A thermistor's operation is a function of the electron-hole pair theory. As the temperature of the semiconductor rises, the electron-hole pair generation increases (due to thermal agitation). Increased electron-hole pairs causes a drop in resistance and a resultant increase in current flow.
There are two classes of thermistors: positive temperature coefficient (PTC) and negative temperature coefficient (NTC). With NTC, an increase in temperature causes a decrease in thermistor resistance
With PTC, a relatively small increase in temperature at the switch temperature point causes an extremely large increase in thermistor resistance. A PTC thermistor will have relatively low resistance levels during its ON state and relatively high resistance levels during its OFF state.
Most thermistors operate on an NTC, although some applications require operation on a PTC.