A-LevelPhysicsElectricity9702/42 · Feb/Mar 2017 · Q115 Marks

Use band theory to explain why, unlike a copper wire, the resistance of an intrinsic semiconductor decreases with an increase of temperature.

Mark scheme

Marking points from the official mark scheme (Feb/Mar 2017). The mark or mark type is shown in brackets.

[B5] any five from:
electrons need energy to enter conduction band (from valence band)
(positively-charged) holes are left in valence band
moving charge carriers/holes/electrons are current
(increase of temperature leads to) more (positive and negative) charge carriers/more holes/more electrons so more current
more charge carriers/holes/electrons gives rise to less resistance
(increase of temperature causes) greater (amplitude of) vibrations of atoms/ions/lattice
effect of more charge carriers/holes/electrons is greater than effect of greater vibrations (and so resistance decreases)

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Topic: Electricity, A-Level Physics (syllabus 9702). Structured question, 5 marks.
Source: A-Level Physics, Feb/Mar 2017 Paper 4 Variant 2, question 11.

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