Transistor

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The transistor is a device used to switch electrical signals on and off, or to amplify them (make them larger). It was invented at Bell Labs in Murray Hill, New Jersey, by scientists William Shockley, Walter Brattain, and John Bardeen from the University of Minnesota.

Contents

Function

Modern transistors are so small that designers make heavy use of quantum theory. However, this isn't important for everyday use of the devices and a simple explanation of transistors is given below. There are two basic types of transistor: field-effect transistors (FETs) and bipolar junction transistors (BJTs).

Field-effect transistors

A field effect transistor contains a thin strip of semiconductor called the "channel". A pair of metal contacts are put at either end of the channel, allowing it to be connected into a circuit. These contacts are called the "source" and the "drain". The rest of the channel is coated with an insulator to stop any unwanted currents from entering or leaving.

Finally, a third metal contact called the "gate" is placed on top of the insulator. When a battery is connected between the gate and source, the insulator stops current getting through. Instead, the electrons in the channel are all pushed away from the gate. In other words, they can only move through a very thin part of the channel, making it difficult for current to flow between the source and drain. The resistance of the channel therefore depends on the voltage at the gate. If a big enough voltage is placed on the gate, the electrons can be pushed completely out of the channel and no current can flow at all.

The first transistors were made of germanium, but more modern devices are made of silicon. Some modern devices use silicon-germanium alloys or gallium arsenide.

History

Brattain and Bardeen usually worked without much supervision, as Shockley worked mostly from home. Their mission was to create the first amplifier using semiconductors.

Bardeen and Brattain were a perfect partnership, as Bardeen was a brilliant theoretician and Brattain a genius at experiments.

In December 1947, Bardeen had the insight that electrons formed a barrier on the surface, and Brattain worked that insight into a point-contact transistor. It used strips of gold foil on a plastic triangle, held in contact by a slab of germanium.

Shockley improved it to become the junction (sandwich) transistor. He mostly built it in a hotel room in Chicago. Once perfected, this was more durable than the point-contact transistor, and far easier to make. Upon this the semiconductor industry was built.

Bell Labs announced the invention to the public on June 30, 1948. Bell Labs engineer John Pierce gave it the name "transistor", a combination of "trans-resistance" with the names like thermistors.

Bardeen, Brattain, and Shockley made very little money from their invention. Others, like Gordon Moore and Robert Noyce in founding Intel, made a fortune. This was true even though Shockley himself saw the potential and started the company upon which Silicon Valley was based.

Japanese engineers like Masaru Ibuka and Akio Morita, who founded Sony Electronics, were successful in improving the manufacturing of transistors.

Significance

Transistors created a revolution in electronics.

Prior to the transistor, electronics relied on vacuum tubes. Vacuum tubes made possible very sophisticated electronics, including broadcast radio and television as we know them, radar, and the first electronic computers.

But vacuum tubes are large, expensive, power-hungry, and unreliable. Vacuum tubes use filaments like light bulbs. A typical tube consumes as much power as a ten or twenty-watt bulb. When turned on, tubes need many minutes to "warm up" and stabilize. During this time, radios do not hold their tuning and "drift" away from stations.

Like light bulbs, vacuum tubes have a short life. Eventually the filament burns out and the tube needs to be replaced.

In a home radio with six tubes, tube failure was only a nuisance. In an electronic computer—like the 17,468-tube ENIAC—it was disastrous, because with that many tubes some would always be failing.

The telephone company was concerned with tubes as well. Telephone signals cannot travel thousands of miles under the sea without amplification. Built into the cable every few miles were repeaters, electronic amplifiers to keep boosting and restoring signal strength. It is not easy to replace a tube when it fails in a cable on the sea floor.

And the military, which was starting to use electronics, was concerned because tubes are made of glass and are fragile, unable to withstand G-forces and vibration.

The transistor ushered in the "solid-state revolution," and another revolution called "miniaturization." Compared to tubes, transistors were at least a hundred times smaller, used a hundred times less power, and were extremely stable and reliable. At first, the transistor changed things in small ways. The book-sized portable radios that used small tubes were displaced by pocket-sized "transistor radios." Over time, transistors completely reshaped the way in which electronics could be used and the things that could be done with it.

A computer made out of transistors did not require continuous component replacement, and because of the stability of transistors it could be turned on and just start working properly within seconds. Transistorized circuits could withstand military conditions, in missiles for example, making electronic guidance systems much more feasible.

Originally "miniaturization" simply meant replacing light-bulb-sized tubes with jelly-bean-sized transistors. But a second breakthrough occurred in the 1960s, when engineers realized that the same technology used to create a single transistor on a semiconductor wafer could be used to create many interconnected transistors. Over time, engineers learned to put dozens, hundreds, thousand, and eventually millions of transistors onto a single chip, launching the "digital revolution."

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