A thermistor is a type of semiconductor that reacts like a resistor sensitive to temperature, meaning it has greater resistance than conducting materials, but lower resistance than insulating materials. The word thermistor is a portmanteau of thermal and resistor. Depending on the materials used, thermistors are classified into two types:

• NTC Thermistors: With NTC thermistors, resistance decreases as temperature rises. This is usually due to an increase in conduction electrons bumped up by thermal agitation from the valence band. An NTC is commonly used as a temperature sensor, or in series with a circuit as an inrush current limiter.

• PTC Thermistors: PTC thermistors have a positive temperature coefficient, meaning their resistance increases as temperature rises. They are used as an overcurrent protection device, as their resistance increases with temperature, which limits the current flow.

Thermistors are made up of metallic oxides, binders, and stabilizers pressed into wafers and then cut to chip size, left in disc form, or made into another shape. The precise ratio of the composite materials governs their resistance/temperature "curve". Manufacturers typically control this ratio with great accuracy, as it determines how the thermistor will function.

Thermistors are easy to use, inexpensive, sturdy, and respond predictably to changes in temperature. While they do not work well with excessively hot or cold temperatures, they are the sensor of choice for applications that measure temperature at a desired base point. They are ideal when very precise temperatures are required. Some common applications for thermistors include digital thermometers, cars to measure oil and coolant temperatures, household appliances such as ovens and refrigerators, and almost any application that requires heating or cooling protection circuits for safe operation.