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Modern Transistors

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In 1945, Shockley had an idea for making a solid state device out of
semiconductors. He reasoned that a strong electrical field could cause the flow
of electricity within a nearby semiconductor. He tried to build one, then had
Walter Brattain try to build it, but it didn’t work.

Three years later, Brattain and Bardeen built the first working transistor, the
germanium point-contact transistor, which was manufactured as the “A” series.
Shockley then designed the junction (sandwich) transistor, which was
manufactured for several years afterwards. But in 1960 Bell scientist John
Atalla developed a new design based on Shockley’s original field-effect
theories. By the late 1960s, manufacturers converted from junction type
integrated circuits to field effect devices. Today, most transistors are
field-effect transistors. You are using millions of them now.

Most of today’s transistors are “MOS-FETs”, or Metal Oxide Semiconductor Field
Effect Transistors. They were developed mainly by Bell Labs, Fairchild
Semiconductor, and hundreds of Silicon Valley, Japanese and other electronics
companies.

Field-effect transistors are so named because a weak electrical signal coming in
through one electrode creates an electrical field through the rest of the
transistor. This field flips from positive to negative when the incoming signal
does, and controls a second current traveling through the rest of the
transistor. The field modulates the second current to mimic the first one — but
it can be substantially larger.

On the bottom of the transistor is a U-shaped section (though it’s flatter than
a true “U”) of N-type semiconductor with an excess of electrons. In the center
of the U is a section known as the “base” made of P-type (positively charged)
semiconductor with too few electrons. (Actually, the N- and P-types can be
reversed and the device will work in exactly the same way, except that holes,
not electrons, would cause the current.)

Three electrodes are attached to the top of this semiconductor crystal: one to
the middle positive section and one to each arm of the U. By applying a voltage
to the electrodes on the U, current will flow through it. The side where the
electrons come in is known as the source, and the side where the electrons come
out is called the drain.

If nothing else happens, current will flow from one side to the other. Due to
the way electrons behave at the junction between N- and P-type semiconductors,
however, the current won’t flow particularly close to the base. It travels only
through a thin channel down the middle of the U.

There’s also an electrode attached to the base, a wedge of P-type semiconductor
in the middle, separated from the rest of the transistor by a thin layer of
metal-oxide such as silicon dioxide (which plays the role of an insulator). This
electrode is called the “gate.” The weak electrical signal we’d like to amplify
is fed through the gate. If the charge coming through the gate is negative, it
adds more electrons to the base. Since electrons repel each other, the electrons
in the U move as far away from the base as possible. This creates a depletion
zone around the base � a whole area where electrons cannot travel. The channel
down the middle of the U through which current can flow becomes even thinner.
Add enough negative charge to the base and the channel will pinch off
completely, stopping all current. It’s like stepping on a garden hose to stop
the flow of water. (Earlier transistors controlled this depletion zone by making
use of how electrons move when two semiconductor slabs are put next to each
other, creating what is known as a P-N junction. In a MOS-FET, the P-N junction
is replaced with metal-oxide, which turned out to be easier to mass produce in
microchips.)

Now imagine if the charge coming through the gate is positive. The positive base
attracts many electrons � suddenly the area around the base which used to be a
no-man’s-land opens up. The channel for current through the U becomes larger
than it was originally and much more electricity can flow through.

Alternating charge on the base, therefore, changes how much current goes through
the U. The incoming current can be used as a faucet to turn current on or off as
it moves through the rest of the transistor.

On the other hand, the transistor can be used in a more complex manner as well
— as an amplifier. Current traveling through the U gets larger or smaller in
perfect synch with the charge coming into the base, meaning it has the identical
pattern as that original weak signal. And, since the second current is connected
to a different voltage supply, it can be made to be larger. The current coming
through the U is a perfect replica of the original, only amplified. The
transistor is used this way for stereo amplification in speakers and
microphones, as well as to boost telephone signals as they travel around the
world.