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Semiconductor device

2007 Schools Wikipedia Selection. Related subjects: Electricity and
Electronics; Engineering

   Semiconductor devices are electronic components that exploit the
   electronic properties of semiconductor materials, principally silicon,
   germanium, and gallium arsenide. Semiconductor devices have replaced
   thermionic devices (vacuum tubes) in most applications. They use
   electronic conduction in the solid state as opposed to the gaseous
   state or thermionic emission in a high vacuum.

   Semiconductor devices are manufactured both as single discrete devices
   and as integrated circuits (ICs), which consist of a number—from a few
   to millions—of devices manufactured and interconnected on a single
   semiconductor substrate.

Semiconductor device fundamentals

   The main reason semiconductor materials are so useful is that the
   behaviour of a semiconductor can be easily manipulated by the addition
   of impurities, known as doping. Semiconductor conductivity can be
   controlled by introduction of an electric field, by exposure to light,
   and even pressure and heat; thus, semiconductors can make excellent
   sensors. Current conduction in a semiconductor occurs via mobile or
   "free" electrons and holes (collectively known as charge carriers).
   Doping a semiconductor such as silicon with a small amount of impurity
   atoms, such as phosphorus or boron, greatly increases the number of
   free electrons or holes within the semiconductor. When a doped
   semiconductor contains excess holes it is called "p-type", and when it
   contains excess free electrons it is known as "n-type". The
   semiconductor material used in devices is doped under highly controlled
   conditions in a fabrication facility, or fab, to precisely control the
   location and concentration of p- and n-type dopants. The junctions
   which form where n-type and p-type semiconductors join together are
   called p-n junctions.

Diode

   The p-n junction diode is a device made from a p-n junction. At the
   junction of a p-type and an n-type semiconductor there forms a region
   called the depletion zone which blocks current conduction from the
   n-type region to the p-type region, but allows current to conduct from
   the p-type region to the n-type region. Thus when the device is forward
   biased, with the p-side at higher electric potential, the diode
   conducts current easily; but the current is very small when the diode
   is reverse biased.

   Exposing a semiconductor to light can generate electron–hole pairs,
   which increases the number of free carriers and its conductivity.
   Diodes optimized to take advantage of this phenomenon are known as
   photodiodes. Compound semiconductor diodes can also be used to generate
   light, as in light-emitting diodes and laser diodes.

Transistor

   Two MOS transistors with common gate (metallic layers and dielectric
   removed for clarity).
   Two MOS transistors with common gate (metallic layers and dielectric
   removed for clarity).

   Bipolar junction transistors are formed from two p-n junctions, in
   either n-p-n or p-n-p configuration. The middle, or base, region
   between the junctions is typically very narrow. The other regions, and
   their associated terminals, are known as the emitter and the collector.
   A small current injected through the junction between the base and the
   emitter changes the properties of the base-collector junction so that
   it can conduct current even though it is reverse biased. This creates a
   much larger current between the collector and emitter, controlled by
   the base-emitter current.

   Another type of transistor, the field effect transistor operates on the
   principle that semiconductor conductivity can be increased or decreased
   by the presence of an electric field. An electric field can increase
   the number of free electrons and holes in a semiconductor, thereby
   changing its conductivity. The field may be applied by a reverse-biased
   p-n junction, forming a junction field effect transistor, or JFET; or
   by an electrode isolated from the bulk material by an oxide layer,
   forming a metal-oxide-semiconductor field effect transistor, or MOSFET.
   Cross-section through a MOS transistor (metallic layers and dielectric
   removed for clarity), foreground.
   Cross-section through a MOS transistor (metallic layers and dielectric
   removed for clarity), foreground.

   The MOSFET is the most used semiconductor device today. The gate
   electrode is charged to produce an electric field that controls the
   conductivity of a "channel" between two terminals, called the source
   and drain. Depending on the type of carrier in the channel, the device
   may be an n-channel (for electrons) or a p-channel (for holes) MOSFET.
   Although the MOSFET is named in part for its "metal" gate, in modern
   devices polysilicon is typically used instead.

