Semiconductor

Semiconductor is a material that conducts electricity better than insulators like glass, but not as well as conductors like copper. Such materials have made possible modern computers and other important electronic devices. The transistors used in tiny pocket radios are semiconductor devices. So are the solar cells that provide electric power in artificial satellites.

Silicon is the most widely used semiconductor material. Other important semiconductor materials include cuprous oxide, germanium, gallium arsenide, gallium phosphide, indium arsenide, lead sulfide, selenium, and silicon carbide.

Electronic devices made of semiconductor materials can perform many functions, including those of vacuum and gas-filled tubes. However, semiconductor devices have a number of advantages over these tubes. Semiconductor devices use much less power than tubes, they last longer, and they can be built much smaller. One example of a tiny semiconductor device is the silicon chip used in computers and calculators.

Like tubes, semiconductor devices can rectify (change alternating current to direct current). They can also amplify weak electric signals. In addition, these devices can oscillate (make alternating current or radio waves) at frequencies from a few hertz to over 100,000 megahertz. Radios, television sets, and other electronic devices depend on rectifiers, amplifiers, and oscillators. Some semiconductor devices can make light, and others can detect light. Most television camera tubes are semiconductor devices.

Basic principles. In ordinary copper wire, the copper atoms have electrons that are free to move from atom to atom. Such a flow of electrons makes up an electric current. In an ideal state, semiconductor materials would be insulators because they would have no free electrons. But if very small amounts of certain impurities such as antimony, arsenic, or phosphorus are present, a few free electrons are produced that can move and form an electric current. These semiconductors are known as n-type semiconductors.

Another type of semiconductor, called p-type, is formed by adding small quantities of other impurities such as aluminum, boron, or gallium. These impurities take electrons away from a few atoms of the semiconductor. This lack of an electron in an atom is called a hole. A hole can pass from one atom to another. A flow of such holes passing from atom to atom also forms an electric current.

The abbreviation n means negative, referring to the negative charge of the electrons in n-type materials. Similarly, p means positive, referring to the positive charge associated with holes in p-type materials.

Semiconductor materials must be exceptionally pure to work properly. Scientists have developed special techniques to obtain pure crystals of semiconductor materials and to add the right amounts of impurities.

Semiconductor devices include semiconductor diodes, semiconductor lamps, semiconductor lasers, semiconductor radiation detectors, solar cells, and transistors. These devices are formed by making certain regions in a semiconductor either p- or n-type.

Semiconductor diodes allow current to flow in only one direction and are used as rectifiers. They have a piece of gallium arsenide, germanium, or silicon with an n-type region and a p-type region. The area where the two regions touch is called a p-n junction. When the p-type region has a positive charge and the n-type region has a negative charge, the p-type attracts electrons from the n-type, and the n-type attracts holes from the p-type. Thus, electric current flows across the p-n junction. If the p-type region is made negative and the n-type region is positive, almost no current will flow across the junction. The p-type then repels electrons in the n-type, and the n-type repels holes in the p-type.

Other semiconductor diodes, such as the Esaki or tunnel, Gunn, IMPATT, and LSA diodes can oscillate. They generate extremely high frequency radio waves that are used for communications, radar, or other purposes.

Semiconductor lamps include tiny gallium phosphide diodes that produce light with little electric power. These lamps are used in some telephone sets.

Semiconductor lasers produce narrow beams of intense light. They are efficient lasers, but their light covers a wider frequency range than light from other lasers.

Semiconductor radiation detectors indicate the presence and intensity of gamma rays and X rays. These devices are widely used in scientific research.

Solar cells change sunlight into electricity. They are made of slabs of silicon with a p-n junction near the surface. Light knocks electrons out of the atoms, producing electrons and holes that flow to make an electric current.

Transistors are used to amplify electric signals, act as oscillators, or make circuits that perform arithmetic and logic operations. Some transistors have more than one p-n junction.

Contributor: Vijai K. Tripathi, Ph.D., Prof. of Electrical and Computer Engineering, Oregon State Univ.

Semiconductor

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