Conduction and Semiconductor Theory

In semiconductors, there is a special valence energy band under the conduction band. With pure (intrinsic) semiconductors, the energy levels in between are forbidden levels, and at room temperature very few electrons statistically have sufficient energy to cross the forbidden band and reach the conduction band that is, the conductivity is low. For example, the energy gap is 0.7 eV for germanium, 1.1 eV for silicon, and 5.2 eV for diamond (an insulator). 

However, with local impurities in the material, local energy centers may exist in the forbidden gap. Here, electrons reside on energy levels that may be characterized as local energy wells. With a certain amount of added energy electrons can come up from the well and reach the conduction band (n-type impurities). This can considerably increase conductivity (extrinsic conduction). According to the nature of the impurities, this added conduction may be by electrons or holes as charge carriers. 

The idea of a possible semiconductive mechanism (electrons and holes) in biomaterials is old, as illustrated by the book by Pethig (1979). Later, Takashima (1989) did not mention semiconductivity as a possible mechanism. Indications of an electronic, semiconductive conduction for the DNA molecule have appeared again (Fink and Schdnenberger, 1999). Such experiments are performed under nonphysiological conditions in vacuum, which implies that every water molecule free to do so has disappeared. 

Ions do not obey these laws of semiconductors. However, the concept of local energy wells can also be adapted to ionic conduction. Debye (1929) proposed a model in which an ion may be translocated between a pair of neighboring energy wells by an applied electric field. With an applied AC field, an ion can be made to hop between these two wells. However, such local hopping does not contribute to DC conduction, only to AC polarization. 

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