Semiconductor materials:

There are several different mixtures of elements which work as semiconductors.  The two most commonly used materials are silicon and a compound of gallium and arsenic called as gallium arsenide (abbreviated GaAs). In early years of semiconductor technology, germanium formed basis for many semiconductors; today it is seen occasionally. Other substances which work as semiconductors are cadmium compounds, selenium, indium compounds, and several metal oxides.Many elements found in semiconductors are mined from earth. Others are grown as crystals under the laboratory conditions.

Silicon

Silicon (symbol Si) is widely used in transistors, diodes, and integrated circuits. Generally, substances, or impurities, should be added to silicon to give it required properties. The best quality of silicon can be obtained by growing crystals in the laboratory. The silicon is fabricated into wafers or chips.

Gallium arsenide

The other common semiconductor is compound gallium arsenide.  Engineers and technicians call this material by the acronym like chemical symbol, GaAs, pronounced as gas. If you hear about gasfets and gas ICs, you are dealing with gallium arsenide technology.

Gallium arsenide works much better than silicon in several ways. It requires less voltage, and will function at the higher frequencies as the charge carrier's move faster. GaAs devices are relatively immune to the effects of ionizing radiation such as X and gamma rays. GaAs is used in the light emitting diodes, laser diodes, infrared emitting diodes, visible-light detectors and infrared detectors, ultra high frequency amplifying the devices, and in a range of integrated circuits.

The basic disadvantage of GaAs is that it is more expensive to generate than silicon.

Selenium

Selenium has resistance which ranges depending on the intensity of light which falls on it. All the semiconductor materials posses this property, called as photoconductivity, to a greater or lesser degree, but the selenium is affected especially. For this very reason, selenium is used for making the photocells.

Selenium is used in various types of rectifiers. This is a device which converts alternating current to direct current. The major advantage of selenium over silicon is that selenium can withstand the brief transients, or surges abnormally high voltage.

Germanium

The pure germanium is very poor electrical conductor.  It becomes a semiconductor when the impurities are added to it. Germanium was used widely in the early years of semiconductor technology.  Some of the diodes and transistors are still using it.

A germanium diode has quite low voltage drop (0.3 V, when compared with 0.6 V for silicon and 1 V for selenium) when it conducts, and this makes it very useful in some of the situations. But germanium can be destroyed easily by heat. Extreme care must be used when soldering the leads of a germanium component.

Metal  oxides

Certain metal oxides have properties which make them useful in manufacture of semiconductor devices. When you hear about MOS or CMOS technology, you are hearing about metal-oxide semiconductor and the complementary   metal-oxide semiconductor devices, respectively. The basic advantage of MOS and CMOS devices is that they require no power to function. They draw so less current that a battery in a MOS or CMOS device lasts as long as it would on shelf. The other advantage of is high speed. This allows operation at the high frequencies, and makes it possible to perform several calculations every second.

Certain types of transistors, and integrated circuits, make use of this technology.  In the integrated circuits, MOS and CMOS allows for a large number of discrete diodes and transistors on the single chip. Engineers would say that MOS/CMOS posses high component density. The major problem with MOS and CMOS is that the devices can be damaged easily by static electricity. WE should  use the components of this type very carefully.