Gallium

Gallium is chemical element 31, symbol Ga, a soft, silvery metal in group 13 of the periodic table that melts at about 29.8 degrees Celsius, so close to room temperature that a sample will liquefy in a warm hand. It is the base of the compound semiconductors gallium arsenide and gallium nitride, a p-type dopant for silicon and germanium, and a metal whose supply has become a matter of industrial and geopolitical attention. Its story links nineteenth-century chemistry, the first transistor crystals and today's power electronics and light-emitting diodes.

Gallium's discovery was a triumph of the periodic table. In 1875 the French chemist Paul-Émile Lecoq de Boisbaudran identified a new element by its spectral lines in zinc ore, and named it for Gaul, the Latin name of France. Its properties matched those that Dmitri Mendeleev had predicted for a missing element beneath aluminium, which he had called eka-aluminium, and that match strengthened confidence in the periodic law. Gallium stays liquid over an unusually wide range, from about 30 degrees to above 2,000 degrees Celsius, and like water it expands when it freezes. It is obtained almost entirely as a by-product of refining aluminium from bauxite, with smaller amounts from zinc processing.

In the transistor's early days, gallium was a practical acceptor for grown crystals. When Gordon Teal and John Little at Bell Labs developed single-crystal growth from the melt in the late 1940s, and when Teal, with Morgan Sparks, produced grown-junction transistors in 1950, the junction was made by adding impurities to the melt in sequence: a donor such as antimony and then an acceptor, with gallium being a common choice, so that the pulled crystal changed from n-type to p-type and back. The same melt-doping approach, with gallium as an acceptor, carried over to Teal's silicon crystal growing, which at Texas Instruments led to the first commercial silicon transistor announced in 1954. It was not the best choice for diffused devices, however: gallium moves quickly through silicon dioxide, so the oxide masks that block boron and phosphorus fail to contain it, and boron became the standard acceptor of planar processing.

Gallium compounds became important in their own right, starting with gallium arsenide. GaAs, a III-V compound whose semiconducting character was established by Heinrich Welker at Siemens in the early 1950s, has high electron mobility and a direct bandgap, so it conducts electrons quickly and emits light efficiently. It is used for microwave amplifiers in mobile phones, infrared light-emitting diodes and laser diodes, and the multi-junction solar cells that power many satellites. Gallium nitride, a wide-bandgap material, allowed the blue light-emitting diode, whose development by Isamu Akasaki, Hiroshi Amano and Shuji Nakamura earned the 2014 Nobel Prize in Physics and enabled white LED lighting. GaN transistors now appear in compact power chargers and radio-frequency equipment because they withstand high voltages and switch fast.

Gallium is also used as a p-type dopant in crystalline silicon solar wafers, where it solves a specific problem. Boron-doped Czochralski silicon contains oxygen, and under illumination boron and oxygen can combine into a defect that reduces carrier lifetime, causing an initial loss of efficiency. Gallium-doped wafers avoid that degradation, and several manufacturers adopted them for that reason, though gallium's distribution along the crystal varies more than boron's, making resistivity harder to control. Beyond that niche, the pitfalls are familiar: gallium contamination can alter wafer electrical properties, and liquid gallium attacks aluminium and other metals by embrittling them, so it cannot be handled with aluminium tools.

Supply is concentrated and has become politically sensitive. Because gallium is recovered as a by-product, its output does not rise simply because demand does. China accounts for most of the world's primary production, and in July 2023 it announced export licensing requirements for gallium and germanium products, effective from August 2023, which prompted other countries to look at recycling and at new recovery projects. A common misconception is that gallium is scarce in the Earth's crust, when it is about as abundant as lead, but is simply dispersed and costly to extract in quantity. Another is that gallium is a rare metal used only in chips, when it also features in liquid-metal alloys and medical imaging compounds.

Read gallium as the near-room-temperature metal that gave the first grown junctions their acceptor atoms and now supplies the light, speed and power-handling of modern compound semiconductors.