ty levels in order of ppm. In intrinsic semiconductors, the conductivity is dominated by the exponential relation between temperature and band-gap. However, the conductivity of extrinsic semiconductors is governed by competing forces: charge carrier density and charge carrier mobility. At low temperatures, the number of charge carriers initially increases with temperature. At intermediate temperatures, for n-type extrinsic semiconductor, most of the donor electrons have been promoted and the number of charge carriers is nearly independent of temperature in what is known as the exhaustion range. At higher temperatures, the number of valence electrons excited to the conduction band greatly exceeds the total number of electrons from the donor atoms, and extrinsic semiconductor behaves like an intrinsic semiconductor. That is, at low temperatures the conductivity varies with temperature as the charge carrier concentration, at higher temperatures the charge carrier mobility dominates and the conductivity decreases with temperature, and at still higher temperatures the extrinsic semiconductor behaves like an intrinsic semiconductor Electrical conduction in ionic ceramics and in polymers In addition to the conduction of charge via electrons, charge can be conducted via ions. Ions are present in most crystalline ceramic materials such as oxides and halides. This process is called ionic conduction, and may occur either in conjunction with or separately from electronic conduction. Several types of compounds show exceptionally high ionic conductivity. Such phases fall into three broad categories: halide and chalcogenides of silver and copper; oxides with β- alumina structure; and oxides of fluorite structure. Conductivity of halides and chalcogenides is result of larger number of available sites than the number of cations. In oxides with β-alumina structure (AM 11 O 17 ) monovalent cation A (such as Na, K, Rb, Li) is mobile. High dopant levels in the fluorite-type solid solutions leads to large defect concentrations and vacancy ordering which results in rapid oxygen migrations. This is the base for development of high-temperature superconductors. For example: La 2 CuO 4 (T c = 30 K), YBC compounds yttrium doped perovskite structure, YBa 2 Cu 3 O 7 (T c = 92 K). Although ceramics are insulators, by properly engineering the point defects in ceramics, it is possible to convert them into semiconductors. For example: Indium tin oxide (ITO) is used as a transparent conductive coating on plate glass, and also for tough screen displays; Ytrria-stabilized zirconia (YSZ) is used as solid electrolyte in solid oxide fuel cells; Lithium cobalt oxide is used as solid electrolyte in lithium ion batteries. Polymers have a band structure with a large energy gap leading to low conductivity because their valence electrons are involved in covalent boding. Hence polymers are used
9 frequently in applications that require electrical insulation. In some cases however, low conductivity is undesirable. For example; static electricity can accumulate on housing for electronic equipment making the polymer vulnerable to electromagnetic radiation that damages the internal devices. These problems can be solved in two approaches: (1) introducing an additive to the polymer to improve conductivity, and (2) creating polymers with inherent conductivity. Adding ionic compound to the polymer can reduce resistivity because the ions migrate to the polymer surface and attract moisture, which in turn, dissipates static charges. Introducing conductive fillers such as carbon black can also dissipate static charges. Some polymers inherently have good conductivity as result of doping or processing techniques. When acetal polymers are doped with agents such as arsenic pentafluoride, electrons or holes are able to jump freely from one atom to another along the backbone of the chain, increasing the conductivity to near that of metals. However, unlike semiconductors, the dopant atoms or molecules do not substitute for or replace the polymer atoms. Other polymers include polyparaph