Carbon. silicon and germanium have same lattice structure. But charbon is an insulator, silicon and germanium are semiconductors because

The 4 bonding electrons of C, Si or Ge respectively, in the second , third and fourth orbit . Hence , energy required to take out and electron from these atoms (i.e., ionization energy ) 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 negligible small for C.