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Asked in RS Academy GUJCET booklet · Energy bands and band gap
Idea: at absolute zero there is no thermal energy at all, so every electron sits in the lowest state still available to it.
In silicon the four valence electrons of each atom go into covalent bonds, and those bond states are exactly the states of the valence band. Four electrons per atom fill the valence band completely.
Getting into the conduction band costs about 1.1 eV, and at 0 K no electron has it, so not one electron is up there.
So at absolute zero the valence band is completely filled and the conduction band completely empty -- which is why pure silicon is a perfect insulator at 0 K and starts to conduct only as it warms up.
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