Gallium nitride (GaN) is a III–V semiconductor built from a direct band gap of about 3.4 eV, a strong chemical bond and a polar wurtzite lattice. Those three attributes together explain why the material has become 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, blue and green, which is how blue and white light-emitting diodes, violet laser diodes and deep-ultraviolet sources are made. The bond strength 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 quickly. And the polar lattice connects it to radio frequency: at an AlGaN/GaN interface the discontinuity of spontaneous and piezoelectric polarization creates a dense sheet of electrons, the two-dimensional electron gas, without any intentional doping — a channel that carries both high current density and high electron velocity. This entry follows the material from the first vapour-grown crystals of 1969 through 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 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 grown on silicon.