Transistors

With the relay you saw how a small current can switch a large one: a coil clicks a contact over. The transistor does exactly the same, but without a single moving part, millions of times faster and so small that billions of them fit on a fingernail. Many consider it the most important invention of the 20th century, and after this chapter you will know why.

Three terminals, one idea

A transistor has three terminals: base, collector and emitter. The idea behind it is a controlled valve. The large current wants to flow between collector and emitter, but the path is normally blocked. Only when a small current flows into the base does the valve open and let the large current through. When the base current stops, the transistor blocks again immediately. The small current at the handle controls the large current through the pipe.

A switch with no moving parts

As a switch, the transistor beats the relay hands down. Nothing moves, nothing wears out, nothing clicks. A relay manages at most a few dozen switching operations per second, a transistor manages billions. This is exactly what computers are built on: a transistor that blocks or conducts represents a 0 or a 1. Many such switches form logic gates, logic gates form arithmetic units, and in the end a whole processor.

The transistor as an amplifier

The transistor can also open the valve only partway: the more base current, the more collector current. The large current follows the small one like a magnified echo. The ratio is called current gain, and typical values are around 100: 1 mA of base current then becomes 100 mA of collector current. This is how an amplifier turns the tiny signal of a microphone into a current that drives a loudspeaker. Without transistors there would be no hearing aids, no phones and no concert sound systems.

Billions of them in a chip

In 1947 researchers at Bell Labs built the first transistor, about the size of a thumb. Today a single transistor measures only a few dozen nanometres, roughly a thousand times thinner than a human hair, and billions of them are printed onto one silicon chip at a time. The type used in chips is a different one, the field effect transistor: it also has three terminals, but here the control terminal is called the gate, and at it a voltage does the controlling instead of a current. The valve principle stays exactly the same. The processor in your phone contains more transistors than there are people on Earth. For decades, the number per chip doubled roughly every two years. That is why the transistor rightly tops every list of important inventions: every computer, every phone and the entire internet consist, at their core, of this one component.