THIS CAPSULE CHALLENGES VACUUM TUBE
(Simple, tiny amplifier for telephones, radio, and television utilizes
"holes" in atoms of rare element.)
For the first time since its invention 41 years ago, the vacuum tube that runs your radio, operates welding machines, counts beans, opens doors, and supports the $3,500,000,000 electronics industry, has a rival. It is a tiny, seemingly simple gadget called a transistor.
A transistor can do nearly everything a vacuum tube can do and then some. It amplifies, oscillates, and detects electric currents. It can amplify telephone conversations and television signals. It makes possible the construction of a complete, high-fidelity radio without any vacuum tubes.
The transistor has no vacuum, grid, plate, or cathode. Since there is no cathode to shoot out electrons when hot, there is nothing to heat up. The transistor starts to work the instant the current flows. Partly for this reason, the transistor uses less power than a vacuum tube. It requires only about a tenth as much as a flashlight bulb.
The device that may start a revolution in electronics is smaller
than a paper match. Inside its little, cylindrical metal case, three wires
are connected to a crystal of germanium, a rare element with curious electrical
properties.
Prompted by a signal from one of the three wires, the germanium
acts as a valve controlling the flow of current between the other two wires.
This valve causes the voltage and amperage in the output circuit to vary as they do in the input circuit. Thus by transferring the signal from one circuit to another, its strength may be increased as much as 100 times.
The first use of the transistor, say the Bell Laboratories scientists who invented it, probably will be in their own business of telephone transmission. Phone calls fade out after traveling on wires a few miles and must be built up again by vacuum-tube amplifiers called repeaters. Transistors can do this building-up without consuming as much power as the vacuum tubes. This will permit the installation of more repeaters, improving service.
Television, too, will benefit. Transistors can amplify over a wide range of frequencies, including the very high frequencies used for video. So, by using transistors as repeaters, television programs may be transmitted over ordinary telephone wires. this would provide a simple means of linking many cities into a television network, just as the telephone lines now furnish radio networks. At present, television can be sent from city to city only over coaxial cable or microwave radio relay systems.
Transistors also may help make many small, portable devices such as "wrist-watch" radios and the tiny amplifiers used by the hard-of-hearing still smaller and lighter. Since transistors require less power than even the smallest vacuum tubes, they can be used with smaller batteries. And in hearing aids and tiny radios it is the batteries that take up most of the weight and space.
Whether transistors will ever replace the vacuum tubes in your home radio will probably depend most on how much it costs to make them. Bell scientists won't even guess what you will have to pay for a transistor, but the simplicity of the device indicates that mass-production economies will be possible, and plans already are being made for a pilot plant to manufacture the new devices.
It is an interesting sidelight to history that the telephone company, which played such a big part in the invention and development of the vacuum tube should also conceive the device that may replace it. Dr. Lee de Forest, the electronics genius who invented the amplifying vacuum tube, was a member of the telephone company staff until a few years before he patented his "audion." And he sold a license for telephone use of that patent to his former employers.
Now, nearly half a century later, a new generation of telephone physicists has found a radically different method for amplifying electric currents. This new method, utilized in the transistor, depends on the strange way in which certain materials - the elements germanium and silicon, oxides of copper and other metals, and a few additional compounds - react to electricity.
Germanium, for example, is a hard, brittle, silver-grey metal that is in between a good conductor and a good insulator - physicists class it as a "semi-conductor." It had little commercial value until someone discovered that it conducted current much better in one direction than in the other. If you apply alternating current (which changes its direction many times a second) at one end of a germanium crystal, direct current (which always goes in the same direction) comes out of the other end. In other words, it is a natural rectifier. Germanium crystals are widely used for this purpose now in radios and other devices.
Such interesting properties have made germanium and its sister materials the subject of much research in laboratories all over the world. At the Bell Telephone Laboratories, Dr. William Shockley, a top-ranking physicist, suspected that germanium's ability to carry electricity might be changed by applying an electric field to it. A varying field would induce a varying current, making the germanium an amplifying device. But this apparently good idea did not work at first.
The task of finding out why was assigned to Drs. John Bardeen and Walter H. Brattain. They developed a new theory to describe the action of germanium. When translated into practice in the workshop, this theory became the transistor, which works very well indeed.
Drs. Bardeen and Brattain explain how the transistor operates by using the concept of electrical "holes." Ordinarily, electric current is considered to be a stream of electrons, very tiny atomic particles each carrying one piece of electrical charge. But if an electron is moved away from the atom to which it belongs, the atom is left with an empty space - a hole. These holes can move around from atom to atom just as bubble moves in a glass of water. And a stream of holes can be produced in a block of germanium just as a stream of bubbles can be produced in water. Since a hole lacks the negative charge of an electron, it is positive and will move from a positive electrode to a negative electrode. A hole can be considered to be a piece of positive electricity, just as an electron is a piece of negative electricity.
In a transistor, the weak signal pulls electrons out of the germanium atoms near the point of the emitter wire. This loss of electrons creates holes that can move around near the surface of the germanium, but only as far as .001 or .002 inches from the emitter point.
The point of the collector wire is placed at this distance. It is maintained at a high negative potential - the battery is trying to force electrons out of the wire and into the germanium crystal. But the strange properties of germanium prevent this. The electrons bunch up around the collector point, becoming so crowded that they prevent the flow of additional electrons from the collector and there is no output. When holes reach the collector point, however, they neutralize part of this bunch of electrons, clearing a path for more electrons to flow out of the collector. Some of the electrons from the collector fill up the holes and are neutralized, but the electrons are so numerous and moving so fast that many more rush right by the holes and reach the base electrode. The electrons that get past the holes make up the output current.
Since the number of electrons able to flow out of the collector depends on the number of holes reaching the collector - which in turn depends on the signal - the output is patterned exactly after the signal. And since more electrons flow out of the collector than holes to the collector, the output is larger than the signal. Thus, the transistor amplifies the signal.
END