white lighting ^[3]. In t. And the polar lattice connects it to radio frequency: at an AlGaN/GaN interface the eledisctronic direction, the ontinuity of spontaneous and piezoelectric polariszation of an aluminium gallium nitride barrier induces acreates a dense sheet of electrons, the two-dimensional electron gas at its interface , without any intentional doping, and — a channel that gas carries theboth high current of a high-density and high electron-mobility transistor velocity. This entry follows the material from the first vapour-grown ^[4].crystals of 1969 through Tthe remaining obstacles are the cost and the size of 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 native substrates, and the trapping and degradation that limit the reliability of power transistors under switching ^[5], 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 grown on silicon.