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semiconductor
Table of Contents
semiconductor

 * Introduction
   
 * 
   Semiconductor materials
   
 * 
   Electronic properties
   
 * 
   The p-n junction
   

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Home Science Physics Matter & Energy


SEMICONDUCTOR

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Table of Contents
conductivities
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Key People: Duncan Haldane Walter H. Brattain Yves-André Rocard ...(Show more)
Related Topics: silicon germanium avalanche effect Gunn effect p-n junction
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semiconductor, any of a class of crystalline solids intermediate in electrical
conductivity between a conductor and an insulator. Semiconductors are employed
in the manufacture of various kinds of electronic devices, including diodes,
transistors, and integrated circuits. Such devices have found wide application
because of their compactness, reliability, power efficiency, and low cost. As
discrete components, they have found use in power devices, optical sensors, and
light emitters, including solid-state lasers. They have a wide range of current-
and voltage-handling capabilities and, more important, lend themselves to
integration into complex but readily manufacturable microelectronic circuits.
They are, and will be in the foreseeable future, the key elements for the
majority of electronic systems, serving communications, signal processing,
computing, and control applications in both the consumer and industrial markets.




SEMICONDUCTOR MATERIALS

Solid-state materials are commonly grouped into three classes: insulators,
semiconductors, and conductors. (At low temperatures some conductors,
semiconductors, and insulators may become superconductors.) The figure shows the
conductivities σ (and the corresponding resistivities ρ = 1/σ) that are
associated with some important materials in each of the three classes.
Insulators, such as fused quartz and glass, have very low conductivities, on the
order of 10−18 to 10−10 siemens per centimetre; and conductors, such as
aluminum, have high conductivities, typically from 104 to 106 siemens per
centimetre. The conductivities of semiconductors are between these extremes and
are generally sensitive to temperature, illumination, magnetic fields, and
minute amounts of impurity atoms. For example, the addition of about 10 atoms of
boron (known as a dopant) per million atoms of silicon can increase its
electrical conductivity a thousandfold (partially accounting for the wide
variability shown in the preceding figure).

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The study of semiconductor materials began in the early 19th century. The
elemental semiconductors are those composed of single species of atoms, such as
silicon (Si), germanium (Ge), and tin (Sn) in column IV and selenium (Se) and
tellurium (Te) in column VI of the periodic table. There are, however, numerous
compound semiconductors, which are composed of two or more elements. Gallium
arsenide (GaAs), for example, is a binary III-V compound, which is a combination
of gallium (Ga) from column III and arsenic (As) from column V. Ternary
compounds can be formed by elements from three different columns—for instance,
mercury indium telluride (HgIn2Te4), a II-III-VI compound. They also can be
formed by elements from two columns, such as aluminum gallium arsenide (AlxGa1 −
xAs), which is a ternary III-V compound, where both Al and Ga are from column
III and the subscript x is related to the composition of the two elements from
100 percent Al (x = 1) to 100 percent Ga (x = 0). Pure silicon is the most
important material for integrated circuit applications, and III-V binary and
ternary compounds are most significant for light emission.



periodic table
Modern version of the periodic table of the elements.
Encyclopædia Britannica, Inc.

Prior to the invention of the bipolar transistor in 1947, semiconductors were
used only as two-terminal devices, such as rectifiers and photodiodes. During
the early 1950s germanium was the major semiconductor material. However, it
proved unsuitable for many applications, because devices made of the material
exhibited high leakage currents at only moderately elevated temperatures. Since
the early 1960s silicon has become by far the most widely used semiconductor,
virtually supplanting germanium as a material for device fabrication. The main
reasons for this are twofold: (1) silicon devices exhibit much lower leakage
currents, and (2) silicon dioxide (SiO2), which is a high-quality insulator, is
easy to incorporate as part of a silicon-based device. Thus, silicon technology
has become very advanced and pervasive, with silicon devices constituting more
than 95 percent of all semiconductor products sold worldwide.

Many of the compound semiconductors have some specific electrical and optical
properties that are superior to their counterparts in silicon. These
semiconductors, especially gallium arsenide, are used mainly for optoelectronic
and certain radio frequency (RF) applications.




