THERMISTORS:
These are devices whose resistance varies quite markelly with temperature. Their name is derived from thermal resistors. Most thermistors have negative coeffiecent of resistance, that is, the resistance of such thermistors decreases when temperature is increased. Thermistors with positive coffiecent of resistance are also available.
Thermistors are made by heating under high pressure semiconductor ceramic made from mixtures of metallic oxides of manganese, iron, nickle, cobalt and copper etc. These are pressed into desired shapes and then backed at high temperature.Different type of transistors are shown below.

They may be in form of rods, discs, washers of beads.
Transistors with high negative temperature coeffiecent are very accurate for measuring low temperature especially near 10K. The high resistance at low temperature enables more accurate measurement.
Thermistors have wide applications as temperature sensor.They convert changes of temperature into electrical voltage signals, which are duly processed.