Band Theory, Defects and Why Some Solids Conduct · seed 1 · A4, ink-friendly. The answer key prints on its own page for grown-ups.

Bands decide who conducts

Science · Chemistry · ages 21-22
Name ______________________   Date ____________

  1. In the simple independent-electron band model, neglect localisation and strong electron correlations. A solid has a partly filled top band. How does it conduct?

    • Like an insulator
    • Not at all
    • Like a metal

  2. How do atomic orbitals produce energy bands in a solid?

    • They interact to form many closely spaced states
    • They remain isolated and all keep exactly one energy
    • They disappear and leave only forbidden energies

  3. In the ideal band model at zero temperature, a band insulator has full occupied bands separated from empty bands by a gap.

    Circle one:   True   False

  4. Shallow donor states just below silicon's conduction band release electrons. Which doped type results?

    • p-type
    • Unchanged silicon
    • n-type

  5. Why does warming intrinsic silicon usually raise conductivity, while warming copper near room temperature lowers it?

    • Silicon gains carriers; copper has more electron scattering
    • Silicon loses all carriers; copper opens a wide forbidden gap
    • Both gain identical carrier numbers and identical mobilities

  6. An electron trapped at an anion vacancy absorbs selected visible wavelengths. Which conclusion is supported?

    • It can change colour without making the electron mobile
    • It must create a mobile conduction electron to change colour
    • It can change colour only by changing the crystal's bulk band gap

  7. Adding impurity atoms to silicon can only lower its electrical conductivity.

    Circle one:   True   False

  8. A pure crystal has full valence bands at zero temperature, a small gap, and thermally generated mobile electrons and holes when warmed. Which band classification fits?

    • A typical metal
    • A semiconductor
    • A wide-gap insulator
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Answer key

For grown-ups. Fold this page away before handing over the rest.

Bands decide who conducts W1-mt_RuPWyBzGOp-s1

  1. Like a metal · Under these model assumptions, a partly filled band has occupied and nearby empty states. An electric field can change their occupation and produce metallic conduction.
  2. They interact to form many closely spaced states · Interacting orbitals form many states. With very many atoms, the allowed energies are closely spaced.
  3. True · The stated zero-temperature band model has no partially occupied band available for ordinary electronic conduction.
  4. n-type · Levels near the conduction band donate electrons, which is the n-type pattern.
  5. Silicon gains carriers; copper has more electron scattering · Silicon gains thermally excited electrons and holes. Copper's stronger lattice vibrations mainly reduce electron mobility.
  6. It can change colour without making the electron mobile · Selective absorption by a localised defect state can change colour without producing mobile carriers or changing the bulk band gap.
  7. False · Shallow donors or acceptors can add mobile electrons or holes and raise conductivity. The outcome also depends on mobility.
  8. A semiconductor · The small gap and thermally generated electron-hole carriers identify the stated semiconductor band model.
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