The maximum possible number of electrons in the
outer orbit is 8 (2s + 6p electrons) So, for the 4N valence electrons there
are 8N available energy states These 8N discrete energy levels can either
form a continuous band or they may be grouped in different bands
depending upon the distance between the atoms in the crystal (see box
on Band Theory of Solids) At the distance between the atoms in the crystal lattices of Si and Ge,
the energy band of these 8N states is split apart into two which are
separated by an energy gap Eg (Fig