^Energy bands in solids

Energy levels of electrons in an isolated atom are well defined & discrete, however the valence energy levels get modified drastically when atoms interact to form the lattice. If N atoms interact, then total 8 N energy valence energy levels are available which split in two bands (called VB & CB) separated by energy gap (called E_g) as shown in the diagram.

As E_g  (Ge) < E_g  (Si), thus Ge is preferred in making semi conducting devices.