C, Si and Ge have same lattice structure. Why is C insulator while Si and Ge intrinsic semiconductors?

Explanation

Atoms require less energy to release electron from the orbit away from nucleus.
There are four electrons in the second orbit in `""_(6)C`, four electrons in third orbit of `""_(14)Si` and four electrons in the fourth orbit of `""_(32)Ge`.
Hence, energy required to take out an electron from these atom (ionisation energy) will be least for Ge, more than that for Si and highest for C or the resistivity of material depends on the energy difference between their conduction and valence band and for C this energy difference is 5.4 eV, for Si it is 1.1 eV and for Ge it is 0.7 eV.
Hence, number of free electrons for conduction in Ge and Si are significant but neligibly small for C so Ge and Si are semiconductor and C is insulator.