Semiconductor

Semiconductors are atoms that contain four valence electrons. Because the number of valence electrons in a semiconductor is halfway between one (for a conductor) and eight (for an insulator), a semiconductor atom is neither a good conductor nor a good insulator. Three common semiconductor elements are silicon (Si), germanium (Ge) and carbon (C). These atoms are of the three, sili­con and germanium have traditionally been used to produce solid-state components. (Carbon is used primarily in the production of resistors and potentiometers.) Silicon is used far more commonly than germanium primarily because it is more tolerant of heat. Semiconductors are important because they have found applications in almost all branches of industry and areas of daily life. Moreover, the study of semiconduc­tors provides an opportunity to apply and test much of the theory of solids and to obtain information on transport properties which could not be obtained from the study of metals alone. There are two reasons why observation and interpretation of electronic processes is easier in semiconductors than in metals and other crys­tals. Semiconductor crystals can be grown with purity far in excess more than the gained effect in the case of insulators and metals. Second, the mobile carrier concentrations in a semiconductor are low, so that the carriers can be treated as distinguishable, noninteracting particles like molecules of an ideal gas.

Semiconductors can be classified in several ways. One classification may be ac­cording to the nature of the current carriers. Thus, we have ionic and electronic semiconductors. In ionic semiconductors, the conduction takes place through the movement of ions and is accompanied by mass transport. In an electronic semi­conductor, current is carried by electrons and no mass transport is involved.

Semiconductors may also be classified as elemental and compound semicon­ductors. In an elemental semiconductor, all the constituent atoms are of the same kind, whereas a compound semiconductor is composed of atoms of two or more different elements. Yet another way of classifying semiconductors is according to their structure. Accordingly, a semiconductor may be amorphous, polycrystalline, or single crys­tal. The nature of the crystalline structure has a significant influence on the prop­erties of the semiconductor. Amorphous semiconductors have a short-range order and can be considered to be quasi-periodic. They display poor characteristics in terms of carrier mobility and lifetime. In the majority of the semiconductor devices, single-crystal materials are used because of their superior electrical char­acteristics. Polycrystalline semiconductors show electrical behavior" similar to that of single crystals, but their conductivity is significantly lower because of the presence of grain boundaries that obstruct the flow of mobile carriers.

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