Antimony
Antimony is chemical element 51, symbol Sb (from the Latin stibium), a brittle, silvery metalloid in group 15 of the periodic table, the same group as nitrogen, phosphorus and arsenic. In semiconductor work it matters in two separate ways: as a group V donor impurity that makes silicon n-type while diffusing much more slowly than phosphorus or boron, and as a component of compound semiconductors, most notably indium antimonide, used for infrared detection. Its long history as an industrial and medicinal material is older than electronics by several thousand years.
Antimony's older history is tied to the mineral stibnite, antimony sulphide, a heavy grey ore with a metallic lustre. Powdered stibnite was used in the ancient world as an eye cosmetic known as kohl, and the metal and its compounds appeared in alchemy and in medicine, often with unfortunate results given their toxicity. A practical use arrived with the printing press, where lead alloyed with antimony and tin made type metal that was harder than lead alone and expanded slightly as it solidified, giving sharp castings. Antimony hardens lead for battery plates, and antimony trioxide is widely used as a flame-retardant synergist in plastics. These are bulk uses, but antimony's metalloid character, between metal and non-metal, is what made it interesting to the semiconductor pioneers.
As a dopant, antimony is an n-type donor chosen for what it does not do, which is move. An antimony atom in silicon has one more valence electron than the four needed for bonding, so it donates an electron, like phosphorus. But antimony is a large atom, and its diffusivity in silicon is far below that of phosphorus or boron at the same temperature, so once placed it stays put through later high-temperature steps. Its electrically active solubility is lower than phosphorus, in the range of a few times 10 to the 19 per cubic centimetre, and it is hard to deliver from the gas phase in diffusion furnaces, so it is typically applied by solid sources or, in modern lines, by ion implantation. Antimony was also among the donors used in the earliest grown-junction germanium transistors, where pellets of donor and acceptor impurities were added to the melt during crystal pulling.
The buried layer is where antimony earned its place in integrated circuits. In a bipolar integrated circuit, current from the collector must travel laterally through lightly doped silicon to reach its contact, adding resistance that slows the transistor and raises its saturation voltage. To avoid this, a heavily doped n+ region is formed first in the p-type substrate, and a lightly doped epitaxial silicon layer is grown on top, so the n+ region sits underneath the transistor as a low-resistance collector path. Antimony is ideal because it will not diffuse upward into the epitaxial layer in large amounts during growth and the following heat treatments, though arsenic is a common alternative. Heavily antimony-doped wafers are also made for some power devices.
Indium antimonide, abbreviated InSb, is the main semiconductor use of antimony outside silicon. InSb is a compound of group III indium and group V antimony whose bandgap is only about 0.17 electronvolts at room temperature, meaning even low-energy photons can lift electrons across it. That makes it an effective detector of mid-wave infrared light at wavelengths of about 3 to 5 micrometres, usually cooled to around 77 kelvin, the temperature of liquid nitrogen, to suppress thermal noise. It has also a very high electron mobility, and is used in thermal imaging, astronomy and magnetic field sensors. Heinrich Welker's work at Siemens in the early 1950s established that compounds of this III-V type are semiconductors, opening the field to which InSb belongs.
A common misconception is that antimony is the same as arsenic or that either can be swapped freely; they differ in diffusivity, solubility and toxicity, and the choice between them in a buried layer involves trade-offs such as autodoping, where dopant evaporates from the substrate and contaminates the growing epitaxial layer. Antimony compounds are toxic and need careful handling in fabs, and supply comes mostly as a mining and smelting by-product concentrated in a few countries.
Read antimony as silicon's anchored donor, an atom heavy and slow enough to stay exactly where it was put, and also as the heavy half of a compound that sees heat as infrared light.