Pure silicon and germanium behaves as

Semiconductors

The pure silicon and germanium atom belong to the group 14 of the periodic table, having four valence electrons forming covalent bonds in the diamond-like crystal lattice, thus making it a poor conductor.

But they are not completely insulators like the carbon atom in the same group, because they have an extra shell, making the outermost electrons loosely bound to the nucleus and so less energy is needed to exchange the electrons.

Thus, the pure silicon and germanium atom have a transitional behaviour between the conductors and the insulator. Thus, making them a semiconductor and are also known as a metalloid.