Thermistors measure temperature as a function of change in potential difference at constant current or a change in current at constant temperature. These temperature sensors are constructed of semiconductor materials, and their resistance is either a direct or inverse function of temperature.
Thermistors with resistance that increases as temperature increases are called positive temperature coefficient thermistors, or posistors. Meanwhile, thermistors with resistance that decreases as temperature increases are called negative temperature coefficient thermistors.
Posistors usually function as temperature sensitive cut-off switches, as their resistance can be made to spike suddenly above a certain critical temperature. Such cut-off posistors are constructed from ceramics containing barium titanate and other compounds to increase thermal stability and sensitivity and modulate cutoff temperature. Posistors can also be constructed from thermally sensitive silicon crystals, which have a positive and linear temperature-resistance correlation. Negative coefficient thermistors are made from pressed disks, chips and beads of semiconducting metal oxides. An increase in temperature increases the number of active charge carriers that are available for electrical conduction, decreasing the overall resistance.
Thermistors can be used as extremely sensitive cryogenic temperature thermometers, enabling the accurate measurement of temperatures. They are also commonly used in automotive applications, where they monitor engine and coolant temperature.