Terry Gambarotto, age 11, of Welland, Ontario, Canada, for his question:

HOW ARE TRANSISTORS MADE?

In 1947, three American physicists invented the transistor. John Bardeen, Walter Brattain and William Shockley then shared the 1956 Nobel prize in physics for their outstanding work. At first the transistor was called a point contact transistor but within a few years it was replaced by the junction type.

Transistors do the work once done by vacuum tubes. They control the flow of electricity in computers, radios, television sets and other electronic equipment.

A transistor is extremely small. As many as a thousand or more can be placed on a piece of material that isn't as large as a postage stamp.

Without transistors we wouldn't have been able to have pocket calculators or high speed computers. Transistors also played an important part in the development of communications satellites that link the world through telephones and television.

Transistors are made from silicon, a solid material called a semiconductor. The atoms of the semiconductor material used in a transistor must be in the form of crystals.

Impurities added to the crystals by the manufacturer control the way electricity flows in the silicon. Some impurities add free, or extra, electrons to the crystals while others do not supply the crystal with enough electrons. This lack of electrons causes empty spaces, known as holes, in the crystal.

A semiconductor material is called n type if it has extra electrons and p type if it has holes.

Electricity flows as a movement of electrons in the 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, a manufacturer can grow, or make, pure crystals and cut them into  thin slices. They then heat these slices and expose them to impurities to form n type and p type layers. They next attach wires to the layers and finally put the finished transistor into a tiny case to protect it.

The usefulness of any transistor comes from its ability to control a strong current with a weak voltage.

There are two main types of transistors: function transistors and field effect transistors.

During the 1960s, manufacturers developed the technique of making transistors on a single piece of semiconductor material. This technique led to complete electronic circuits, called integrated circuits, on one piece of semiconductor material. In the 1970s, larger integrated circuits were developed and the cost of transistors was greatly reduced.