Energy Band Theory Questions and Answers

Answer: D

The band gap of SiO₂ is about 8 eV.

Silicon dioxide is an excellent insulator, with a band gap usually quoted as about 8 to 9 eV. This wide gap is why thermally grown SiO₂ serves as the gate dielectric and as an insulating and masking layer in silicon technology.

1.12 eV is the band gap of silicon and 1.42 eV that of gallium arsenide, both semiconductors, and 5 eV is far too small for SiO₂; it is closer to the gap of diamond.

Answer: B

The material that is not a semiconductor is indium.

Indium is a metal of group III (group 13). It conducts like a metal, with no band gap, and in semiconductor work it is used as a dopant (an acceptor in germanium and silicon) or as part of compounds such as indium phosphide, not as a semiconductor on its own.

The other three are semiconductors. Germanium and silicon are the classic elemental semiconductors of group IV, with band gaps of about 0.67 eV and 1.1 eV. Gallium arsenide (spelt 'Galium' in the paper) is a III-V compound semiconductor with a direct band gap of about 1.43 eV, used in LEDs and high-speed devices.

Answer: C

The electron is surrounded by a large number of unoccupied energy states.

The conduction band of a semiconductor contains a very large number of allowed states, and at ordinary temperatures only a tiny fraction of them hold electrons. An electron excited into the band therefore finds plenty of empty states next to it in energy, which is why it can gain a little energy from an applied field, move into a neighbouring state, and conduct.

The options describing a very small number of states, or a band full of occupied states, describe the valence band of an insulator-like case, where a full band leaves no room for the electron to move.