Temperature, Thermistors & LDRs

• In a metal, resistance comes from electrons colliding with vibrating ions. Heat the metal and the ions vibrate more. More collisions, slower drift, more resistance.
• In a semiconductor, most electrons are fixed to their atoms. Heat frees more of them, so $n$ increases a lot. The extra carriers matter more than the extra collisions, so the resistance falls.
• In a metal, heating only adds vibration, so resistance rises. In a semiconductor, heating also adds carriers, so resistance falls.

• A thermistor (the NTC type the syllabus wants) is a semiconductor made to use this effect: its resistance drops steeply as it warms. Thousands of ohms cold, tens of ohms hot.
• That makes it a temperature sensor: car water-temperature gauges, fire alarms, ice detectors on aircraft wings.
• The exam also asks about thermistors inside circuits, where their I-V curve is combined with a fixed resistor. (9702/11/M/J/25 Q35)

• An LDR (light-dependent resistor) is a semiconductor where light, not heat, frees the electrons. Brighter light, more carriers, less resistance.
• In the dark an LDR's resistance is around a million ohms. In bright light it falls to a few hundred. That is why it can switch street lights on when it gets dark.

Both sensors use the same semiconductor effect: give the electrons energy (heat for the thermistor, light for the LDR) and n rises, so R falls.