Extrinsic Semiconduction

Semiconducting Ceramics. Most oxide semiconductors are either doped to create extrinsic defects or annealed under conditions in which they become non stoichiometric. Although the resulting defects have been carefully studied in many oxides, the precise nature of the conduction is not well understood. Mobihty values associated with the various charge transport mechanisms are often low and difficult to measure. In consequence, reported conductivities are often at variance because the effects of variable impurities and past thermal history may overwhelm the dopant effects. 

In the case of intrinsic band conduction the experimental activation energy SA is identified with half the band gap (Eq. (2.37)) in the case of extrinsic or impurity semiconductivity, SA is either half the gap between the donor level and the bottom of the conduction band or half the gap between the acceptor level and the top of the valence band, depending upon whether the material is n or p type. In such cases the temperature dependence is determined by the concentration of electronic carriers in the appropriate band, and not by electron or hole mobility. 

Defects that introduce extra electrons, or that give missing electrons or holes , have a large influence on electronic conduction in nonmetallic solids. Most semiconductor devices use doped or extrinsic semiconductors rather than the intrinsic semiconduction of the pure material. Doping Si with P replaces some tetrahedrally bonded Si atoms in the diamond lattice (see Topic D2) with P. Each replacement provides one extra valence electron, which requires only a small... 

The poor stability of some semiconducting polymers and their sensitivity to external factors often make it difficult to deduce true intrinsic properties of a certain material [101,102]. Most commonly, the off-currents in a TFT are governed by extrinsic factors and it also has an impact on subthreshold slope, threshold voltages, and bias stress. In fact, this sensitivity prompted some researchers to use OSC materials as gas and chemical sensors in TFT devices [103,104]. The observed selective sensitivity is related either to the chemical nature of the semiconductor functionalities or the interaction of the grain... 

The conclusion to be drawn from these results is that inasmuch as polypeptides can be accommodated by the proposed model, pure proteins should be rather quite good insulators. The observed semiconductivity, which, following the quoted work of Eley, Rosenberg and collaborators, involves an energy gap of 2-3 eV, is probably extrinsic, originating from extra energy levels, due to the presence of defects or impurities. 

Trans-polyacetylene has the unusual property of exhibiting no extrinsic dimerization, and thus has no extrinsic band gap. The dimerization arises entirely from TT-electrons coupling to the lattice. Consequently, the A and B phases are degenerate. Most polymers, however, have an extrinsic semiconducting band gap as a result of their stereochemistry independent of the of 7r-electrons. Examples of polymers that are extrinsically semiconducting include, cis-polyacetylene (because of the structure caused by the a orbitals), polydiacteylene (because of the... 

The phenyl-based conjugated polymers are extrinsically semiconducting as a consequence of the chemical structure determined by the a bonds. Thus, with all bond lengths equal there is still a semiconducting band gap, as shown by Figs 11.8 and 11.11. However, coupling of the 7r-electrons to the lattice is still important as it causes the types of excited state structures described in Chapters 4 and 7 for extrinsically semiconducting polymers. 

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