There are numerous different mixtures of elements that work 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 various metal oxides.

Many of the elements found in semiconductors can be mined from the earth. Others are “grown” as crystals under laboratory conditions.

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.

Gallium arsenide
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.

Gallium arsenide works better than silicon in several ways. It needs less voltage, and will function at higher frequencies because the charge carriers move faster. 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, infrared-emitting diodes, laser diodes, visible-light and infrared detectors, ultra-high-frequency amplifying devices, and a variety of integrated circuits. The primary disadvantage of GaAs is that it is more expensive to produce than silicon.

Selenium has resistance that varies depending on the intensity of light that falls on it. All semiconductor materials exhibit this property, known as photoconductivity, to a greater or lesser degree, but selenium is especially affected. For this reason, selenium is useful for making photocells.

Selenium is also used in certain types of rectifiers. This is a device that converts ac to dc. The main advantage of selenium over silicon is that selenium can withstand brief transients, or surges of abnormally high voltage.

Pure germanium is a poor electrical conductor. It becomes a semiconductor 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.

Metal oxides
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.

One advantage of MOS and CMOS devices is 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, and makes it possible to perform many calculations per second.

Certain types of transistors, and many kinds of integrated circuits, make use of this technology. In integrated circuits, MOS and CMOS allows 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 is that the devices are easily damaged by static electricity. Care must be used when handling components of this type.

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