Indium
Indium is chemical element 49, symbol In, a soft, silvery, low-melting metal in group 13 of the periodic table, just beneath gallium and above thallium. It melts at about 157 degrees Celsius, is so soft that it can be marked with a fingernail, and wets and sticks to glass and many other surfaces. Indium plays three notable roles in the history and practice of electronics: as the metal dots of the alloy-junction transistor, as the indium in the transparent conductor used in nearly every flat-panel display, and as a component of indium phosphide semiconductors for optical and high-speed devices.
Indium was discovered in 1863 by Ferdinand Reich and Hieronymus Theodor Richter in Freiberg, Germany, and named for the bright indigo line in its spectrum. The element is rare in the Earth's crust and has few ores of its own, so it is obtained mainly as a by-product of zinc smelting. Its uses long remained modest: low-melting solder and bearing coatings, seals for vacuum and cryogenic equipment where its softness allows it to flow into surface irregularities, and thermal interface layers. Its role in transistor history began in the early 1950s, when its properties turned out to be well matched to germanium.
In the alloy-junction transistor, small dots of indium were melted into a germanium wafer to form the emitter and collector. In the process developed at General Electric and RCA in the early 1950s, a thin n-type germanium wafer served as the base, and indium pellets were placed on both faces and heated. The molten indium dissolved some germanium, and on cooling the germanium regrew as a layer doped p-type by indium, an acceptor, creating p-n-p junctions on each face. The thickness of the remaining base was set by how deep the dots were alloyed, and it could be thin enough for useful gain. Philco's surface-barrier transistor of 1953 also used indium, electroplated by jets onto an etched germanium wafer. Indium works well with germanium because it alloys readily at low temperature, but it never suited silicon, which required higher temperatures and different methods, leading to the diffused and planar processes.
Indium tin oxide, ITO, extended indium's importance far beyond the transistor. ITO is a mixture of indium oxide with roughly one-tenth tin oxide by weight, deposited as a thin film by sputtering. It is one of the few materials that is both transparent to visible light and electrically conductive, because it is a heavily doped oxide semiconductor with a wide bandgap, and that combination is needed for the electrodes of liquid-crystal and organic light-emitting displays, touch screens and thin-film solar cells. The growth of flat-panel televisions and touch phones made display manufacture a leading consumer of indium, and the recycling of used sputtering targets became an important source.
Indium phosphide, InP, and related compounds fill a different niche. InP is a III-V semiconductor with a direct bandgap that, in alloys with gallium and arsenic, emits and detects light at wavelengths of about 1.3 and 1.55 micrometres, the windows in which optical fibre loses the least light, so lasers and photodetectors for long-distance communication are made on InP substrates. InP transistors also run at very high frequencies, in millimetre-wave circuits. As a dopant in silicon, indium is a deep acceptor, with an energy level about 0.16 electronvolt above the valence band, so it is only partly ionized at room temperature, and it is used rarely, mostly in some infrared detectors and in specialised implants where its heavy mass and slow diffusion are desirable, while boron is the normal choice.
A common misconception is that indium is a rare-earth element or that supply is simple, when it is a by-product metal whose availability follows zinc production and which is concentrated in a few producing countries. Another is that soft, low-melting indium makes poor electrical contacts; alloyed indium contacts were in fact among the first reliable ones on germanium, and indium bumps are still used to join infrared detector arrays to their readout circuits.
Read indium as the soft metal that made the first mass-produced junction transistors by melting into germanium and now, as a transparent film and a phosphide crystal, lights, senses and connects modern displays and networks.