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Speaking in terms of energy levels:

At 0K (absolute zero) all states below valence energy band of electrons are occupied and all states above the conduction energy band are empty.

At higher temperatures some valence electrons are excited from their parent atom.

My question is: Are the valence electrons which are teared away from their parent atom moved to conduction energy band?

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This is correct! At finite temperature, the valence electrons acquire a specific probability, given by the Fermi-distribution, to enter the conduction band of the semiconductor.

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  • $\begingroup$ Since it looks like you know much in this field of study, I would ask you one more question if you do not mind: In 5-valence atoms added to pure silicon, the fifth electron is weakly bounded to parent atom and at room temperature is most probably excited to conduction band. After the lost of fifth valence atom, the parent atom becomes a positive ion right? And same thing applies for 3-valence atoms which after gaining extra electron in the 4th valence (covalent) bond become negative ions right? $\endgroup$
    – lucenzo97
    Commented Dec 26, 2016 at 10:10
  • $\begingroup$ @Keno - You are completely right! The atom with 5 valence electrons is a donor and releases one electron into the conduction band. The remaining donor atom, which occupies a site in the semiconductor lattice, becomes a positive ion. This requires only a very small energy (donor ionization energy) relative to the band gap energy. A an atom with 3 valence electrons is an acceptor accepting an electron so that it has 4 electrons in the lattice which means that a positive hole is released into the valence band and the atom remains as a negatively charged ion in the lattice. $\endgroup$
    – freecharly
    Commented Dec 26, 2016 at 19:26

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