The Point Contact
Point Contact Transistor
Point Contact Transistor
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.
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