nner, with increase of either defects or impurities, resistivity increases. Thus, in general, for pure metals, the resistivity approaches zero at absolute zero temperature. Some metals, however, such as Pb, lose all resistivity abruptly and completely at some low temperatures. This phenomenon is called superconductivity, and the materials that exhibit it are called superconductors. The temperature at which the resistivity vanishes is called the critical transition temperature, T c. Many elemental metals, solid-solution alloys, some ceramics and intermetallic compounds exhibit superconductivity. For example: Ti, V, Zn, W, Al, Hg, NbTi, Nb 3 Sn, MgB 2, La-Sr-Cu oxide, YBa 2 Cu 3 O 7-x, carbon nanotubes, etc. BCS (Bardeen, Cooper and Schrieffer) theory explains that the super conductivity is caused by electron-lattice interaction and that the superconducting electrons consist of paired ordinary electrons called a Cooper pair Semiconductivity Conductivity of semiconductors is not as high as metals / conductors. However, their electrical properties are unique, in the sense that their electrical properties are extremely sensitive to even minute concentrations of impurities. For intrinsic semiconductors, their electrical behavior is based on inherent electronic structure of the pure material. On the other hand, if the electrical properties are dominated by impurities, they are called extrinsic semiconductors. Intrinsic semiconduction: Energy band gap between the valence and conduction bands can be used to classify materials as conductors, insulators, and semiconductors. The magnitude of the band gap is characteristic of the lattice alone, and varies widely for different crystals. In semiconductors, the valence and conduction bands do not overlap as in metals, but they possess enough electrons in the valence band those can be promoted to the conduction band at a certain temperature. Promotion of electrons leaves behind positively charged holes in the valence band which maintain the charge neutrality. These holes can be regarded as moving positive charges through the crystal with a charge magnitude and ass as that of an electron. Conduction that arises from thermally / optically excited electrons is called intrinsic semiconduction. It usually takes place at elevated temperatures since sufficient thermal agitation is necessary to transfer a reasonable number of electrons from the valence band to the conduction band. At still higher temperatures, the concentration of thermally excited electrons in the conduction band becomes so high that the semiconductor behaves more like a metal. The elements capable of intrinsic conduction are limited, and include: Si, Ge, Sn, Pb, etc.
6 Figure 14-2: Band structure and intrinsic conductivity. In intrinsic semiconductors, the number of holes equals the number of mobile electrons. The resulting conductivity can be expressed as σ = n q. μ + n. q. μ = n. q.( μ + μ )..(12) e. e e h h h e e e h as n e = n h, and both electron and hole possess equal magnitude of charge. The mobility of holes is always less than those of electrons i.e. μ h < μ e. In Si and Ge, the ratio μ e / μ h is approximately three and two respectively. Extrinsic semiconduction: The charge carrier density can also be increased by adding impurities of either higher or lower valence. This addition of impurities is known as doping, and impure atoms in the element are called donor atoms. When a pentavalent substitutional atoms like P, As, or Sb are placed into a covalently bonded tetravalent materials such as Si or Ge, only four of their five valence electrons are required to participate in covalent bonding, while the fifth electron remains weakly bound to the donor atom. However, this binding energy is in order of 0.01 ev, much less than that of a covalently bonded electron. The energy state of this electron is indicated as E d. Energy gap between the donor and conducting bands is much smaller than that between the valence band and conduction band, E g. Thermal agitation, even at room temperature, is good enough to excite this electron