Semiconductor device materials

   By far, silicon (Si) is the most widely used material in semiconductor
   devices. Its combination of low raw material cost, relatively simple
   processing, and a useful temperature range make it currently the best
   compromise among the various competing materials. Silicon used in
   semiconductor device manufacturing is currently fabricated into boules
   that are large enough in diameter to allow the production of 300 mm (12
   in.) wafers.

   Germanium (Ge) was a widely used early semiconductor material but its
   thermal sensitivity makes it less useful than silicon. Today, germanium
   is often alloyed with silicon for use in very-high-speed SiGe devices;
   IBM is a major producer of such devices.

   Gallium arsenide (GaAs) is also widely used in high-speed devices but
   so far, it has been difficult to form large-diameter boules of this
   material, limiting the wafer diameter to sizes significantly smaller
   than silicon wafers thus making mass production of GaAs devices
   significantly more expensive than silicon.

   Other less common materials are also in use or under investigation.

   Silicon carbide (SiC) has found some application as the raw material
   for blue light-emitting diodes (LEDs) and is being investigated for use
   in semiconductor devices that could withstand very high operating
   temperatures and environments with the presence of significant levels
   of ionizing radiation. IMPATT diodes have also been fabricated from
   SiC.

   Various indium compounds (indium arsenide, indium antimonide, and
   indium phosphide) are also being used in LEDs and solid state laser
   diodes. Selenium sulfide is being studied in the manufacture of
   photovoltaic solar cells.

Semiconductor device applications

   All transistor types can be used as the building blocks of logic gates,
   which are fundamental in the design of digital circuits. In digital
   circuits like microprocessors, transistors act as on-off switches; in
   the MOSFET, for instance, the voltage applied to the gate determines
   whether the switch is on or off.

   Transistors used for analog circuits do not act as on-off switches;
   rather, they respond to a continuous range of inputs with a continuous
   range of outputs. Common analog circuits include amplifiers and
   oscillators.

   Circuits that interface or translate between digital circuits and
   analog circuits are known as mixed-signal circuits.

   Power semiconductor devices are discrete devices or integrated circuits
   intended for high current or high voltage applications. Power
   integrated circuits combine IC technology with power semiconductor
   technology, these are sometimes referred to as "smart" power devices.
   Several companies specialize in manufacturing power semiconductors.

Component identifiers

   The type designators of semiconductor devices are often manufacturer
   specific. Nevertheless, there have been attempts at creating standards
   for type codes, and a subset of devices follow those. For discrete
   devices, for example, there are three standards: JEDEC JESD370B in USA,
   Pro Electron in Europe and JIS in Japan.

History of semiconductor device development

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 a galena (lead sulfide) or carborundum
   (silicon carbide) crystal until it suddenly started working. Then, over
   a period of a few hours or days, the cat's whisker 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. The "cat's whisker" is a primitive example
   of a special type of diode still popular today, called a Schottky
   diode.

World War II

   During World War II, radar research quickly pushed radar receivers to
   operate at ever higher frequencies and the traditional tube based radio
   receivers no longer worked well. The introduction of the cavity
   magnetron from Britain to the United States in 1940 during the Tizard
   Mission resulted in a pressing need for a practical high-frequency
   amplifier.

   On a whim, Russell Ohl of Bell Laboratories decided to try a cat's
   whisker. By this point they had not been in use for a number of years,
   and no one at the labs had one. After hunting one down at a used radio
   store in Manhattan, he found that it worked much better than tube-based
   systems.

   Ohl investigated why the cat's whisker functioned so well. He spent
   most of 1939 trying to grow more pure versions of the crystals. He soon
   found that with higher quality crystals their finicky behaviour 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 caused
   more conductance in the crystal. He invited several other people to see
   this crystal, and Walter 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 (what he called) an
   "insulator". Because the two parts of the crystal were in contact with
   each other, the electrons could be pushed out of the conductive side
   which had extra electrons (soon to be known as the emitter) and
   replaced by new ones being provided (from a battery, for instance)
   where they would flow into the insulating portion and be collected by
   the whisker filament (named the collector). However, when the voltage
   was reversed the electrons being pushed into the collector would
   quickly fill up the "holes" (the electron-needy impurities), 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. The mechanism of action
   when the diode is off has to do with the separation of charge carriers
   around the junction. This is called a " depletion region".

