A-LevelPhysicsQuantum physics9702/42 · Feb/Mar 2018 · Q117 Marks
Some electron energy bands in a solid are shown in [Figure 11.1]. The width of the forbidden band and the number density of charge carriers occupying each band depends on the nature of the solid. Use band theory to explain why (a) the resistance of a metal at room temperature increases gradually with temperature, (b) the resistance, at constant temperature, of a light-dependent resistor (LDR) decreases with increasing light intensity.
- (a)the resistance of a metal at room temperature increases gradually with temperature,[3]
- (b)the resistance, at constant temperature, of a light-dependent resistor (LDR) decreases with increasing light intensity.[4]
This question uses a diagram from the original paper, which isn't shown here. Open the full paper from the past papers archive to see it.
Mark scheme
Marking points from the official mark scheme (Feb/Mar 2018). The mark or mark type is shown in brackets.
- [B1] no forbidden band / valence and conduction bands overlap
- [B1] no change in number of charge carriers (as temperature rises)
- [B1] increased lattice vibrations so resistance increases
- [B1] photons captured / absorbed by electrons in valence band
- [B1] electrons promoted to conduction band
- [B1] leaving holes in the valence band
- [B1] more holes and / or electrons so resistance decreases
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