Formation of Electron Bands in a Semiconductor Crystal
The figure above shows a sequence that indicates how electrons bands are formed in a crystal from atomic states.
Electron bands are formed due to the regular ordering of atoms in semiconductor crystal. Within a band the electrons are allowed to move "freely" throughout the whole crystal. The concepts can be grasped in such simplified single band models. However, quantitative modeling of electron transport through heterostructures requires a more detailed understanding of the bandstructure physics.