Bill Hamilton, age 11, of St. John's, Newfoundland, Canada, for his question:

HOW ARE TRANSISTORS MADE?

Transistors are tiny devices used in computers, radios, television sets and other electronic equipment. They control the flow of electric current in such equipment.

Transistors do the work once done by vacuum tubes. They are smaller and more dependable and they use far less electricity.

Manufacturers make transistors from silicon, which is a solid material called a semiconductor. Semiconductors conduct electricity, but not so well as do true conductors, such as copper or iron. The atoms of the semiconductor material used in a transistor must be in the form of crystals.

The transistor manufacturers add small amounts of certain impurities to the crystals of semiconductors. The impurities control the way electricity flows in the silicon. .

Some impurities add free or extra electrons to the crystals. Other impurities do not supply the crystal with enough electrons. This lack of electrons causes empty spaces known as holes, in crystals.

A semiconductor material called an n type if it has extra electrons, and p type if it has holes. Electricity flows as a movement of electrons in n type material and as a movement of holes in p type material.

Transistors are made of layers of n type and p type materials. To make a transistor, manufacturers may grow or make pure crystals and then cut them into thin slices. They then heat these slices and expose them to impurities to form n type and p type layers.

Finally the manufacturers attach wires to the layers and finish the transistor by putting it into a tiny case to protect it.

A transistor is so small that manufacturers can put many thousands of them on a piece of material no larger than a postage stamp. A single transistor may be the size of a pea.

Electronic equipment has been revolutionized by transistors and almost all such equipment made today uses them instead of vacuum tubes. Without transistors, manufacturers could not make pocket calculators or high speed computers.

Battery operated radios and television sets would be much larger today and they would also cost much more to operate if we didn't have transistors.

The small size and weight of transistors has also led to the development of communications satellites that link continents through telephones and TV.

The transistor was invented in 1947 by three American physicists: John Bardeen, Walter Brattain and William Shockley. They shared the 1956 Nobel prize in physics for their work.

The first transistor was called a point contact transistor. Then in 1948, Shockley developed the theory of the junction transistor. Within a few years, junction transistors had replaced the point contact type.

In 1952, Shockley published the theory that led to the field effect transistor.