9.20 Thermistors
A thermistor is a heat sensitive resistor. Most thermistors have negative temperature coefficient of resistance, i.e., the resistance of such thermistors decreases when their temperature is increased. Thermistors with positive temperature coefficient are also available.
In the thermistors, resistance decreases as temperature increases. This is because increasing temperature provides more energy to the charge carriers (electrons or holes), enabling them to move more freely and thus reducing resistance.
Thermistors are made by heating under high pressure semiconductor ceramic made from mixtures of metallic oxides of manganese, nickel, cobalt, copper, iron, etc. These are pressed into desired shapes and then baked at high temperature. Different types of thermistors are shown in Fig. 9.34. They may be in the form of beads, rods or washers.