Conductors, Insulators and Semiconductors

An electric current can be defined as a flow of electrons. In conductors, such as metals, the attraction between the outer electrons and the nucleus of the atom is weak, the outer electrons can move readily and consequently metals are good conductors of electricity. In other materials, electrons are strongly bonded to the nucleus and are not free to move. Such materials are insulators (or dielectrics). In semiconductors, the conductivity falls between those of conductors and insulators. Table 13.1 lists the characteristics of all three groups. 

Device Typical material Electron mobility Resistivity ohm-cm 

Conductor Copper Silver Gold Tungsten Silicides Free to move 10-5 to 10-6 

Semiconductor Silicon Germaniun III-V II-VI compounds Diamond Silicon carbide Partially able to move 10-2 to 109 

Insulator (dielectrics) Alumina Silicon oxide Silicon nitride Glass Bound to nucleus 1012 to 1022 

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Semiconductors. The basic material employed in LEDs is the semiconductor, a soHd which possesses a conductivity intermediate between that of a conductor and an insulator. Unlike conductors, semiconductors and insulators possess an energy gap, E, between two energy bands, the... 

CVD in Electronic Applications Semiconductors 347 2.1 Conductors, Semiconductors, and Insulators... 

The processible organic conductors, semiconductors, and insulators (not discussed in this chapter but well known historically for saturated polymers with sp3 electronic configuration) form fundamental material set for device applications. In the following sections, we discuss how to construct a PLED with such material set. 

Wilson ((>85) has pointed out that if a Brillouin zone is full, the electrons occupying the states of this zone can make no contribution to the electric current. This fact follows from the definition of the zone as a region enclosing all reduced wave vectors. Imagine all electrons of Figure 6 shifted Akx by an external field. The electrons in states within Akx of the zone boundary are reflected to the opposite zone boundary, so that the zone remains filled and there is no transfer of charge. This observation permits a sharp distinction between metallic conductors, semiconductors, and insulators. Because of the high density of states in a band, a crystal with partially filled bands is a metallic conductor. If all occupied zones (or bands) are filled, the crystal is a semiconductor if Ef kT, is an insulator if kT. 

The electric conductivity specifies the electric character of the material. Solid materials, in three groups of conductors, semiconductors, and insulators, exhibit a wide range of electric conductivities. Metals have conductivities on the order of 107 (fi m)-1, insulators have conductivities ranging between 10 10 and 10 20 (O m), and the conductivities of semiconductors range from 10 6 to 104 (O m). ... 

With respect to their conductivity, materials can roughly be divided into three categories metallic conductors, semiconductors and insulators. 

What is the energy gap in band theory Compare its size in superconductors, conductors, semiconductors, and insulators. 

The difference between conductors, semiconductors, and insulators is determined by how easily electrons (or holes) can move through the crystalline material. The movement (delocalization) of electrons, or their localization on or between particular atoms, is determined by the chemical bonding. In a crystal, it is necessary to consider the repeating bonding interactions between the many atoms rather than just the bonding interactions between two atoms in an isolated molecule. 

The features of metallic conductors, semiconductors, and insulators that we ve outlined here apply to more than just elemental forms of substances. Alloys are combinations of metallic elements (with an occasional minor component that is a nonmetal), and their bonding resembles what we have just described for metals. Semiconductors are often made from combinations of elements, too. Gallium arsenide is a particularly important example. Insulators, too, are often made from combinations of elements, and many ceramic materials are insulators. 

FIGURE 10.1 Comparison of conductivity in various materials utilized as conductor, semiconductor, and insulator [1],... 

The band theory of the electronic structure of solids was discussed. In this theory the electrons are assumed to occupy delocalized orbitals that comprise bands of energy levels. The differences between conductors, semiconductors, and insulators... 

A simple way to imderstand the electronic conductivity is to examine the available energies for electrons in the materials. Instead of having discrete energies, the available energy states form bands. Figure 18.1 shows the schematic of the electronic band stmctures of conductors, semiconductors, and insulators. The lower band is called the valence band, by analogy with the valence electrons of individual atoms. Electrons in the valence band are bound to individual atoms. The upper band is called the conduction band because only when electrons are... 

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