Example

Question:

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

Solution:

The 4 bonding electrons of C, Si, or Ge lie in the second, third, and fourth orbit, respectively. The energy required to remove an electron (ionisation energy \(E_g\)) is least for Ge, followed by Si, and highest for C. Therefore, the number of free electrons for conduction in Ge and Si is significant, but negligibly small for C.