Various elements, compounds, and mixtures can function as semiconductors. The two most common materials are silicon and a compound of gallium and arsenic known as gallium arsenide (often abbreviated GaAs). In the early years of semiconductor technology, germanium formed the basis for many semiconductors; today it is seen occasionally, but not often. Other substances that work as semiconductors are selenium, cadmium compounds, indium compounds, and the oxides of certain metals.
Silicon (chemical symbol Si) is widely used in diodes, transistors, and integrated circuits. Generally, other substances, or impurities, must be added to silicon to give it the desired properties. The best quality silicon is obtained by growing crystals in a laboratory. The silicon is then fabricated into wafers or chips.
Another common semiconductor is the compound gallium arsenide. Engineers and technicians call this material by its acronym-like chemical symbol, GaAs, pronounced “gas.” If you hear about “gasfets” and “gas ICs,” you’re hearing about gallium-arsenide technology.
GaAs devices require little voltage, and will function at higher frequencies than silicon devices because the charge carriers move faster through the semiconductor material. GaAs devices are relatively immune to the effects of ionizing radiation such as X rays and gamma rays. GaAs is used in light-emitting diodes (LEDs), infrared-emitting diodes (IREDs), laser diodes, visible-light and infrared (IR) detectors, ultra-high-frequency (UHF) amplifying devices, and a variety of integrated circuits.
Selenium exhibits conductivity that varies depending on the intensity of visible light or IR radiation that strikes it. All semiconductor materials exhibit this property, known as photoconductivity, to some degree; but in selenium the effect is especially pronounced. For this reason, selenium is useful for making photocells. Selenium is also used in certain types of rectifiers. A rectifier is a component or circuit that converts ac to pulsating dc.
A significant advantage of selenium is the fact that it is electrically rugged. Selenium-based components can withstand brief transients, or spikes, of abnormally high voltage, better than components made with most other semiconductor materials.
Pure elemental germanium is a poor electrical conductor. It becomes a semiconductor only when impurities are added. Germanium was used extensively in the early years of semiconductor technology. Some diodes and transistors still use it.
A germanium diode has a low voltage drop (0.3 V, compared with 0.6 V for silicon and 1 V for selenium) when it conducts, and this makes it useful in some situations. But germanium is easily destroyed by heat. Extreme care must be used when soldering the leads of a germanium component.
Certain metal oxides have properties that make them useful in the manufacture of semiconductor devices. When you hear about MOS (pronounced “moss”) or CMOS (pronounced “sea moss”) technology, you are hearing about metal-oxide semiconductor and complementary metal-oxide semiconductor devices, respectively.
An advantage of MOS and CMOS devices is the fact that they need almost no power to function. They draw so little current that a battery in a MOS or CMOS device lasts just about as long as it would on the shelf. Another advantage is high speed. This allows operation at high frequencies in RF equipment, and makes it possible to perform many switching operations per second for use in computers.
Certain types of transistors, and many kinds of ICs, make use of this technology. In integrated circuits, MOS and CMOS allow for a large number of discrete diodes and transistors on a single chip. Engineers would say that MOS/CMOS has high component density.
The biggest problem with MOS and CMOS technology is the fact that the devices are easily damaged by static electricity. Care must be used when handling components of this type. Technicians working with MOS and CMOS components must literally ground themselves by wearing a metal wrist strap connected to a good earth ground. Otherwise, the electrostatic charges that normally build up on their bodies can destroy MOS and CMOS components when equipment is constructed or serviced.