Code_ The Hidden Language of Computer Hardware and Software - Charles Petzold [91]
Since 1912, the Bell System had worked with vacuum tube amplification, and a considerable amount of research and engineering went into improving vacuum tubes for use by the telephone system. Despite this work, vacuum tubes still left much to be desired. Tubes were large, consumed a lot of power, and eventually burned out. But they were the only game in town.
All that changed December 16, 1947, when two physicists at Bell Labs named John Bardeen (1908–1991) and Walter Brattain (1902–1987) wired a different type of amplifier. This new amplifier was constructed from a slab of germanium—an element known as a semiconductor—and a strip of gold foil. They demonstrated it to their boss, William Shockley (1910–1989), a week later. It was the first transistor, a device that some people have called the most important invention of the twentieth century.
The transistor didn't come out of the blue. Eight years earlier, on December 29, 1939, Shockley had written in his notebook, "It has today occurred to me that an amplifier using semiconductors rather than vacuum is in principle possible." And after that first transistor was demonstrated, many years followed in perfecting it. It wasn't until 1956 that Shockley, Bardeen, and Brattain were awarded the Nobel Prize in physics "for their researches on semiconductors and their discovery of the transistor effect."
Earlier in this book, I talked about conductors and insulators. Conductors are so called because they're very conducive to the passage of electricity. Copper, silver, and gold are the best conductors, and it's no coincidence that all three are found in the same column of the periodic table of the elements.
As you'll recall, the electrons in an atom are distributed in shells that surround the nucleus of the atom. What characterizes these three conductors is a lone electron in the outermost shell. This electron can be easily dislodged from the rest of the atom and hence is free to move as electrical current. The opposites of conductors are insulators—like rubber and plastic—that barely conduct electricity at all.
The elements germanium and silicon (as well as some compounds) are called semiconductors, not because they conduct half as well as conductors, but because their conductance can be manipulated in various ways. Semiconductors have four electrons in the outermost shell, which is half the maximum number the outer shell can have. In a pure semiconductor, the atoms form very stable bonds with each other and have a crystalline structure similar to the diamond. Such semiconductors aren't good conductors.
But semiconductors can be doped, which means that they're combined with certain impurities. One type of impurity adds extra electrons to those needed for the bond between the atoms. These are called N-type semiconductors (N for negative). Another type of impurity results in a P-type semiconductor.
Semiconductors can be made into amplifiers by sandwiching a P-type semiconductor between two N-type semiconductors. This is known as an NPN transistor, and the three pieces are known as the collector, the base, and the emitter.
Here's a schematic diagram of an NPN transistor:
A small voltage on the base can control a much larger voltage passing from the collector to the emitter. If there's no voltage on the base, it effectively turns off the transistor.
Transistors are usually packaged in little metal cans about a quarter-inch in diameter with three wires poking out:
The transistor inaugurated solid-state electronics, which means that transistors don't require vacuums and are built from solids, specifically semiconductors and most commonly (these days)