Conductors, semiconductors, insulators

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... 

Metals and semiconductors are electronic conductors in which an electric current is carried by delocalized electrons. A metallic conductor is an electronic conductor in which the electrical conductivity decreases as the temperature is raised. A semiconductor is an electronic conductor in which the electrical conductivity increases as the temperature is raised. In most cases, a metallic conductor has a much higher electrical conductivity than a semiconductor, but it is the temperature dependence of the conductivity that distinguishes the two types of conductors. An insulator does not conduct electricity. A superconductor is a solid that has zero resistance to an electric current. Some metals become superconductors at very low temperatures, at about 20 K or less, and some compounds also show superconductivity (see Box 5.2). High-temperature superconductors have enormous technological potential because they offer the prospect of more efficient power transmission and the generation of high magnetic fields for use in transport systems (Fig. 3.42). 

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

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. 

CVD is a maj or process in the production of thin films of all three categories of electronic materials semiconductors, conductors, and insulators. In this chapter, the role of CVD in the fabrication of semiconductors is reviewed. The CVD production of insulators, conductors, and diffusion barriers is reviewed in the following chapter. The major semiconductor materials in production or development are silicon, germanium, ni-V and II-VI compounds, silicon carbide, and diamond. 

The description derived above gives useful insight into the general characteristics of the band structure in solids. In reality, band structure is far more complex than suggested by Fig. 6.16, as a result of the inclusion of three dimensions, and due to the presence of many types of orbitals that form bands. The detailed electronic structure determines the physical and chemical properties of the solids, in particular whether a solid is a conductor, semiconductor, or insulator (Fig. 6.17). 

By the nature of conduction and values of conductivity, materials can be classified as conductors, semiconductors, or insulators (dielectrics). It is a special attribute of conductors that free electric charges are present in them. The migration of these free charges in an applied electric field manifests itself as electric current. 

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. 

The very first information obtained is whether a given compound is a conductor, semiconductor or insulator. Such a basic answer is of importance concerning the 5 f localiza-... 

Materials can be classified as conductors, semiconductors or insulators. Conductors typically have resistivity in the range 10 2-103 xQ cm, semiconductors approximately 106-10n iQ cm, and insulators about 1013-1018 (xQ cm. Table 1.5 compares the electrical resistivity of the elements and compounds at room temperature. Although the carbides and nitrides have somewhat higher resistivity than do the pure metals, they still have resistivity in the regime of metallic conductors. In comparison the ceramic materials have much higher values, and are typically insulators. 

The SEMICONDUCTOR, insulator, or conductor layers in microscale or larger scale electronic devices such as a photovoltaic cell are created in a reactor. The reactor needs to be designed and operated to produce materials that have the desired optical and electronic properties. The design of reactors is a nontrivial research and design problem. In this chapter, some of the theoretical and experimental framework for this research and for more-effective designs of physical-vapor-deposition-type reactors will be developed. 

Symmetrical placement of the ion-selective membrane is typical for the conventional ISE. It helped us to define the operating principles of these sensors and most important, to highlight the importance of the interfaces. Although such electrodes are fundamentally sound and proven to be useful in practice, the future belongs to the miniaturized ion sensors. The reason for this is basic there is neither surface area nor size restriction implied in the Nernst or in the Nikolskij-Eisenman equations. Moreover, multivariate analysis (Chapter 10) enhances the information content in chemical sensing. It is predicated by the miniaturization of individual sensors. The miniaturization has led to the development of potentiometric sensors with solid internal contact. They include Coated Wire Electrodes (CWE), hybrid ion sensors, and ion-sensitive field-effect transistors. The internal contact can be a conductor, semiconductor, or even an insulator. The price to be paid for the convenience of these sensors is in the more restrictive design parameters. These must be followed in order to obtain sensors with performance comparable to the conventional symmetrical ion-selective electrodes. 

Semiconductors are materials with electrical conducting properties somewhere between those of insulators and conductors. Semiconductors are prepared from semimetals, most commonly silicon. Semiconductors are used in many electronic devices including computers. What makes these materials so popular is the ability to control the conductivity by the addition of small amounts of impurities called doping agents. 

Unlike metals, the conductivity of semi-conductors and insulators is mainly due to the presence of interstitial electrons and positive holes in the solids due to imperfections. The conductivity of semiconductors and insulators increases with increase in temperature while that of metals decreases. 

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 band theory of solids provides a clear set of criteria for distinguishing between conductors (metals), insulators and semiconductors. As we have seen, a conductor must posses an upper range of allowed levels that are only partially filled with valence electrons. These levels can be within a single band, or they can be the combination of two overlapping bands. A band structure of this type is known as a conduction band. 

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. 

Materials that have plenty of electrons in the conduction band are, logically enough, called conductors, and energy supplied by a battery can cause these electrons to move in what is generally called an electric current. Other solids do not have electrons in a conduction band and do not conduct electricity. They are called insulators. Materials that are intermediate between conductors and insulators are called semiconductors, a fairly innocuous-sounding name for a revolutionary material. 

The invention of transistors, based on semiconductors, has changed the world. What are semiconductors As the name implies, they are materials with electrical properties between those of conductors and insulators. Semiconductors are materials that conduct some of the time. 

If the vacuum tube is the grandfather, then the transistor is the parent of today s electronic components. Transistors (Figure 1.2) work in basically the same manner as vacuum tubes. A small voltage applied to one pole controls a larger voltage on the other poles. The difference between a transistor and a vacuum tube is that a transistor uses a sandwich of silicon instead of tube filled with gas to do the switching. Silicon falls into a family of elements that are neither conductor nor insulator they re called semiconductors. This type of element will be either a conductor or an insulator depending on some condition. In the case of transistors, an electrical current will cause the silicon to be a conductor. 

In Chapter 3, the Hiickel model of linear and closed polyene chains is used to explain the origin of band structure in the one-dimensional crystal, outlining the importance of the nature of the electronic bands in determining the different properties of insulators, conductors, semiconductors and superconductors. 

Another possible way of classifying soUds is according to their electrical properties as conductors, semiconductors or insulators. This aspect of the solid state is discussed briefly in Section 1.5. below. 

Figure 5.7 Density of states diagrams for (i) and (iii) conductors and (ii) semiconductor/insulator...

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