NTC Thermistor Materials

This chapter describes the electrical properties of negative temperature coefficient (NTC) thermistor materials. The electrical properties of NTC thermistors result from the process of semiconduction in the polycrystalline oxide materials used in their construction. A controlled impurity addition leads to the formation of stable semiconducting materials and this has been the principle employed for all NTC thermistor material formulations. The introduction of foreign atoms into oxide structures not only changes the cation distribution but also causes small changes in the physical distances between cations and in their net positive charge. These latter changes alter the binding energy of a hopping electron, thus changing the activation energy of the semiconduction process. By suitable selection of both the basic oxide and the impurity additions, together with good control over the preparative conditions, a wide range of resistivities and temperature coefficients can be achieved. The voltage–current or current–time characteristics of an NTC thermistor are of greater importance than the zero-power resistance–temperature behavior. It is found that any change in the thermal conductivity of the environment surrounding a self-heated thermistor will change its equilibrium temperature, as indicated by a large resistance change.