Code_ The Hidden Language of Computer Hardware and Software - Charles Petzold [92]
The first commercial application of the transistor was, however, a hearing aid. In commemorating the heritage of Alexander Graham Bell in his lifelong work with deaf people, AT&T allowed hearing aid manufacturers to use transistor technology without paying any royalties. The first transistor television debuted in 1960, and today tube appliances have almost disappeared. (Not entirely, however. Some audiophiles and electric guitarists continue to prefer the sound of tube amplifiers to their transistor counterparts.)
In 1956, Shockley left Bell Labs to form Shockley Semiconductor Laboratories. He moved to Palo Alto, California, where he had grown up. His was the first such company to locate in that area. In time, other semiconductor and computer companies set up business there, and the area south of San Francisco is now informally known as Silicon Valley.
Vacuum tubes were originally developed for amplification, but they could also be used for switches in logic gates. The same goes for the transistor. On the next page, you'll see a transistor-based AND gate structured much like the relay version. Only when both the A input is 1 and the B input is 1 will both transistors conduct current and hence make the output 1. The resistor prevents a short circuit when this happens.
Wiring two transistors as you see below in the diagram on the right creates an OR gate. In the AND gate, the emitter of the top transistor is connected to the collector of the bottom transistor. In the OR gate, the collectors of both transistors are connected to the voltage supply. The emitters are connected together.
So everything we learned about constructing logic gates and other components from relays is valid for transistors. Relays, tubes, and transistors were all initially developed primarily for purposes of amplification but can be connected in similar ways to make logic gates out of which computers can be built. The first transistor computers were built in 1956, and within a few years tubes had been abandoned for the design of new computers.
Here's a question: Transistors certainly make computers more reliable, smaller, and less power hungry. But do transistors make computers any simpler to construct?
Not really. The transistor lets you fit more logic gates in a smaller space, of course, but you still have to worry about all the interconnections of these components. It's just as difficult wiring transistors to make logic gates as it is wiring relays and vacuum tubes. In some ways, it's even more difficult because the transistors are smaller and less easy to hold. If you wanted to build the Chapter 17 computer and the 64-KB RAM array out of transistors, a good part of the design work would be devoted to inventing some kind of structure in which to hold all the components. Most of your physical labor would be the tedious wiring of millions of interconnections among millions of transistors.
As we've discovered, however, there are certain combinations of transistors that show up repeatedly. Pairs of transistors are almost always wired as gates. Gates are often wired into flip-flops or adders or selectors or decoders. Flip-flops are combined into multibit latches or RAM arrays. Assembling a computer would be much easier if the transistors were prewired in common configurations.
This idea seems to have been proposed first by British physicist Geoffrey Dummer (born 1909) in a speech in May 1952. "I would like to take a peep into the future," he said.
With the advent of the transistor and the work in semiconductors