**2.1 Electrical behavior of semiconductors**

By definition, a semiconductor is characterized by the absence of a continuum between the states (as for metals) but shows a band structure. The filled levels, called "valence band" (VB), are an energetically closely spaced array of orbitals composed by the valence electrons of the material. A similar, higher energetic, spaced array is formed by the unoccupied orbitals and it is called "conduction band" (CB). The gap existing between the top edge of VB (Ec) and the lower edge of CB (Ec) is called "band-gap" (Chattopadhyay & Rakshit, 2010).
