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Asked in CBSE AIPMT 2004 · Energy bands and band gap
Idea: at 0 K a semiconductor's valence band is full and its conduction band empty, which is why it behaves as an insulator then.
The gap is small, about 1.1 eV in silicon, so at room temperature thermal energy lifts some electrons across it.
Each electron that leaves takes its place in the conduction band and leaves a hole behind.
So the valence band ends up partially empty and the conduction band partially filled, and conduction becomes possible.
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