Chuck Catnerer, age 13, of Eugene, Oregon, for his question:

How do transistors work?

The basic principle is simple enough, but things get pretty complicated when it comes to making it work. This is understandable because the basic idea has been adapted to create transistors that perform a multitude of very different jobs. What’s more, this rather recent breakthrough in electronics required new substances called semiconductors and new ways to use them. The terms and letters used for various transistor systems are intended to explain the basic principle on which they work.

In early radio sets, the flow of electrons was controlled by a crystal. It was durable and worked fairly well, though nobody knew how. Later, it was replaced by the vacuum tube. It was fragile and used a lot of electricity, but it was a dependable worker.. Then, in the 1940s, the transistor was invented to control the flow of electrons. It was small, durable and used much less energy than the old vacuum tube. Strange to say, it is an updated version of the old crystal, using skillful new methods to make it work to the limit.

This realm of electronics is governed by charged atomic particles. Its traffic laws dictate that two positive charges repel each other and so do two negative charges. But a pair of opposites attract each other and move through certain substances. Copper is a good conductor because its many loosely attached electrons can be pushed through a wire circuit. Glass is an insulator because its electrons refuse to bridge. A transistor uses tricky semiconductors to create positive and negative charges, which start moving.

Semiconductors are insulators with traces of other materials added to free a few of their negative electrons. When an electron comes loose, it leaves behind a particle with a surplus positive charge. This is called a hole. N type semiconductors have a surplus of negative electrons, p types have a surplus of positive holes. Both holes and electrons can move around, governed by the traffic law that makes opposite charges attract each other.

All we have to do is to set the pieces in place and provide energy to start the traffic moving. Thin layers of n type and p type materials are sandwiched together in tiny wafers. Holes and electrons move across the junctions between the layers, and this traffic  the transistor. The wafer may be NPN, PNP or some other type of sandwich. The transistor may be made to amplify or oscillate a current or even switch itself on and off. All the various kinds depend on the motion of opposite charges across junctions between layers or semiconductor materials.

These materials must be made very precisely. The basic ingredients used to make semiconductors for most transistors are silicon and germanium, plus traces of one or several substances to free the electrons. Other ingredients used may be cuprous oxide, gallium arsenide, lead sulphide and silicon carbide    to name a few possibilities. The recipes must be accurate and the wafers sealed precisely in place to make the transistor do its best.