Question
C, Si and Ge have same lattice structure. Why is $C$ insulator while $Si$ and Ge intrinsic semiconductors?
✓
Answer
The 4 bonding electrons of $C , Si$ or Ge lie, respectively, in the second, third and fourth orbit. Hence, energy required to take out an electron from these atoms (i.e., ionisation energy $E_q$ ) will be least for Ge, followed by $Si$ and highest for $C$. Hence, number of free electrons for conduction in Ge and Si are significant but negligibly small for $C$.
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