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The band gaps of a conductor, a semiconductor and an insulator are respectively E_g1, E_g2 and E_g3. The relationship between them can be given as ________.

Asked in GSEB Board July 2015 · Energy bands and band gap

• (1) E_g1 = E_g2 = E_g3
• (2) E_g1 > E_g2 > E_g3
• (3) E_g1 < E_g2 < E_g3
• (4) E_g1 < E_g2 > E_g3

Step-by-step solution

Why the other options are wrong

• (1) Equal gaps would make the three materials conduct alike. The size of the gap is the very thing that separates a conductor from a semiconductor from an insulator.
• (2) This is the correct order reversed. It would give the conductor the widest gap, so electrons could hardly reach its conduction band at all, which is the opposite of what a conductor is.
• (4) This makes the semiconductor's gap the largest of the three. Silicon's 1.1 eV sits between the metal's effectively zero gap and the several eV of an insulator, so the semiconductor is in the middle, not at the top.

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