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Asked in GUJCET 2018 · Energy bands and band gap
Idea: the three classes of solid differ in the width of the forbidden gap between the valence and conduction bands. Read the symbols in the order the question lists them -- insulator, conductor, semiconductor.
In a conductor the two bands overlap, so there is no gap at all to cross: E_g2≈0, the smallest of the three.
In a semiconductor the gap is narrow but real -- about 1.1 eV in silicon and 0.72 eV in germanium -- so E_g3 is small, but not zero.
In an insulator the gap is wide -- about 5.4 eV in diamond -- so E_g1 is the largest.
The true order is therefore E_g1 > E_g3 > E_g2, with the conductor at the bottom.
Only one choice is consistent with that: E_g1 > E_g2 < E_g3, which says the conductor's gap is smaller than both of the others.
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