C (diamond), Si, and Ge share a similar tetrahedral lattice structure, with each carbon, silicon, and germanium atom covalently bonded to four neighboring atoms. However, their electronic properties differ significantly.

C (diamond) acts as an insulator because its wide band gap (~5.5 eV) prevents electrons from moving freely, as all valence electrons are involved in strong covalent bonds. In contrast, Si and Ge are intrinsic semiconductors with smaller band gaps (about 1.1 eV for Si and 0.66 eV for Ge). This allows thermal energy at room temperature to excite some electrons from the valence band to the conduction band, enabling conductivity.

Thus, while the lattice structure is identical, the band gap differences lead to distinct electrical behaviors, classifying C as an insulator and Si and Ge as semiconductors.