Discussion
A phosphorous doped silicon semiconductor (doping density : 10\(^{17}\)/cm\(^{3}\)) is heated from 100°C to 200°C. Which one of the following statements is correct ?
Answer: C
The correct statement is that the Fermi level moves towards the middle of the energy gap.
In n-type silicon the Fermi level lies near the conduction band at low temperature, because the donors supply most of the electrons. As the temperature rises, more electron-hole pairs are created across the gap, the material moves towards intrinsic behaviour, and the Fermi level moves down towards mid-gap.
The donor (phosphorus) level itself is fixed by the impurity, about 0.045 eV below the conduction band, and does not move with temperature, which rules out options B and D. Option A gives the wrong direction.
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