The resistance of a pure metal increases with temperature but the resistance of certain other conducting materials, e.g., carbon decreases with temperature. Various metal alloys, notably manganin and constantan which are used for making standard resistors, show very small resistance changes with temperature under normal laboratory conditions. Certain other substances notably germanium, silicon, and selenium are classed as semiconductors. The resistance of these substances decreases with rising temperature, especially when they contain certain impurities. In single-crystal form with controlled amounts of impurity such as arsenic or indium they are used in the manufacture of solid-state diodes and the transistors used in radio and computer circuits and for many other purposes. See also Chapter 45.
A semiconducting device known as a thermistor and which consists of a mixture of oxides of manganese and nickel has a resistance which can decrease by a factor of many hundreds from room temperature to 100°C. Thermistors can be used as sensitive thermometers and also as devices to compensate for the increase in resistance with rising temperature of other components in a circuit.