• The most important result of the application of quantum mechanics to the description of electrons in a solid is the allowed energy levels of electrons will be grouped into bands.
• The bands are separated by regions of energies the electrons in the solid can not possess. Theses regions are called forbidden gaps or band gaps.
• The electrons in the outer most shell of the atoms in a solid, the valence electrons, are shown in the picture below in their lowest energy state. The band of these states is called the valence band.

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• Conduction or the motion of electrons is the principal intrest in the study of semiconductor physics.
• Conduction is only possible if one can impart kinetic energy to an electron.
• Let's look at three different classes of materials in terms of both the atomistic representation and from the energy band representation on if we can impart energy to an electron.