A perfect semiconductor crystal with no impurities or lattice defect is called an intrinsic semiconductor. At zero Kelvin, there is no charge carrier, because the valence band is filled and conduction band is empty. At higher temperature electron - hole pairs are generated, the valence band electron is excited across the band gap to the conduction band. The electron hole pairs are the only charge carriers in the intrinsic semiconductor.
In order to understand the electrical conduction property in this, consider silicon (or germanium) having four valence electrons as shown in Fig.1
At 0 K all the valence electrons are engaged in forming covalent bonds with the neighbours and no free electron is available. At this stage the available silicon does not conduct and the material behaves like an insulator. At room temperature, the thermal energy is sufficient to liberate electrons from valence band and produce free electrons. When an electric field is applied, the free electron acquires sufficient energy and shifts to conduction band. Since the electrons and holes are created in pairs, the conduction band electron concentration is equal to the concentration of holes in valence band. Due to opposite charges, the movement of electrons and holes give rise to electric current in the same direction.
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