Semiconductors fall into a gray area between good conductors and poor conductors (i.e., insulators). The conductivity of these materials can be varied by doping with impurities, changes in temperature and by light (as in the case of phototransistors). Prior to the development of microwave transistors, most devices were made of Group IV-A materials, such as silicon (Si) and germanium (Ge). The term Group IV-A refers to the group occupied by these materials on the Periodic Table of chemical elements. Some modern microwave transistors are made of Group III-A semiconductors, such as gallium and indium (see Table 22-1).

Figure 22-19 shows an energy level diagram for semiconductor materials. Two permissible bands represent states that are allowed to exist: the conduction band and the valence band. The region between these permitted bands is a forbidden band. This band represents energy states that are not allowed to exist. The width of the forbidden band is the difference between the conduction band energy ( E _c) and valence band energy ( E _v). Called the bandgap energy ( E _BG), this parameter is unique for each type of material. For silicon at 25 C the bandgap energy is 1.12 electron volts (eV ); for germanium, it is 0.803 eV. A Group III-A material called gallium arsenide (GaAs) is used in...