 to the conduction band, leaving behind a positively charged hole in the donor band. Conductivity is due to motion of negatively charged electrons in the conduction band, and so the material is called n-type semiconductor. Fermi level (E f ) of n-type extrinsic semiconductor is higher than that of intrinsic semiconductor i.e. it shifted upward in the band gap, to within the vicinity of the donor state; and its exact position is a function of both temperature and donor concentration.
7 Figure 14-3: Band structure and n-type semiconductivity. Other possibility of susbstituion involves impurity with lower valence. For example trivalent elements like B, Al, Ga, In, Th doped to Si or Ge. Since the donor atoms are deficient in bonding electrons, one of their bonding orbitals will contain a hole that is capable of accepting an electron from elsewhere in the crystal. Binding energy is small, and promotion of an electron from the valence band to the acceptor band, E a, leaves a positively charged hole in the valence band that can act as a charge carrier. Materials with this p-type extrinsic semiconduction are called p-type semiconductors. Fermi level (E f ) for p-type semiconductors is positioned within the band gap and near to the acceptor level. Figure 14-4: Band structure and p-type semiconductivity. For an n-type semiconductor, electrons in the conduction band are charge carriers and holes participate in bonding, whereas for a p-type semiconductor, the electrons participate in bonding, and the holes are the charge carriers. Conductivity equations for these extrinsic semiconductors are written as: n-type semiconductor: p-type semiconductor: σ n. q. μ.(13) e σ n. q. μ..(14) h
8 In both types of extrinsic semiconductors, the doping elements are chosen so that both acceptor and donor levels are located closer to the corresponding energy bands and only a small energy gap in involved when exciting electrons. Intrinsic semiconductors are pure to the order greater than 10 7, while extrinsic semiconductors are made with impurity 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 vac