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Point Contact Transistor

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The first transistor was about half an inch high. That’s mammoth by today’s
standards, when 7 million transistors can fit on a single computer chip. It was
nevertheless an amazing piece of technology. It was built by Walter Brattain.
Before Brattain started, John Bardeen told him that they would need two metal
contacts within .002 inches of each other — about the thickness of a sheet of
paper. But the finest wires then were almost three times that width and couldn’t
provide the kind of precision they needed.

Instead of bothering with tiny wires, Brattain attached a single strip of gold
foil over the point of a plastic triangle. With a razor blade, he sliced through
the gold right at the tip of the triangle. Voila: two gold contacts just a
hair-width apart.

The whole triangle was then held over a crystal of germanium on a spring, so
that the contacts lightly touched the surface. The germanium itself sat on a
metal plate attached to a voltage source. This contraption was the very first
semiconductor amplifier, because when a bit of current came through one of the
gold contacts, another even stronger current came out the other contact.

Here’s why it worked: Germanium is a semiconductor and, if properly treated, can
either let lots of current through or let none through. This germanium had an
excess of electrons, but when an electric signal traveled in through the gold
foil, it injected holes (the opposite of electrons) into the surface. This
created a thin layer along the top of the germanium with too few electrons.

Semiconductors with too many electrons are known as N-type and semiconductors
with too few electrons are known as P-type. The boundary between these two kinds
of semiconductors is known as a P-N junction, and it’s a crucial part of a
transistor. In the presence of this junction, current can start to flow from one
side to the other. In the case of Brattain’s transistor, current flowed towards
the second gold contact.

Think about what that means. A small current in through one contact changes the
nature of the semiconductor so that a larger, separate current starts flowing
across the germanium and out the second contact. A little current can alter the
flow of a much bigger one, effectively amplifying it.

Of course, a transistor in a telephone or in a radio has to handle complex
signals. The output contact can’t just amplify a steady hum of current, it has
to dutifully replicate a person’s voice, or an entire symphony. Luckily, a
semiconductor is perfectly suited to this job. It is exquisitely sensitive to
how many extra or missing electrons are inside. Each time the input signal
shoves more holes into the germanium, it changes the way current flows across
the crystal — the output current instantly gets larger and smaller, perfectly
mimicking the input.