Band model of a semiconductor

1 ELECTROCHEMICAL PROPERTIES OF A SEMICONDUCTOR 9.1.1 Band model of a semiconductor 

The electronic properties of solids were described in Chapter 2 using the band model. A characteristic feature of semiconductors is the separation of the electron energy levels into two bands, the valence band with occupied energy levels and the conduction band with unoccupied energy levels. Both bands are separated by an energy gap. The band gap energy determines the intrinsic conductivity because electricity can only be transported through the semiconductor if some electrons are excited from the valence band to the conduction band. Then either holes in the valence band or electrons in the conduction band become mobile. The mobility of valence band holes and conduction band electrons 

In metals the Fami energy is the enetgy of the highest occupied energy level (at T = 0 K). In an intrinsic semiconductor the Fermi energy is per definition midway between the valence and conduction bands. Therefore, the Fermi energy for an intrinsic semiconductor is 

Conductivity can also be achieved by doping. By adding small amounts of substances with excess electrons, these electrons can be inserted into the conduction band (n-doping, n-semiconductor). Adding substances with electron deficiency attracts the electrons from the valence band and creates holes in the valence band (p-doping, p-semiconductor). The properties of the semiconductor are then determined by the new majority careers, either by the electrons in the conduction band or the electron holes in the valence band. 

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Figure 9.18 Band model of a semiconductor aud tip iu an electrochemical ceU under illumination. (A) n-Type semiconductor and (B) p-type semiconductor (according to Allongue). ...

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