Discussion
In a silicon crystal containing N atoms, at absolute zero the 4 N energy states of
Answer: B
At absolute zero the valence band is completely occupied and the conduction band is completely empty.
Each of the N silicon atoms has 4 valence electrons, 4N in all, and the bands formed from these levels have 4N states each. At 0 K all 4N electrons fill the lower (valence) band and none has the energy to cross the gap of about 1.1 eV, so the upper (conduction) band is empty.
That is why pure silicon is an insulator at absolute zero. A partly filled conduction band needs thermal energy or doping, and the other options describe states that do not occur at 0 K.
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