Explain why the electrical conductivity of metals decreases as temperature increases, whilst the electrical conductivity of semiconductors increases as temperature increases.

IB DP Chemistry Higher Level (2023 syllabus) — S2.3 The metallic model · Explain · 4 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Model answer (4 marks)

Metals consist of a lattice of positive ions surrounded by delocalised electrons that carry charge. As temperature rises the ions vibrate more strongly, increasing the frequency of collisions with the delocalised electrons and impeding their flow, so conductivity falls.

In semiconductors the electrons are covalently bonded and localised at low temperature, giving a low conductivity. When the temperature increases the electrons gain enough thermal energy to be promoted across the small band gap, creating more charge carriers and therefore increasing conductivity.

Examiner tips

  • Use the terms ‘delocalised electrons’ and ‘band gap’ to show understanding
  • Explain the role of ion vibration in metals and thermal excitation in semiconductors
  • Keep the answer concise and directly address the two materials

Common mistakes

  • Confusing the direction of conductivity change for metals and semiconductors
  • Lacking the term ‘band gap’ or ‘delocalised electrons’
  • Over‑explaining the lattice structure without linking to conductivity

Mark scheme (4 marks)

  1. Metals contain a lattice of positive ions (cations) surrounded by delocalised electrons that carry charge / act as charge carriers.
  2. As temperature increases, the positive ions in the metallic lattice vibrate with greater amplitude, increasing the frequency/degree of collisions with delocalised electrons and impeding electron flow, so conductivity decreases.
  3. In semiconductors, electrons are (covalently) bonded / localised at low temperatures and are not available as charge carriers, so conductivity is initially low.
  4. As temperature increases, electrons in semiconductors gain sufficient thermal energy to be promoted / excited across the (small) band gap, increasing the number of charge carriers and therefore increasing conductivity.

Key terms in this question

semiconductor · electrical conductivity

Related

More The metallic model questions

▶ Try answering this question with AI marking (free) →