固体物理第五章.ppt

CHAPTER 5FREE ELECTRON THEORY Free Electron Theory Many solids conduct electricity. There are electrons that are not bound to atoms but are able to move through the whole crystal. Conducting solids fall into two main classes; metals and semiconductors. and increases by the addition of small amounts of impurity. The resistivity normally decreases monotonically with decreasing temperature. and can be reduced by the addition of small amounts of impurity. Semiconductors tend to become insulators at low T. Why mobile electrons appear in some solids and others? Why mobile electrons appear in some solids and not others? According to free electron model (FEM), the valance electrons are responsible for the conduction of electricity, and for this reason these electrons are termed conduction electrons. Na11 ? 1s2 2s2 2p6 3s1 This valance electron, which occupies the third atomic shell, is the electron which is responsible chemical properties of Na. When we bring Na atoms together to form a Na metal, Na has a BCC structure and the distance between nearest neighbours is 3.7 A? The radius of the third shell in Na is 1.9 A? Solid state of Na atoms overlap slightly. From this observation it follows that a valance electron is no longer attached to a particular ion, but belongs to both neighbouring ions at the same time. The removal of the valance electrons leaves a positively charged ion. The charge density associated the positive ion cores is spread uniformly throughout the metal so that the electrons move in a constant electrostatic potential. All the details of the crystal structure is lost when this assunption is made. Fermi-Dirac distribution function at various temperatures, Fermi-Dirac distribution function is a symmetric function; at finite temperatures, the same number of levels below EF is emptied and same number of levels above EF