Intrinsic Semiconduction

Table 5.4 Intrinsic semiconducting properties of eeramie oxides...

There are two experimental investigations of resistivity of germanium under elastic shock compression. The work of Graham et al. [66G01] interpreted limited measurements on [111] Ge on the basis of intrinsic semiconduction... 

Semiconductors are materials that contain a relatively small number of current carriers compared to conductors such as metals. Intrinsic semiconductors are materials in which electrons can be excited across a forbidden zone (bandgap) so that there are carriers in both the valence (holes, p-type) and conduction (electrons, ra-type) bands. The crucial difference between a semiconductor and an insulator is the magnitude of the energy separation between the bands, called the bandgap (Eg). In the majority of useful semiconducting materials this is of the order of 1 eV some common semiconductors are listed in Table 1. 

The most striking implication of the electron lattice coupling in ID chains is the appearance of the semiconducting state the equal bond ID lattice (metallic state) is unstable (33) with respect to a lattice distorsion and this so called static Peierls instability is the origin of the opening of the intrinsic band gap at the edge of the B.Z. with an infinite density of states there and the presence of band alternation. 

Aliovalent additives are often called donor dopants, when they tend to provide electrons and enhance intrinsic n-type semiconducting behavior, or acceptor dopants, when they tend to give a population of mobile holes and enhance /j-typc semiconducting behavior. The process of creating electronic defects in a crystal in this way is called valence induction. 

Reaction between C in methanol and RTCNQ in acetonitrile yielded three kinds of ionic solids (1) insulators composed of methoxy substituted RTCNQ anions such as (CHC )[F4TCNQ-0Me ](H20) (Fig. 6) [136], (2) semiconducting CT solids with fully ionic RTCNQ radical anions such as (CHC )(TCNQ ) [137, 138], and (3) conducting CT solids of partially ionic or mixed valent RTCNQ radical anions such as (CHC"XMeTCNQ° >2 [138], where CHC" is the hemiprotonated cytosine pair (Fig. 6b). Cation units in aU products were found to be protonated cytosine species, most commonly CHC, where comes from methanol. This result suggests that the intrinsic transport properties of DNA should be studied not in protic solvents but under strictly dried conditions. 

Intrinsic Semiconduction. We return now to our description of band gaps from Section 6.1.1.1 in order to elaborate upon those materials with conductivities in the range 10 to 10" (f2 m) . At first, it may not seem important to study materials that neither readily conduct electricity nor protect against it, as do insulators. However, the development of semiconducting materials has, and continues to, revolutionize our lives. 

Conduction that arises from thermally or optically excited electrons is called intrinsic semiconduction. The conduction of intrinsic semiconductors usually takes place at elevated temperamres, since sufficient thermal agitation is necessary to transfer a reasonable number of electrons from the valence band to the conduction band. The elements that are capable of intrinsic semiconduction are relatively limited and are shown in Figure 6.12. The most important of these are silicon and germanium. 

Figure 6.12 The intrinsic semiconducting elements. Reprinted, by permission, from I. Amato, Stuff, p. 68. Copyright 1997 by Ivan Amato.

Chapter 4 discussed semiconductivity in terms of band theory. An intrinsic semiconductor has an empty conduction band lying close above the filled valence band. Electrons can be promoted into this conduction band by heating, leaving positive holes in the valence band the current is carried by both the electrons in the conduction band and by the positive holes in the valence band. Semiconductors, such as silicon, can also be doped with impurities to enhance their conductivity. For instance, if a small amount of phosphorus is incorporated into the lattice the extra electrons form impurity levels near the empty conduction band and are easily excited into it. The current is now carried by the electrons in the conduction band and the semiconductor is known as fl-type n for negative). Correspondingly, doping with Ga increases the conductivity by creating positive holes in the valence band and such semiconductors are called / -type (p for positive). 

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