ABSTRACT
Semiconductors are defined as materials with resistivity (ρ) between ~10−4 and ~103 Ω · cm. An approximate range of sensitivities exhibited by silicon (Si)-based semiconductors is shown in Figure 3.1. Elements showing semiconductivity are called elemental semiconductors (e.g., silicon), and the compounds that show semiconducting behavior are known as compound semiconductors (e.g., gallium arsenide [GaAs]). A band diagram of a typical semiconductor is shown in Figure 3.2. For most semiconductors, the band gap energy (Eg) is between ~0.1 and 4.0 eV. If the band gap is larger than 4.0 eV, we usually consider the material to be an insulator or a dielectric. In this chapter, we will learn that the composition of such dielectric materials can be altered so that they exhibit semiconductivity (see Section 3.21). As shown in Figure 3.2, the top of the valence band is known as the valence band edge (Ev), and the bottom of the conduction band is known as the conduction band edge (Ec). Recall from Chapter 2 that the band diagram shows the outermost part of the overall electron energy levels of a solid. The vertical axis shows the increasing electron energy. Thus, the magnitude of the band gap energy (Eg) is given by
Eg = Ec − Ev (3.1)
Let us consider the origin of the semiconducting behavior in semiconductors such as silicon. Silicon has covalent bonds; each silicon atom bonds with four other silicon atoms, which leads to the formation of a three-dimensional network of tetrahedra arranged in a diamond cubic crystal structure (Figure 3.3).
