We know that the atomic size of elements increase as we move down in a group in the period table. This increase in size is accompanied by a decrease in ionization energy required. Thus, the energy gap between the conduction band and valence band decreases down the group. For Sn it is 0eV, for C its 5.4 eV and for Si and Ge its 1.1eV and 0.7eV respectively, thus Sn is a conductor, C is an insulator while Si and Ge are semiconductors.