Editing Ceramic (section)

====Semiconductors====
Some ceramics are [[semiconductor]]s. Most of these are [[transition metal oxides]] that are II-VI semiconductors, such as [[zinc oxide]]. While there are prospects of mass-producing blue [[light-emitting diode]]s (LED) from zinc oxide, ceramicists are most interested in the electrical properties that show [[grain boundary]] effects. One of the most widely used of these is the varistor. These are devices that exhibit the property that resistance drops sharply at a certain [[threshold voltage]]. Once the voltage across the device reaches the threshold, there is a [[Electrical breakdown|breakdown]] of the electrical structure{{clarification needed|date=November 2021}} in the vicinity of the grain boundaries, which results in its [[electrical resistance]] dropping from several megohms down to a few hundred [[Ohm (unit)|ohm]]s. The major advantage of these is that they can dissipate a lot of energy, and they self-reset; after the voltage across the device drops below the threshold, its resistance returns to being high. This makes them ideal for [[Surge protector|surge-protection]] applications; as there is control over the threshold voltage and energy tolerance, they find use in all sorts of applications. The best demonstration of their ability can be found in [[electrical substation]]s, where they are employed to protect the infrastructure from [[lightning]] strikes. They have rapid response, are low maintenance, and do not appreciably degrade from use, making them virtually ideal devices for this application. Semiconducting ceramics are also employed as [[gas sensor]]s. When various gases are passed over a polycrystalline ceramic, its electrical resistance changes. With tuning to the possible gas mixtures, very inexpensive devices can be produced.