Learning Objectives Explain the physical basis of the Hubbard and Mott models of metal-insulator transitions. Understand why good superconductors derive from bad metals. Know the structures and the periodic trends in band gaps and colors of semiconductors. Obtain the band gap of an intrinsic semiconductor from the temperature dependence of the conductivity. Predict the doping type when impurities or defects are introduced into a semiconductor. Correlate the band picture and Fermi level with n- or p-type doping. Understand the physical principles of operation of diodes, LEDs, solar cells, and FETs. Explain the differences in structures and electronic properties of crystalline and amorphous semiconductors. The band model (like MO theory) is based on a one-electron model. This was an approximation we made at the very beginning of our discussion of MO theory: we used hydrogen-like (one-electron) solutions to the Schrödinger equation to give us the shapes of s, p, d, and f atomic orbitals. In a one-electron atom, these orbitals are degenerate within a given shell, and the energy differences between, e.g., 2s and 2p orbitals arise only when we consider the energy of an electron in the field of other electrons in the atom. Moving from atoms to molecules, we made linear combinations to generate one-electron molecular orbitals (and, in solids, one-electron energy bands). But as in multi-electron atoms, life is not so simple for real molecules and solids that contain many electrons. Electrons repel each other, and so their movement in molecules and in solids is correlated . 7.1.1: Prelude to Electronic Properties of Materials - Superconductors and Semiconductors Correlated electron effects give rise to metal-insulator transitions that are driven by small changes in temperature, pressure, or composition, as well as to superconductivity - the passage of current with zero resistance at low temperatures. In this chapter we will develop some simple models to understand these interesting and important electronic properties of solids. 7.1.2: Metal-Insulator Transitions Under experimentally accessible temperatures and pressures, Si and Ge are always semiconducting (i.e., insulating), and Pb is always metallic. Why is Sn different? The reason has to do with orbital overlap. Theory tells us in fact that any (and all) insulators should become metallic at high enough pressure, or more to the point, at high enough density. For most insulators, however, the pressures required are far beyond those that we can achieve in the laboratory. 7.1.3: Periodic Trends- Metals, Semiconductors, and Insulators 7.1.4: Semiconductors- Band Gaps, Colors, Conductivity and Doping There are a number of places where we find semiconductors in the periodic table. 7.1.5: Semiconductor p-n Junctions 7.1.6: Diodes, LEDs and Solar Cells Diodes are semiconductor devices that allow current to flow in only one direction. Diodes act as rectifiers in electronic circuits, and also as efficient light emitters (in LEDs) and solar cells (in photovoltaics). The basic structure of a diode is a junction between a p-type and an n-type semiconductor, called a p-n junction. Typically, diodes are made from a single semiconductor crystal into which p- and n-dopants are introduced. 7.1.7: Amorphous Semiconductors Amorphous semiconductors are disordered or glassy forms of crystalline semiconductor materials with network structures involving primarily covalent bonding. Crystalline silicon, which has the diamond structure, is an ordered arrangement of fused six-membered silicon rings and the local bonding environment of the silicon atoms is tetrahedral. The silicon atoms in amorphous silicon (a-Si) are also predominantly tetrahedrally coordinated, but there is no long-range order in the structure. 7.1.8: Discussion Questions 7.1.9: Problems 7.1.10: References