I know the operation of PN junction diode. But I wonder how they use it as a temperature sensor.
Effect of temperature on forward characteristics :
The characteristics curve of a Si diode shifts to the left at the rate of -2.5 mV per degree centigrade change in temperature in forward bias region.
Refer to the graph shown above. The curves are shown far apart just for illustration purpose and are not to scale. The curve shifts to the left at the rate of -2.5 mV per degree centigrade change in temperature. Hence if the temperature increases from room temperature (25° C) to 80° C, the voltage drop across the diode will be (80-25) x 2.5 mV = 137.5 mV.
Note:The values will change with respect to diodes,not all diodes are made up of same parameters and composition.
Hope this helps...