Transistors—Tiny Titans of Electronics
MINIATURE radios, TV sets, hearing aids—these owe much of their existence to those tiny titans of electronics called transistors. What is behind these electronic marvels? The ideas have much to do with a branch of physics called quantum mechanics, which deals with very small objects such as atoms and electrons.
Just what do transistors do? What are their advantages? How are they made?
Basically a transistor does the same jobs that a vacuum tube does. Many of its applications center around its role as an amplifier. That is, the transistor strengthens the signals picked up, for example, by the antennas of radios and TV sets.
This amplifying device can be thought of as taking a small amount of an electrical signal into one side of the transistor, copying it and putting out large amounts of the electrical pattern on the other side. The transistor used as an amplifier takes in an electrical image in the form of current and emits perhaps twenty times the input current having the same electrical pattern.
One might wonder, If transistors do basically the same thing as a tube, why bother with them? Because the transistor has advantages over its ancestor, the vacuum tube.
The first advantage is the transistor’s tiny size. It is about one one-hundredth the size of a vacuum tube of similar performance; in other words, a tube may be as large as a man’s thumb, but a transistor is about the size of a pea. Because of transistors, all kinds of electronic devices can be miniaturized.
Another advantage of these tiny titans of electronics is that they can operate on much less power than tubes. This is because transistors have no filament or heater. In order for a tube to work, it has to have a heater called the filament (like a burner on an electric range only much smaller) to “boil” off electrons from the cathode or the electron-emitting region in the tube. The transistor does not need such a heater. And since the transistor produces almost no heat, it does not become hot. Anytime a tube becomes hot, it is using up energy.
Other advantages are: the transistor, not needing a warm-up period as tubes do, starts to work instantly. The transistor also is more durable, since it has no fine wires suspended in it as the tube does. As a result the transistor has a higher reliability. Some persons have estimated that a transistor operating all day and all night, every day of the year, would last eight to ten years. Actually there is little reason for these tiny titans of electronics to wear out; however, bumps, temperature changes and moisture do have an adverse effect on them.
Because of their many advantages, one of the things that transistors have made possible is the communications satellite. On July 3, 1962, the Telstar communications satellite was used in transmitting live television from the United States to Europe. The Telstar received signals from a ground station in the United States, amplified these signals and then retransmitted them so they could be detected at another distant ground station. Since transistors operate on very little power, batteries energized by the sun’s light could be used for power. The Telstar satellite uses one vacuum tube, 1,064 transistors and other solid-state devices. Communications satellites that have been launched since Telstar have all used transistors. But of what are transistors made?
Made of Semiconductor Materials
Materials that conduct electricity very easily are called conductors. Silver, aluminum and copper, for example, are conductors. Now, why is it that a certain material is a good conductor? It is because of the large number of free electrons in the material. Just what is meant by “free” electrons? Well, the electrons are free in these materials in that they can wander easily from one atom, which comprises the conductor, to another.
In contrast with materials that are good conductors of electricity, some materials are called insulators. These materials have no free electrons. As a result, electricity will not flow easily through them. Understandably such materials are used on home appliances to prevent shock. Thus we have rubber-covered electrical plugs and plastic light switches.
There is still a third class of materials—a type of solids known as semiconductors. Materials of this class do not conduct electricity very well, and they are not good insulators either. Hence such materials are called semiconductors. Germanium (discovered by a German chemist and named after Germany) and silicon are the most widely known semiconductor materials.
Now, why is it that the third class of materials are not good at serving as either conductors or insulators? The reason they are only halfway good conductors is that they lack free electrons. And they are not good insulators either because it does not take much energy to produce free electrons. In fact, the number of free electrons increases about a million times when the temperature is raised from 0° F. to about 350° F.
Transistors start with pure crystalline semiconductor material, and because this material is in the solid state of matter, contrasted with the liquid and gaseous states, transistors are spoken of as “solid-state” devices.
Impurities Need to Be Added
Strangely enough, semiconductor material cannot be put to work very hard in its pure state; but when the right amount of impurities are added, it can be made to work very hard indeed.
But why do impurities need to be added? Because a slight trace of certain impurities produces a few free electrons or a lack of electrons. Thus some impurities do not produce free electrons but rather take electrons away from a few atoms of the semiconductor. The result? Lack of an electron in an atom. This is called a hole. Now, the advantage of a “hole” is that it can move from one atom to another. And a flow of these “holes,” moving from atom to atom, forms an electric current. The “hole” becomes a carrier of positive electricity, which is the opposite of the negatively charged electron.
Semiconductor material that has free electrons is called n-type (because of the negative charge). When the material has “holes” or electron deficits it is called p-type (because of the positive charge).
To illustrate: If arsenic is dissolved in very pure molten silicon or germanium, then there is an abundance of electrons that can be considered as almost free electrons. The result is n-type material because the arsenic atom has five outer electrons per atom whereas germanium has only four, so there is an abundance of electrons. These electrons are very easily excited to become free electrons.
Now, what if boron or aluminum is added to the semiconductor material? Well, these two elements have only three outer electrons. So there is a shortage of electrons compared to the germanium; thus a “hole” exists. The result is p-type material.
Made of Layers of Material
The transistor, then, consists of a layer of p-type material sandwiched between two n-types. This is called a n-p-n transistor. Or a transistor may consist of a layer of n-type material between two p-types. This is called a p-n-p transistor.
The junctions where these materials meet are where the amplifying action takes place. They can be thought of as valves that pass current freely or not, depending on which way the electrical potential or voltage is placed across these two junctions.
Even though the transistor is small in size and uses little power compared with the tube, new developments have made smaller electronic packages than is possible even with the transistors. These are called integrated circuits or simply ICs.
In this new development transistors as well as other circuit elements are put together in a series of layers. These little packages are whole circuits rather than just one component (say a transistor) of a circuit. Integrated circuits permit microminiaturization.
Says the World Book Science Annual Science Year (1968): “Today’s ICs are a tenth of an inch square and a few thousandths of an inch thick. Like transistors, they waste almost no electric power as heat, and thus need relatively little cooling. . . . A television set made entirely with ICs, except for the picture tube and the loudspeaker, would fit into a small matchbox.”
To illustrate the difference between whole circuits and individual components of a circuit, let us think of a box as large as a half-gallon milk container. Now a circuit containing perhaps one hundred conventional parts could be put into that box. But with integrated circuits, how many parts could be put into that same space? About one billion (one thousand million).
So the new developments are truly amazing. Man’s progress in the art of miniaturization owes much indeed to transistors, those tiny titans of electronics. Yet the art of microminiaturization itself is not new. Man’s Creator microminiaturized the human brain. He designed it so that about one hundred billion (one hundred thousand million) parts can be utilized in that space.