ABSTRACT
In Chapter 1, you were introduced to the physical structure of solids-how their atoms and ions are arranged in space. We now turn to a description of the bonding in solids-the electronic structure. Some solids consist of molecules bound together by very weak forces. We shall not be concerned with these because their properties are essentially those of the molecules. In the nal section we shall be concerned with ‘purely ionic’ solids bound by electrostatic forces between ions, as discussed in Chapter 1. The solids considered here are mostly those in which all the atoms can be regarded as bound together. We shall be looking at the basic bonding theories of these solids and how the theories account for the very different electrical conductivities of different groups of solids. We shall cover both theories based on the free-electron model: a view of a solid as an array of ions held together by a sea of electrons; and on the molecular orbital theory: a crystal seen as a giant molecule. Of these solids, some of the most important are the semiconductors. Many solid-state devices-transistors, photocells, light-emitting diodes (LEDs), solid-state lasers and solar cells-are based on semiconductors. We shall introduce examples of some devices and explain how the properties of semiconductors make them suitable for these applications. We start with the free-electron theory of solids and its application to metals and their properties.
