History of
the first transistor
“The
first transistor was about half an inch high.
Before Brattain started, John Bardeen told him that they would need two
metal contacts within 0.002 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.”
Adopted
from a Public Broadcasting System (PBS) television series (1999)