Metals, Semiconductors, and Insulators

The fundamental division of materials when electrical properties are considered is into metals, insulators, and semiconductors. An insulator is a material that normally shows no electrical conductivity. Metals and semiconductors were originally classified more or less in terms of the magnitude of the measured electrical conductivity. However, a better definition is to include in metals those materials for which the 

Metals are defined as materials in which the uppermost energy band is only partly filled. The uppermost energy level filled is called the Fermi energy or the Fermi level. Conduction can take place because of the easy availability of empty energy levels just above the Fermi energy. In a crystalline metal the Fermi level possesses a complex shape and is called the Fermi surface. Traditionally, typical metals are those of the alkali metals, Li, Na, K, and the like. However, the criterion is not restricted to elements, but some oxides, and many sulfides, are metallic in their electronic properties. 

Rather surprisingly, an equal contribution to the conductivity will come from positive charge carriers equal in number to the electrons promoted into the conduction band. These are vacancies in the valence band and are called positive holes or more generally holes. Each time an electron is removed from the full valence band to the conduction band, two mobile charge carriers are therefore created, an electron and a hole. 

2Naturally, if an enormous voltage is applied, as in a thunderstorm, the electrons are given so much energy that they are ripped from the valence band and can transfer to the conduction band. In these conditions the insulator is said to break down. 

The position of the Fermi level in an extrinsic semiconductor depends upon the dopant concentrations and the temperature. As a rough guide the Fermi level can be taken as half way between the donor levels and the bottom of the valence band for n-type materials or half way between the top of the valence band and the acceptor levels for p-type semiconductor, both referred to 0 K. As the temperature rises the Fermi level in both cases moves toward the center of the band gap. 

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FIGURE 6.3 Conductivities of doped poly acetylenes conductivities of insulators, semiconductors and metals are given for comparison. 

Design and fabrication of ISFET was described in Ref. [88] The interest in ISFET arises chiefly from their application as pH and ion sensors. A graphical procedure to find PZC from capacitance-voltage characteristics of electrolyte-insulator-semiconductor and metal-insulator-semiconductor structures was discussed [89]. Due to the choice of electrolyte (2 mol dm Na2S04) the PZC values reported in this study (2.5 for Si02, 2.8 for Ta20s and 3-3.4 for Si3N4) are not likely to be the pristine values due to specific adsorption of anions. 

Stein A, Ozin GA (1993) Sodalite supralattices from molecules to clusters to expanded insulators, semiconductors and metals. In von Ballmoos R, Higgins JB, Treacy MMJ (eds) Proceedings from the Ninth International Zeolite Conference. Butterworth-Heinemann, Boston London Oxford Singapore Sydney Toronto Wellington, vol I, p 93... 

Fig. 7.29 Band theory for insulators, semiconductors, and metals. The shaded areas represent electronic states occupied by electrons. Eg is the energy gap between occupied and empty states. Metals have partly filled bands. Typically, Eg is over 4 eV for an insulator and below 2 eV for a semiconductor.

Up to this point, all electronic-structure methods have been concerned with the movement of the electrons only. This approach was justified by the idea of light electrons moving in the field of the fixed nuclei with large masses, and this simplification is a very reasonable one for most solids, especially if the properties to be predicted do not depend on the atomic movement. For example, to theoretically characterize insulators, semiconductors, and metals, atomic dynamics is imimportant likewise, magnetic phenomena (linked to the electronic spin) are largely independent of atomic movement. 

A calculated band structure, with information about the position of the Fermi level, tell us a lot about the electric properties of the material being looked at here (insulator, semiconductor, and metal). They tell us also about basic optical properties e.g., the band gap indicates what kind of absorption spectrum we may expect. We can calculate any measurable quantity because we have at our disposal the computed (approximate though) wave function. 

Based on colloidal monolayers of polystyrene spheres, we have prepared various two-dimensional nano-structured arrays by solution routes and electrodeposition. Many ordered structured arrays generated using these methods are of surface roughness on the nano- and micro-scales, and could be superhydrophobic or superhydrophilic. The nano-devices based on such nano-structured arrays would be waterproof and selfcleaning, in addition to their special device functions. In this article, taking silica, ZnO and gold as examples of the insulators, semiconductors and metals, respectively, we report some of our recent results to demonstrate controlled wettability and superhydrophobicity of two-dimensional ordered nano-stmctured arrays with centimeter square-size based on colloidal monolayers. 

Fermi level and energy gap insulators, semiconductors and metals... 

FERMI LEVEL AND ENERGY GAP INSULATORS, SEMICONDUCTORS AND METALS... 

The chapter is divided into three parts. In the first part, the operation of the transistor is described. Special attention is devoted to the problem of parameter extraction. The second and third parts focus on the specific problems related to the insulator-semiconductor and metal-semiconductor interfaces, respectively. 

As stated in the introduction, oxides may cover a wide range of electronic properties, in particular, from insulating ionic to superconducting materials. Consequently, the electronic structure of oxides covers wide band gap insulators, semiconductors, and metals. Examples are MgO or AI2O3, which show properties of insulators with band gaps of 7.5 and 8.5 eV [123], respectively Ti02 or TiOj with semiconducting... 

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