Transistor

The transistor is a solid-state semiconductor device used for amplification
and switching, and has three terminals: a small current or voltage applied
to one terminal controls the current through the other two. It is the key
component in all modern electronics. In digital circuits, transistors are
used as very fast electrical switches, and arrangements of transistors can
function as logic gates, RAM-type memory and other devices. In analog
circuits, transistors are essentially used as amplifiers.

Transistor was also the common name in the sixties for a transistor radio, a
pocket-sized portable radio that utilized transistors (rather than vacuum
tubes) as its active electronics. This is still one of the dictionary
definitions of transistor.

Invention

The transistor was invented at Bell Laboratories in December 1947 by John
Bardeen, Walter Houser Brattain, and William Bradford Shockley, who were
awarded the Nobel Prize in physics in 1956. Ironically, they had set out to
manufacture a field-effect transistor (FET) predicted by Julius Edgar
Lilienfeld as early as 1925 but eventually discovered current amplification
in the point-contact transistor that subsequently evolved to become the
bipolar junction transistor (BJT).

How Does a Transistor Work?

A transistor is electrically a three-terminal device. In a BJT, an
electrical current is fed into the base (B) and modulates the current flow
between the other two terminals known as the emitter (E) and collector (C).
In FETs, the three terminals are called gate (G), source (S) and drain (D)
respectively, and it is the voltage applied to the gate terminal that
modulates the current between source and drain.

Bipolar Junction Transistor (BJT)

Conceptually, one can understand a bipolar junction transistor as two diodes
placed back to back, connected so they share either their positive or their
negative terminals. The forward-biased emitter-base junction allows charge
carriers to easily flow out of the emitter. The base is made thin enough so
that most of the injected carriers will reach the collector rather than
recombining in the base. Since small changes in the base current affect the
collector current significantly, the transistor can work as an electronic
amplifier. The rate of amplification, usually called the current gain
(β), is roughly one hundred for most types of BJTs. That is, one
milliampere of base current usually induces a collector current of about a
hundred milliamperes. BJTs prevail in all sorts of amplifiers from audio to
radio frequency applications and are also popular as electronic switching
devices.

Field-Effect Transistor (FET)

The most common variety of field-effect transistors, the enhancement-mode
MOSFET (metal-oxide semiconductor field-effect transistor) can also be
viewed as two back-to-back diodes that separate the source and drain
terminals. The volume in between is covered by an extremely thin insulating
layer that carries the gate electrode. When a voltage is applied between
gate and source, an electric field is created in that volume, causing a thin
conductive channel to form between the source and drain and allowing current
to flow across. The amount of this current can be modulated, or completely
turned off, by varying the gate voltage. Because the gate is insulated, no
DC current flows to or from the gate electrode. This lack of a gate current
(as compared to the BJT's base current), and the ability of the MOSFET to
act like a switch, allows particularly efficient digital circuits to be
created. Hence, MOSFETs have become the dominant technology used in
computing hardware such as microprocessors and memory devices such as RAM.

The most common form of MOSFET transistor in use today is the CMOS
(complementary metallic oxide semiconductor) which is the basis for
virtually all integrated circuits produced.

Advantages of Transistors over Thermionic Valves

Before the transistor, the thermionic valve or vacuum tube, was the main
component of an amplifier. The key advantages that have allowed transistors
to replace their valve predecessors in almost all applications are

   * smaller size,
   * simpler manufacture and, hence,
   * lower cost,
   * lower operating voltages,
   * absence of a heated filament and, as a consequence,
   * lower power dissipation,
   * higher reliability and greater endurance.

Valves are still used in very high-power applications such as broadcast
radio signal amplification. Some audio amplifiers also use them, their
enthusiasts claiming that their sound is superior. In particular, some argue
that the larger numbers of electrons in a valve behave with greater
statistical accuracy. Other detect a distinctive "warmth" to the tone. The
"warmth" is actually distortion caused by the valves, but some audiophiles
find a certain amount of "fuzziness" pleasing.

The "second generation" of computers through the late 1950s and 1960s
featured boards filled with individual transistors. Subsequently,
transistors, other components and the necessary wiring, were integrated into
a single, mass-manufactured component in the integrated circuit. In modern
digital electronics, single transistors are very rare, though they remain
common in power and analog applications.

History

All transistors rely on the ability of certain materials, known as
semiconductors, to change their electrical resistance under the control of
an electric field. In bipolar transistors, the semiconductor is formed into
structures called p-n junctions that allow electricity to flow in only one
direction through them – that is they are a conductor when voltage is
applied in one direction, and an insulator when it is applied in the other
direction.

1900s

Semiconductors had been used in the electronics field for some time before
the invention of the transistor. Around the turn of the 20th century they
were quite common as detectors in radios, used in a device called a "cat's
whisker". These detectors were somewhat troublesome, however, requiring the
operator to move a small tungsten filament (the whisker) around the surface
of the crystal until it suddenly started working. Then, over a period of a
few hours or days, the crystal would slowly stop working and the process
would have to be repeated. At the time their operation was completely
mysterious. After the introduction of the more reliable and amplified vacuum
tube based radios, the cat's whisker systems quickly disappeared.

World War II

In WWII radar research quickly pushed the frequencies of the radio receivers
inside them into the area where traditional tube based radio receivers no
longer worked well. On a whim, Russell Ohl of Bell Laboratories decided to
try a cat's whisker. After hunting one down at a used radio store in
Manhattan, he found that it worked much better than tube-based systems.

