C and si both have a same lattice structure, having 4 bonding electrons in each. However, C is insulator whereas Si is an intrinsic semiconductor. This is because

• in the case of C, the valence band is not completely filled at absolute zero temperature

• in a case C, the conduction band is partly filled even at absolute zero temperature

• the four bonding electrons in the case of C lie in the second orbit, whereas in case of Si they lie in the third

• the four bonding electrons in the case of C lie in the third orbit, whereas for Si they lie in the fourth orbit