Eli Hampton, age 14, of Daytona Beach, Fla., for his question:

WHAT IS A SEMICONDUCTOR?

A 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 important electronic devices.

Germanium and silicon are the two most widely used semiconductor materials. Others include cuprous oxide, gallium arenide, gallium phosphide, lead sulfide, selenium and silicon carbide.

The transistors used in small pocket radios are semiconductor devices. So are the solar batteries that provide electric power in artificial satellites.

Electronic devices made of semiconductor materials can perform a great 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. Some semiconductor devices are smaller than pinheads.

Like tubes, seminconductor devices can rectify or change alternating current to direct current. They can also amplify weak electric signals.

In addition, these devices can oscillate, or 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 while others can detect light. Many television camera tubes are semiconductor devices.

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. Semiconductor diodes allow current to flow in only one direction, and are used as rectifiers. They have a piece of gallium phosphide, germanium or silicon.

Semiconductor lamps are 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 the light from other lasers.

Solar batteries change sunlight into electricity. They are made of slabs of silicon. Light knocks electrons out of the atoms, producing electrons and holes that flow to make an electric current. Many types of transistors are used to amplify electric signals or to oscillate.