Semiconductor statistics

As we have seen, the particular properties of semiconductor materials are based on the existence of a bandgap of forbidden energies in the band structure, which separates the last occupied band (valence band) from the first unoccupied band (conduction band). Direct interband excitation of electrons from the valence band to the conduction band can occur either by optical excitation (as we shall see in the next chapter) or by thermal excitation. The latter process occurs when the thermal energy https://www.w3.org/1998/Math/MathML" display="inline"> k B T is on the order of the bandgap E g . In the particularly important case of silicon, https://www.w3.org/1998/Math/MathML" display="inline"> E g = 1.17 eV and intrinsic thermal excitation occurs only at temperatures well above 1000 K. It is therefore misleading to say that semiconductors are materials of reduced bandgap. What distinguishes semiconductor materials is the ability to generate appreciable concentrations of charge carriers (electrons in the conduction band, electron holes in the valence band) at room temperature, not so much because of direct interband excitation, but essentially because of the contribution of defects and impurities present in the material. Semiconductor physics is therefore strongly intertwined with the physics of defects and impurities in semiconductors.