We know that C, Si and Ge are four valeance atoms. Their four valence electrons are present in the second, third and forth orbit of the respective atoms. The energy gap of Ge is `0.72 eV`, of Si is` 1.1 eV` and of C is `5.54 eV`. The energy required to take out an electron from these atoms (i.e.,ionisation energy `E_(g))` will be least for Ge, more Si and maximum for C. Due to which the number of free electrons for conduction will be significant in Ge and Si but will be negligibly small for C. That is why C is aan insulator while Si and Ge are intrinsic semiconductor.
