Only negative temperature coefficient (ntc) thermistors will be considered.

The idea

A thermistor is a semiconductor resistor whose resistance changes markedly with temperature. This specification restricts study to the negative temperature coefficient (ntc) type, so you must know that its resistance falls as temperature rises and be able to explain why. In an ntc thermistor, thermal energy releases more charge carriers, so the number density of charge carriers increases. Although lattice vibrations also increase, the dominant effect is the rise in carrier number, so resistance decreases. You should recognise the component symbol, describe its behaviour in words, and use it in circuits such as a potential divider. You are not required to analyse positive temperature coefficient thermistors, so focus your revision on ntc behaviour, its graph and its applications.

What earns the marks

• States that an ntc thermistor is a semiconductor device whose resistance decreases as its temperature increases.
• Explains that increasing temperature releases more charge carriers, increasing the number density of charge carriers and so increasing current for a given potential difference.
• Uses the relationship R = V / I to show that a larger current at the same potential difference corresponds to lower resistance.
• Describes the shape of the resistance–temperature graph for an ntc thermistor: resistance falls steeply at low temperatures and more gradually at higher temperatures.
• Recognises and uses the correct circuit symbol for a thermistor in diagrams and circuit descriptions.
• Applies ntc behaviour in a potential divider, explaining how the output potential difference changes as temperature changes.

Common mistakes and how to put them right

• Thinking that resistance increases with temperature for all thermistors.

  Put it right by stating that only ntc thermistors show a decrease in resistance as temperature rises.

• Confusing the effect of increased lattice vibrations with the dominant carrier effect.

  Put it right by explaining that the rise in the number of charge carriers outweighs the increased scattering in an ntc thermistor.

• Treating the thermistor as an ohmic component with constant resistance.

  Put it right by stating that its resistance changes with temperature and its current–potential difference graph is non-linear.

Exam tips

• Sketch the resistance–temperature curve with resistance on the vertical axis and temperature on the horizontal axis, showing a decreasing curve that is steep at first and then flattens.
• When explaining the decrease in resistance, always link temperature to the release of charge carriers and then to current or resistance.
• In circuit questions, identify whether the thermistor is in the upper or lower part of a potential divider before predicting how the output potential difference changes.