6.1 COMPOUND SEMICONDUCTORS AND ALLOYS

The best-developed semiconductor materials are the elemental semiconductors of column IV of the periodic chart, shown in Fig. 6.1. These include germanium, from which the first transistor was made, and especially silicon, which is the best for most modern commercial microelectronics and electronic integrated circuitry. These materials are perhaps most interesting when formed into pn junction diodes by the addition of small, controlled amounts of impurities. The p-type regions are formed by adding acceptors, which are column III elements (i.e., B, Al, Ga, In) and lack one electron with respect to the column IV host, and n-type regions are formed by adding donors, which are column V elements (i.e., P, As, Sb) and have an extra electron. The lattice structure of the semiconducting forms of the column IV elements is a tetrahedral covalent crystal with a diamond lattice as shown in Fig. 6.2. The lattice constant a _o for these crystalline materials is defined as the size along one edge of the unit cell. All semiconductor materials are characterized by a gap in energy E _g between the conduction and valence bands in the material. These materials are generally transparent to light of lower energy than the energy gap. Shown at the top of Table 6.1 are these parameters for the column IV elemental semiconductors silicon and germanium.

FIGURE 6.1: Portion of the periodic chart showing the constituents of elemental and compound semiconductors and their dopants.