rmits compounds to be developed. The ability of an electron to flow as current depends on the molecu- lar structure of a material, including its valence shell. Specifically, an electron must break its bond to an atom to obtain the ability to flow as current. When it breaks its bond, the electron is said to be in the conduc- tive band. CONDUCTORS The degree of movement of an atom from a valence band to the conductive band determines whether the material is a good or poor conductor. For example, copper, gold, and silver have only one valence electron. This means that those elements require a minimal amount of energy for an electron to break its band. Thus, this explains why those materials are relatively good conductors. THE p-n JUNCTION In the world of electronics the semiconductor positive-negative (p-n) junction represents the basic material used to devel- op LEDs and lasers. Because p-n junction operation is easily described in terms of the use of silicon material, let's continue our discussion of atoms by focusing on the silicon atom. A silicon atom has four valence electrons. Those electrons form the bonds that hold silicon atoms together, which in turn forms the crys- talline structure of silica. The silicon bonds are also referred to as cova- Light Sources and Detectors Nucleus Maximum number Figure 4.3 Shell of electrons The electronic struc- ture of an atom. valence lent bonds because the atoms share their electrons. Figure 4.4 illustrates the covalent bonds in a series of silicon atoms. Note that each silicon atom has access to eight valence electrons: four of its own and four from surrounding atoms. All the electrons are covalent bonds, and initially none are available as free electrons. THE FREE ELECTRON AND n-TYPE MATERIAL To see how a p-n semicon- ductor junction operates, assume that an atom with five valence elec- trons is added to a silicon crystal. Since only four electrons are required for the covalent bond, this results in a free electron. Assume that this electron represents a free electron in