NTC Thermistor: negative temperature coefficient

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The NTC thermistor is widely used in many applications for a variety of purposes where a negative temperature coefficient is required.

Being an NTC thermistor the resistance falls as the temperature increases, making it particularly useful in a number of different areas.

NTC thermistor basics

As the name indicates, the NTC thermistor provides a reduction in resistance for an increase in thermistor body temperature.

The change in body temperature of the NTC thermistor can be brought about in two main ways:

• Raising external temperature:   Raising the temperature of the external fluid, possibly air in which the NTC thermistor is placed will cause the body of the device to change its temperature and hence its resistance will change. To make thermistors responsive when used in this way, they must be in a position where the temperature of the surroundings can be detected as well as possible. Good thermal conduction to the thermistor is needed, either by placing it in a flow of the fluid, e.g. air, or by ensuring it is thermally bonded to the chassis or other mechanical item on which the temperature needs to be sensed.
• Passage of current through the device:   Passing current through any resistor including an NTC thermistor will cause heat to be dissipated (Watts = Volts x Amps). This will cause the temperature to rise.

Typically NTC thermistors exhibit a change in resistance of between about -3%/°C to -6%°C at 25°C. The actual relationship follows a curve that is approximately exponential, with much higher changes in resistance at lower temperatures, and lowering considerably at higher temperatures.

The type of material used will govern many of the properties, but at temperatures around -40°C, the resistance change can be up to -8%/°C but at in the flatter part of the NTC thermistor curve it can be as low as -1%/°C at temperatures above 200°C or so.

NTC thermistor structure & materials

Thermistors can physically take a number of forms. NTC thermistors can be manufactured in the form of pressed discs, rods, plates, beads or even a semiconductor chip for example using a sintered metal oxide.

Often metallic oxide NTC thermistors are made from fine powers that are compressed and sintered at high temperature. Materials used include Mn2O3, NiO, Co2O3, Cu2O, Fe2O3, TiO2 and the like. They may also be manufactured from silicon or germanium crystals that are doped to provide the required level of conduction.

NTC thermistors operate because a rise in temperature results in the number of active charge carriers increasing as they are freed from the crystal lattice.

The method of conduction varies according to the type of material. In the case of ferric oxide, Fe2O3 doped with titanium gives an N-type semiconductor and in this case the majority charge carriers are electrons. In other materials like nickel oxide, NiO doped with lithium, Li form a p-type semiconductor where the majority charge carriers are holes. Either way, the same basic characteristics of an NTC thermistor are exhibited.

the choice of material for the NTC thermistor depends on many factors, although one of the major ones is the temperature range required.

Germanium NTC thermistors are generally used for temperatures in the range 1 - 100°K (i.e. degrees absolute). Silicon ones for temperatures up to 250°K - they cannot be used above this because a positive temperature coefficient takes over above this temperature. Metallic-oxide NTC thermistors are used for the range 200 - 700°K. For higher temperatures still, very stable compounds are required and NTC thermistors for these temperatures may be made from materials including: Al2O3, BeO, MgO, ZrO2, Y2O3 and Dy2O3.

NTC thermistors are widely used within the electronics industry for many basic temperature sensing uses. The thermistors themselves may be very small, often the size of a small bead but with two leads emanating from them. Other types and sizes exist which provide a variety of characteristics.

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