Conductivity and Doping in Semiconductors

The conductivity is proportional to the number of electrons in the conduction band, thus given by a Boltzmann factor. The conductivity (in a weak field) is thus proportional to 

Whether activation energy is necessary for the motion of the electrons in the conduction band depends on the electronic structure. In the inorganic semiconductors previously mentioned (silicon, GaAs, etc.), the electron is delocalized and no further thermal activation is necessary. 

Semiconductors are used in the electronics industry. One important property is that the conductivity increases with the temperature (due to activation to the conduction band) while it decreases for metals. As can be understood from the band gaps, the conductivity at room temperature is very small for listed semiconductors. This 

FIGURE 16.11 Energy levels in an electric field E of a semiconductor. Dashed orbitals are unoccupied full-drawn are occupied. 

Alternatively, diamond may be doped by boron. There is now one missing electron in the sp valence band. The hole is quite delocalized, as the electron at n-doping. The positive hole is centered around the B atom, since the boron core is more negative than the carbon core, while the energy will be at the top of the valence band since the orbital is half filled. This type of doping is called p-doping, since the free particle (the hole) is p(ositively) charged. 

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