Development of the diode

   Armed with the knowledge of how these new diodes worked, a vigorous
   effort began in order 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.

Development of the transistor

   After the war, William Shockley decided to attempt the building of a
   triode-like semiconductor device. He secured funding and lab space, and
   went to work on the problem with Brattain and John Bardeen.

   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 of this
   newly discovered diode, 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 number of electrons (or holes) required to
   be injected would have to be very large -– making it less than useful
   as an amplifier because it would require a large injection 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 depletion region. The key appeared to be to place the input and
   output contacts very close together on the surface of the crystal on
   either side of this region.

   Brattain started working on building such a device, and tantalizing
   hints of amplification continued to appear as the team worked on the
   problem. Sometimes the system would work but then stop working
   unexpectedly. In one instance a non-working system started working when
   placed in water. Ohl and Brattain eventually developed a new branch of
   quantum mechanics known as surface physics to account for the
   behaviour. 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 with the
   application of a small amount of charge from any other location on the
   crystal. Instead of needing a large supply of injected electrons, a
   very small number in the right place on the crystal would accomplish
   the same thing.

   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 leads would both be placed very close
   together on the top, with the control lead placed on the base of the
   crystal. When current was applied to the "base" lead, the electrons or
   holes would be pushed out, across the 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.

The first transistor

   A stylized replica of the first transistor
   A stylized replica of the first transistor

   The Bell team 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, and would work briefly, if at all. 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.

   While the device was constructed a week earlier, Brattain's notes
   describe the first demonstration to higher-ups at Bell Labs on the
   afternoon of 23 December 1947, often given as the birthdate of the
   transistor. The "PNP point-contact germanium transistor" operated as a
   speech amplifier with a power gain of 18 in that trial. Known generally
   as a point-contact transistor today, John Bardeen, Walter Houser
   Brattain, and William Bradford Shockley were awarded the Nobel Prize in
   physics for their work in 1956.

Origin of the term "transistor"

   Bell Telephone Laboratories needed a generic name for their new
   invention: "Semiconductor Triode", "Solid Triode", "Surface States
   Triode" [sic], "Crystal Triode" and "Iotatron" were all considered, but
   "transistor", coined by John R. Pierce, won an internal ballot. The
   rationale for the name is described in the following extract from the
   company's Technical Memoranda ( May 28, 1948) calling for votes:

     Transistor. This is an abbreviated combination of the words
     "transconductance" or "transfer", and "varistor". The device
     logically belongs in the varistor family, and has the
     transconductance or transfer impedance of a device having gain, so
     that this combination is descriptive.

Improvements in transistor design

   Shockley was upset about the device being credited to Brattain and
   Bardeen, who he felt had built it "behind his back" to take the glory.
   Matters became worse when Bell Labs lawyers found that some of
   Shockley's own writings on the transistor were close enough to those of
   an earlier 1925 patent by Julius Edgar Lilienfeld that they thought it
   best that his name be left off the patent application.

   Shockley was incensed, and decided to demonstrate who was the real
   brains of the operation. Only a few months later he invented an
   entirely new type of transistor with a layer or 'sandwich' structure.
   This new form was considerably more robust than the fragile
   point-contact system, and would go on to be used for the vast majority
   of all transistors into the 1960s. It would evolve into the bipolar
   junction transistor.

   With the fragility problems solved, a remaining problem was purity.
   Making germanium of the required purity was proving to be a serious
   problem, and limited the number of transistors that actually worked
   from a given batch of material. Germanium's sensitivity to temperature
   also limited its usefulness. Scientists theorized that silicon would be
   easier to fabricate, but few bothered to investigate this possibility.
   Gordon K. Teal was the first to develop a working silicon transistor,
   and his company, the nascent Texas Instruments, profited from its
   technological edge. Germanium disappeared from most transistors by the
   late 1960s.

   Within a few years, transistor-based products, most notably radios,
   were appearing on the market. A major improvement in manufacturing
   yield came when a chemist advised the companies fabricating
   semiconductors to use distilled water rather than tap water: calcium
   ions were the cause of the poor yields. " Zone melting", a technique
   using a moving band of molten material through the crystal, further
   increased the purity of the available crystals.

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