Compound Semiconductor Materials:
Most of the compound semiconductor materials are formed from the combinations of group III and group V elements.
A portion of the periodic table listing the more common semiconductors is given in Table 6.2.
A list of some semiconductor materials is given in Table 6.3.
Two element compounds are called the binary compounds where as three element compounds are called ternary compounds. The wide variety of electronic and optical properties of semiconductors enable greater flexibility in the design of electronic and optoelectronic functions.
Compound semiconductor are widely employed in high-speed devices and devices requiring the emission or absorption of light. The two element (binary) III-V compounds such as gallium nitride (GaN), gallium phosphide (GaP) and gallium arsenide (GaAs) are commonly used in light-emitting diodes (LEDs). Ternary compounds such as GaAsP and quarternary (four element) compounds like InGaAsP can be grown to give added flexibility.
Television screen fluorescents are usually II-VI compound semiconductor like ZnS. Light detectors are usually compounds like InSb, CdSe, PbTe, HgCdTe etc. Silicon and germanium are also widely employed as infrared and nuclear radiation detectors. Semiconductor lasers are usually made using ternary/quarternary compounds like GaAs, A1GaAs etc. Microwave devices (such as Gunn diodes) are usually made of GaAs or InP.
The electronic and optical properties of semiconductor materials are strongly affected by impurities, which are added in precisely controlled amounts. Such impurities cause variations in the conductivities of semiconductors over wide ranges and even alter the conduction processes from conduction by negative charge carriers to positive charge carriers. An impurity concentration variation of even one part per million can change the property of silicon (Si) from a poor conductor to a good conductor of electric current.
Since even slight variations of impurities produce dramatic changes in electrical properties, the nature and specific arrangement of atoms in each semiconductor takes greater importance.
One of the most important characteristics of a semiconductor, which distinguishes it from conductors and insulators, is its energy band gap. This property determines among other things the wavelengths of light that can be absorbed or emitted by the semiconductor. For example, the energy band gap of gallium arsenide is about 1.43 eV, that corresponds to light wavelengths in the near infrared. In contrast, gallium phosphide (GaP) has an energy band gap of about 2.3 eV, corresponding to wavelengths in the green portion of the spectrum. Because of the wide variety of semiconductor energy band gaps, light-emitting diodes (LEDs) and lasers can be constructed with wavelengths over a broad range of the infrared and visible portions of the spectrum.