Thermistors are often called negative temperature coefficient (NTC) sensors. A thermistor is a semiconductor device made of materials whose resistance varies as a function of temperature.  They can be used to compensate for temperature variation in other components of a circuit. Because of their low cost and the simple circuitry required, they are preferred in many applications to more expensive thermocouples and RTDs. In addition, they can be produced in very small designs with a fast response and low thermal mass.