THERMISTORS

THERMISTORS:

The thermistors is a combination of a thermal and resistor. A thermistor is a resistor with definite thermal characteristics.
most thermistors have a negative temperature coefficient,but positive temperature devices are also available. the reason of negative temperature coefficient of resistance of a semiconductor is that when its temperature increased, the concentration of charge carriers increases resulting in a decrease in resistance.

Thermistors are essentially semiconductor devices, that behave as resistors with high negative temperature coefficient. Thermistor has a very nonlinear resistance-temperature relation is

                                  β/T
                   R=Roα e

symbol of thermistor

 where,
 R is a resistance of a thermistor.
T is a temperature.
α and β are constants depending upon the material and manufacturing technique employed.

positive temperature coefficient (PTC) Thermistors.
such thermistor are resistors with a high positive temperature coefficient of resistance. They differ from NTC  thermistors is the following aspects.
1. the temperature coefficient of a PTC thermistor is positive  only between a certain temperature  range,outside this range the temperature coefficient is either zero or negative.
2.the absolute value of the temperature coefficient of PTC thermistor is usually much higher than of NTC thermistors.

 PTC thermistors are employed as current limiters, temperature  sensors, and protectors again ts overheating in equipment such as electric motors.They are also employed as level indicators,time delay devices,thermostats,compensation resistors etc.
PTC thermistors are prepared from BaTiO3  and  SrTiO3 in a similar way as  NTC thermistors.If prepared in the absence of Oxygen, these semiconductors have a weak negative temperature coefficient.A strong positive temperature coefficient is obtained by firing the sample in an oxygen-rich atmosphere. this is  caused by the penetration of oxygen along pores and crystal boundaries during cooling after the firing process.
The oxygen atoms, absorbed on the crystal surface attract electrons from a thin Zone of the semiconducting crystals.In this way  electrical potential barriers are formed consisting of a negative surface charge with,on both sides,thin layers having a positive space charge resulting from the now uncompensated foreign ions.These 
                                                                                                                                                  barriers to because an extra resistance of the thermistor. At very high temperatures,i.e.,above 160 degree c to 200 degree c the electrons captured at the boundaries  are gradually liberated causing the potential barriers to decrease in strength.This means  that the PTC loses its properties and may eventually act as an NTC if the temperature becomes too thigh.Therefore the applications of PTC thermistors are restricted by a certain temperature .