Semiconductors are materials whose electrical conductivity is lower than that of conductors but higher than that of non-conductors. Silicon (Si), which is the most widely used semiconductor material today, is abundantly available around the globe and ecologically friendly. The other semiconductor materials that can be used for technical applications are germanium (Ge), selenium (Se), gallium arsenide (GaAs), gallium phosphide (GaP), indium phosphide (InP), cadmium sulphide (CdS), cadmium telluride (CdTe) and copper indium diselenide (CuInSe2 or CIS, sometimes with a small amount of gallium added to form copper indium gallium diselenide, Cu(In, Ga)Se2, or CIGS). One of the key parameters for the characterization of semiconductor properties is band gap energy EG. Table 3.1 shows the band gap energy and absorption mechanisms for some of the most important semiconductors.
Figure 3.3 shows the spatial structure of a silicon semiconductor crystal. Silicon has four valence electrons in its outermost shell. In order to establish a stable electron configuration (rare-gas configuration with eight electrons), each silicon atom along with four adjacent atoms form a covalent bond, where each atom controls an electron at each bond; thus a bond consists of two electrons. In a silicon crystal, eight electrons are arrayed around each silicon atom, which means that the desired electron configuration has been attained.
In view of the relative complexity of the spatial structure shown in Figure 3.3, the schematic diagram of a crystal structure as in Figure 3.4 is often used.