On carbon the outer shell valence electrons are closer to the nucleus and so are too tightly bound for carbon to be a semiconductor. Silicon and Germanium have the valence electrons in more distant outer shells which Carbon lacks and so they're freer to slip away and be freely exchanged with adjoining atoms. Silicon in it's pure form isn't a semiconductor anyway, it has to be doped with other elements to become semiconducting.
Please note that this is a vastly oversimplified answer, but the principle is correct.
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On carbon the outer shell valence electrons are closer to the nucleus and so are too tightly bound for carbon to be a semiconductor. Silicon and Germanium have the valence electrons in more distant outer shells which Carbon lacks and so they're freer to slip away and be freely exchanged with adjoining atoms. Silicon in it's pure form isn't a semiconductor anyway, it has to be doped with other
elements to become semiconducting.
Please note that this is a vastly oversimplified answer, but the principle is correct.