He then started to try to figure out why they were so "odd". He spent most
of 1939 trying to grow more pure versions of the crystals. He soon found
that with higher quality crystals the "oddness" went away — but so did
their ability to operate as a radio detector. One day he found one of his
purest crystals nevertheless worked well, and interestingly, it had a
clearly visible crack near the middle. However as he moved about the room
trying to test it, the detector would mysteriously work, and then stop
again. After some study he found that the behaviour was controlled by the
light in the room – more light, more conductance. He invited several
other people to see it, and Brattain immediately realized there was some
sort of junction at the crack.

Further research cleared up the remaining mystery. The crystal had cracked
because either side contained very slightly different amounts of the
impurities Ohl could not remove – about 0.2%. One side of the crystal
had impurities that added extra electrons (the carriers of electrical
current) and made it a conductor. The other had impurities that wanted to
bind to these electrons, making it an insulator. When the two were placed
side by side the electrons could be pushed out of the side with extra
electrons (soon to be known as the emitter) and replaced by new ones being
provided (say from a battery) where they would flow into the insulating
portion and be collected by the filament (the collector). However, when the
voltage was reversed the electrons being pushed into the collector would
quickly fill up the "holes", and conduction would stop almost instantly.
This junction of the two crystals (or parts of one crystal) created a
solid-state diode, and the concept soon became known as semiconduction.

Development

Armed with the knowledge of how these new diodes worked, a crash effort
started to learn how to build them on demand. Teams at Purdue University,
Bell Labs, MIT, and the University of Chicago all joined forces to build
better crystals. Within a year germanium production had been perfected to
the point where military-grade diodes were being used in most radar sets.

The key to the development of the transistor was the further understanding
of the process of the electron mobility in a semiconductor. It was realized
that if there was some way to control the flow of the electrons from the
emitter to the collector, one could build an amplifier. For instance, if you
placed contacts on either side of a single type of crystal the current would
not flow through it. However if a third contact could then "inject"
electrons or holes into the material, the current would flow.

Actually doing this appeared to be very difficult. If the crystal were of
any reasonable size, the amount of electrons (or holes) supplied would have
to be very large – making it less than useful as an amplifier because
it would require a large current to start with. That said, the whole idea of
the crystal diode was that the crystal itself could provide the electrons
over a very small distance. The key appeared to be to place the input and
output contacts very close together on the surface of the crystal.

Brattain started working on building such a device, and tantalizing hints of
amplification continued to appear as the team worked on the problem. One day
the system would work and the next it wouldn't. In one instance a
non-working system started working when placed in water. The two eventually
developed a new branch of quantum mechanics known as surface physics to
account for the behaviour.

Essentially the electrons in any one piece of the crystal would migrate
about due to nearby charges. Electrons in the emitters, or the "holes" in
the collectors, would cluster at the surface of the crystal where they could
find their opposite charge "floating around" in the air (or water). Yet they
could be pushed away from the surface from any other location with the
application of a small amount of charge. So instead of needing a large
supply of electrons, a very small number in the right place would do the
trick.

Their understanding solved the problem of needing a very small control area
to some degree. Instead of needing two separate semiconductors connected by
a common, but tiny, region, a single larger surface would serve. The emitter
and collector would both be placed very close together on one side, with the
control lead on the other. When current was applied to the control lead, the
electrons or holes would be pushed out, right across the entire block of
semiconductor, and collect on the far surface. As long as the emitter and
collector were very close together, this should allow enough electrons or
holes between them to allow conduction to start.

First transistor

The first transistor took some time to construct. They made many attempts to
build such a system with various tools, but generally failed. Setups where
the contacts were close enough were invariably as fragile as the original
cat's whisker detectors had been. Eventually they had a practical
breakthrough. A piece of gold foil was glued to the edge of a plastic wedge,
and then the foil was sliced with a razor at the tip of the triangle. The
result was two very closely spaced contacts of gold. When the plastic was
pushed down onto the surface of a crystal and voltage applied to the other
side (on the base of the crystal), current started to flow from one contact
to the other as the base voltage pushed the electrons away from the base
towards the other side near the contacts. The point-contact transistor had
been invented, a primitive variation of the BJT.

Such a system was of limited practical use, no better physically than the
cat's whisker of old. Soon newer methods and understanding allowed for much
more robust versions. Within a few years, transistor-based products, most
notably radios were appearing on the market.

Origin of Name

In a vacuum tube (British: valve), changes in plate current are proportional
to changes in grid voltage, to a first approximation. The ratio (current /
voltage) has the dimensions of a conductance. Since the current and voltage
are not measured at the same terminals, it is referred to as the
"transconductance" rather than "conductance," and is an important figure of
merit for a vacuum tube. Had vacuum tubes been named for their function
rather than their structure, they might have been called "transconductors."

John R. Pierce coined the name "transistor" in 1949. It was originally
thought that the transistor could be usefully considered to be the
electronic dual of a vacuum tube. The property equivalent to
transconductance would have been "transresistance" and the device would then
have been a "transresistor," or "transistor" for short. It transpired that
the transistor was not close enough to being a vacuum-tube dual for the
concept to have any quantitative usefulness, and the concept of
"transresistance" lives on only in the name "transistor."

Miniaturization

The first CMOS transistor circuit was introduced by RCA in 1963.

Another level of miniaturization later became possible with the invention of
the integrated circuit, which included many transistors on one chip of
silicon, and led to a new generation of devices such as pocket calculators
and digital watches.