a semiconductor ceramic

Materials sort roughly into three electrical camps: insulators, which block current; metals, which conduct it freely; and semiconductors, which sit in between and — crucially — can have their conductivity dialled up or down by doping, by temperature, or by their surroundings. Many oxide ceramics that you might assume are insulators are in fact semiconductors, and it is precisely this tunable, sensitive middle-ground conductivity that makes them useful. A semiconductor ceramic is an oxide whose ability to conduct electrons (not ions) can be controlled and is exquisitely sensitive to its defect chemistry and its environment.

The conductivity here comes from electronic defects tied to the oxide's stoichiometry. Make an oxide slightly oxygen-deficient and it gains free electrons and becomes an n-type semiconductor — this is how zinc oxide (ZnO), tin oxide (SnO2), and reduced titanium dioxide (TiO2) conduct; make it oxygen-rich or dope it the other way and it can gain holes and become p-type, as in nickel oxide (NiO). Often the electrons do not glide in a broad band as they do in silicon but hop from one mixed-valence cation to the next — for instance an electron shuttling between Fe2+ and Fe3+ — a slow, thermally-activated motion called small-polaron hopping. A telling signature separates these ceramics from metals: their conductivity rises as they get hotter (more carriers are thermally freed), the opposite of a metal, following an Arrhenius law.

This defect-driven, environment-sensitive semiconduction is the engine behind a surprising range of devices. It gives the NTC thermistor whose resistance drops with temperature (used to sense and compensate temperature); the zinc-oxide varistor that stays insulating until a voltage surge makes it suddenly conduct and clamp the spike; the PTC thermistor built from doped barium titanate; and the gas sensor, where a tin-oxide surface changes its resistance when a gas molecule lands on it — the heart of household smoke and gas alarms. The same family gives transparent conducting oxides such as indium tin oxide for touchscreens and solar cells, and photocatalysts such as TiO2. Two honest caveats: the line between 'semiconductor' and 'insulator' is one of degree — it is really about band gap and doping level, not a sharp divide — and in many of these ceramics it is the grain boundaries, not the grain interiors, that actually control the electrical behaviour (as in the varistor and the PTC thermistor).