Crystal Properties and Growth of Semiconductors
In studying solid state electronic devices we are interested primarily in the electrical behavior of solids. However, we shall see in later chapters that the transport of charge through a metal or a semiconductor depends not only on the properties of the electron but also on the arrangement of atoms in the solid. In the first chapter we shall discuss some of the physical properties of semiconductors compared with other solids, the atomic arrangements of var- ious materials, and some methods of growing semiconductor crystals. Topics such as crystal structure and crystal growth technology are often the subjects of books rather than introductory chapters; thus we shall consider only a few of the more important and fundamental ideas that form the basis for under- standing electronic properties of semiconductors and device fabrication. Semiconductors are a group of materials having electrical conductivities in- termediate between metals and insulators. It is significant that the conduc- tivity of these materials can be varied over orders of magnitude by changes in temperature, optical excitation, and impurity content. This variability of electrical properties makes the semiconductor materials natural choices for electronic device investigations. Semiconductor materials are found in column IV and neighboring columns of the periodic table (Table 1-1).The column IV semiconductors, sil- icon and germanium, are called elemental semiconductors because they are composed of single species of atoms. In addition to the elemental materials, compounds of column III and column V atoms, as well as certain combina- tions from II and VI, and from IV, make up the compound semiconductors. 1.1 SEMICONDUCTOR MATERIALS
As Table 1-1 indicates, there are numerous semiconductor materials. As we shall see, the wide variety of electronic and optical properties of these semicon- ductors provides the device engineer with great flexibility in the design of elec- tronic and optoelectronic functions.The elemental semiconductor Ge was widely used in the early days of semiconductor development for transistors and diodes. Silicon is now used for the majority of rectifiers, transistors, and integrated cir- cuits. However, the compounds are widely used in high-speed devices and devices requiring the emission or absorption of light. The two-element (binary) III-V compounds such as GaN, GaP, and GaAs are common in light-emitting diodes (LEDs). As discussed in Section 1.2.4, three-element (ternary) compounds such as GaAsP and four-element (quaternary) compounds such as InGaAsP can be grown to provide added flexibility in choosing materials properties. Fluorescent materials such as those used in television screens usually are II-VI compound semiconductors such as ZnS. Light detectors are com- monly made with InSb, CdSe, or other compounds such as PbTe and HgCdTe. Si and Ge are also widely used as infrared and nuclear radiation detectors. An important microwave device, the Gunn diode, is usually made of GaAs or InP. Semiconductor lasers are made using GaAs, AlGaAs, and other ternary and quaternary compounds. One of the most important characteristics of a semiconductor, which distinguishes it from metals and insulators, is its energy band gap. This prop- erty, which we will discuss in detail in Chapter 3, determines among other things the wavelengths of light that can be absorbed or emitted by the semi- conductor. For example, the band gap of GaAs is about 1.43 electron volts (eV), which corresponds to light wavelengths in the near infrared. In contrast, Table 1-1.
Common semiconductor materials: (a) the portion of the periodic table where semiconductors occur; (b) elemental and compound semiconductors. (a) II
BcN Al Si P S
Binary lll-V compounds
AlP AlAs AlSb GaN GaP GaAs GaSb InP InAs InSb
As Se Sb
Binary II-VI compounds...
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