Explain why the electrical conductivity of silicon increases as temperature increases, whereas the electrical conductivity of copper decreases as temperature increases.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
Silicon is a semiconductor; at low temperatures very few electrons occupy the conduction band, so conductivity is low.
As temperature rises, more electrons gain enough thermal energy to cross the band gap into the conduction band, increasing the number of charge carriers and therefore the conductivity.
Copper is a metallic conductor with a partially filled conduction band; it already has a large number of delocalised electrons that act as charge carriers.
With increasing temperature the copper lattice ions vibrate more strongly, causing more frequent electron–phonon scattering, which raises the resistance and reduces the conductivity.
As temperature rises, more electrons gain enough thermal energy to cross the band gap into the conduction band, increasing the number of charge carriers and therefore the conductivity.
Copper is a metallic conductor with a partially filled conduction band; it already has a large number of delocalised electrons that act as charge carriers.
With increasing temperature the copper lattice ions vibrate more strongly, causing more frequent electron–phonon scattering, which raises the resistance and reduces the conductivity.
Examiner tips
- Use the terms "semiconductor" and "metallic conductor". Explain the role of charge carriers for each material. Mention thermal energy and lattice vibrations. Show the opposite trends clearly.
Common mistakes
- Confusing the direction of conductivity change for copper. Saying that silicon has a large number of carriers at low temperature. Ignoring the role of lattice vibrations in metals.
Mark scheme (4 marks)
- Silicon is a semiconductor; at low temperatures there are few charge carriers / electrons in the conduction band.
- As temperature increases, more electrons gain sufficient thermal energy to cross the band gap into the conduction band, increasing the number of charge carriers and thus conductivity.
- Copper is a metallic conductor with a partially filled conduction band / large number of delocalised electrons already available as charge carriers.
- As temperature increases, greater thermal vibration of the copper lattice ions increases resistance / impedes electron flow, so conductivity decreases.
Related
- All IB DP Chemistry Higher Level (2023 syllabus) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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