Germanium (Ge)

Germanium is the element that had the semiconductor industry first and then handed it over. The point-contact transistor that John Bardeen and Walter Brattain assembled at Bell Labs in December 1947 was a slab of germanium with two gold contacts pressed against it, and for a decade afterwards germanium was what transistors were made of. Silicon took the business away for reasons that had almost nothing to do with electronics and everything to do with rust.

Why silicon won on an oxide

Silicon's advantage is that when you heat it in oxygen it grows a layer of silicon dioxide — glass, chemically inert, tightly bonded to the surface underneath, and an excellent insulator. That layer can be patterned and used to mask which parts of the wafer get doped, which is the entire basis of the planar process and therefore of the integrated circuit. Germanium's own oxide is water-soluble and structurally useless; nothing equivalent could be built on it.

The second problem is the band gap. Germanium's is about 0.67 eV against silicon's 1.12 eV, which is why germanium conducts more easily at a given voltage — the original attraction — and also why it leaks badly as it warms up. Early germanium radios genuinely misbehaved on hot days. Silicon tolerates the temperatures inside a running computer; germanium does not.

Germanium never left entirely. Silicon–germanium alloy layers strain the silicon lattice and speed up carriers, and SiGe heterojunction transistors handle the high-frequency analogue work in phones and radar. The element that lost the digital fight kept the radio.

Seven per cent that would not add up

Clemens Winkler was handed a new silver mineral from the Himmelsfürst mine near Freiberg in Saxony in 1885, later named argyrodite. He analysed it thoroughly and found silver, sulfur, mercury and iron — and a total that fell about seven per cent short of the sample's mass. A good analyst treats a persistent seven per cent hole as an unknown substance rather than a mistake, and in February 1886 he pulled a new element out of it.

Winkler's first instinct for a name was neptunium, after the planet whose discovery had also been predicted before it was seen. The name had already been attached to a spurious element some years earlier, so he settled on germanium, after his country. There was some grumbling that the name sounded like a flower; Winkler pointed out that nobody complained about ruthenium.

Which of Mendeleev's gaps it filled

Everyone agreed the new element belonged in the table. Nobody could agree where. Winkler initially placed it as eka-antimony. Mendeleev suggested eka-cadmium. Lothar Meyer argued for the empty square below silicon, and Winkler's own further measurements proved Meyer right: germanium was eka-silicon, predicted in 1871 with an atomic weight near 72, a grey metallic appearance, a specific gravity of 5.5 and a tetrachloride boiling below 100 °C. The match was so close that this, more than gallium, is the case usually cited as the moment the periodic law stopped being controversial.

Light, heat and drink bottles

Modern germanium demand has three legs, none of them electronic in the popular sense.

• Optical fibre. Germanium tetrachloride is burned into the silica preform to raise the refractive index of the core relative to the cladding. Without a dopant the fibre cannot confine light by total internal reflection; germanium is the standard choice, and there is a little of it in essentially every long-haul data link on the planet.
• Infrared optics. Germanium is opaque to visible light and beautifully transparent in the 8–14 micrometre band, which is exactly where objects at human and engine temperatures emit. Nearly every thermal imaging camera, from a building surveyor's to a targeting pod's, looks through a germanium lens. Its very high refractive index makes those lenses shallow and their anti-reflection coatings essential.
• Polymerisation catalysts. Germanium dioxide catalyses the production of polyethylene terephthalate. It is used mainly in Japan, where it is preferred over antimony compounds for bottles because it leaves no visible haze and no metal residue of concern.

The purest solid routinely manufactured

Gamma-ray spectrometers are built from single crystals of high-purity germanium, and "high purity" here means something extreme: impurity concentrations around 10^10 atoms per cubic centimetre, roughly one foreign atom in a trillion. It is the cleanest bulk material anyone makes on a regular basis, and it lets a detector resolve gamma lines a fraction of a per cent apart, which is how nuclear forensics tells one reactor's fingerprint from another's.

That purity has a cost in convenience — the detectors have to be operated cold, in liquid nitrogen or on a mechanical cooler, because the narrow band gap that hurt germanium transistors also generates thermal noise in a detector.

Germanium-76 and a question about the neutrino

Of germanium's five natural isotopes, the interesting one is germanium-76, the heaviest and rarest. It cannot decay by ordinary beta emission, but it can undergo double beta decay, emitting two electrons and two antineutrinos at once — an absurdly slow process with a measured half-life around 10^21 years.

The prize is a version of that decay with no neutrinos emitted, which is only possible if the neutrino is its own antiparticle. Experiments at Gran Sasso — Heidelberg-Moscow, then GERDA, now LEGEND — have built detectors out of germanium enriched in the 76 isotope, so that the source and the detector are the same crystal. Nothing has been found yet, and each round pushes the half-life limit further out.

A passenger on zinc and coal

There is no germanium deposit worth mining for germanium. It concentrates slightly in sphalerite — zinc sulfide — and is recovered from the residues of zinc refining, and it accumulates in the ash of certain coals, notably in Inner Mongolia and in Siberia, where fly ash is a genuine commercial source. Recycled scrap from optical and infrared manufacturing supplies a large share of demand.

China produces most of the world's refined germanium, and placed it under export licence alongside gallium in July 2023; a full prohibition on American-bound cargoes followed eighteen months after that. For an element with no dedicated mine anywhere, the alternatives are slow to appear: a new supply means persuading a zinc smelter to install a recovery circuit it has managed without.