ELECTRONIC PROPERTIES

The semiconductor materials described here are single crystals; i.e., the atoms
are arranged in a three-dimensional periodic fashion. Part A of the figure shows
a simplified two-dimensional representation of an intrinsic (pure) silicon
crystal that contains negligible impurities. Each silicon atom in the crystal is
surrounded by four of its nearest neighbours. Each atom has four electrons in
its outer orbit and shares these electrons with its four neighbours. Each shared
electron pair constitutes a covalent bond. The force of attraction between the
electrons and both nuclei holds the two atoms together. For isolated atoms
(e.g., in a gas rather than a crystal), the electrons can have only discrete
energy levels. However, when a large number of atoms are brought together to
form a crystal, the interaction between the atoms causes the discrete energy
levels to spread out into energy bands. When there is no thermal vibration
(i.e., at low temperature), the electrons in an insulator or semiconductor
crystal will completely fill a number of energy bands, leaving the rest of the
energy bands empty. The highest filled band is called the valence band. The next
band is the conduction band, which is separated from the valence band by an
energy gap (much larger gaps in crystalline insulators than in semiconductors).
This energy gap, also called a bandgap, is a region that designates energies
that the electrons in the crystal cannot possess. Most of the important
semiconductors have bandgaps in the range 0.25 to 2.5 electron volts (eV). The
bandgap of silicon, for example, is 1.12 eV, and that of gallium arsenide is
1.42 eV. In contrast, the bandgap of diamond, a good crystalline insulator, is
5.5 eV.



semiconductor bonds
Three bond pictures of a semiconductor.
Encyclopædia Britannica, Inc.

At low temperatures the electrons in a semiconductor are bound in their
respective bands in the crystal; consequently, they are not available for
electrical conduction. At higher temperatures thermal vibration may break some
of the covalent bonds to yield free electrons that can participate in current
conduction. Once an electron moves away from a covalent bond, there is an
electron vacancy associated with that bond. This vacancy may be filled by a
neighbouring electron, which results in a shift of the vacancy location from one
crystal site to another. This vacancy may be regarded as a fictitious particle,
dubbed a “hole,” that carries a positive charge and moves in a direction
opposite to that of an electron. When an electric field is applied to the
semiconductor, both the free electrons (now residing in the conduction band) and
the holes (left behind in the valence band) move through the crystal, producing
an electric current. The electrical conductivity of a material depends on the
number of free electrons and holes (charge carriers) per unit volume and on the
rate at which these carriers move under the influence of an electric field. In
an intrinsic semiconductor there exists an equal number of free electrons and
holes. The electrons and holes, however, have different mobilities; that is,
they move with different velocities in an electric field. For example, for
intrinsic silicon at room temperature, the electron mobility is 1,500 square
centimetres per volt-second (cm2/V·s)—i.e., an electron will move at a velocity
of 1,500 centimetres per second under an electric field of one volt per
centimetre—while the hole mobility is 500 cm2/V·s. The electron and hole
mobilities in a particular semiconductor generally decrease with increasing
temperature.



electron hole: movement
Movement of an electron hole in a crystal lattice.
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Electrical conduction in intrinsic semiconductors is quite poor at room
temperature. To produce higher conduction, one can intentionally introduce
impurities (typically to a concentration of one part per million host atoms).
This is called doping, a process that increases conductivity despite some loss
of mobility. For example, if a silicon atom is replaced by an atom with five
outer electrons, such as arsenic (see part B of the figure), four of the
electrons form covalent bonds with the four neighbouring silicon atoms. The
fifth electron becomes a conduction electron that is donated to the conduction
band. The silicon becomes an n-type semiconductor because of the addition of the
electron. The arsenic atom is the donor. Similarly, part C of the figure shows
that, if an atom with three outer electrons, such as boron, is substituted for a
silicon atom, an additional electron is accepted to form four covalent bonds
around the boron atom, and a positively charged hole is created in the valence
band. This creates a p-type semiconductor, with the boron constituting an
acceptor.




THE P-N JUNCTION

If an abrupt change in impurity type from acceptors (p-type) to donors (n-type)
occurs within a single crystal structure, a p-n junction is formed (see parts B
and C of the figure). On the p side, the holes constitute the dominant carriers
and so are called majority carriers. A few thermally generated electrons will
also exist in the p side; these are termed minority carriers. On the n side, the
electrons are the majority carriers, while the holes are the minority carriers.
Near the junction is a region having no free charge carriers. This region,
called the depletion layer, behaves as an insulator.


p-n junction characteristics
(A) Current-voltage characteristics of a typical silicon p-n junction. (B)
Forward-bias and (C) reverse-bias conditions. (D) The symbol for a p-n junction.
Encyclopædia Britannica, Inc.

The most important characteristic of p-n junctions is that they rectify. Part A
of the figure shows the current-voltage characteristics of a typical silicon p-n
junction. When a forward bias is applied to the p-n junction (i.e., a positive
voltage applied to the p-side with respect to the n-side, as shown in part B of
the figure), the majority charge carriers move across the junction so that a
large current can flow. However, when a reverse bias is applied (as in part C of
the figure), the charge carriers introduced by the impurities move in opposite
directions away from the junction, and only a small leakage current flows. As
the reverse bias is increased, the leakage current remains very small until a
critical voltage is reached, at which point the current suddenly increases. This
sudden increase in current is referred to as the junction breakdown, usually a
nondestructive phenomenon if the resulting power dissipation is limited to a
safe value. The applied forward voltage is typically less than one volt, but the
reverse critical voltage, called the breakdown voltage, can vary from less than
one volt to many thousands of volts, depending on the impurity concentration of
the junction and other device parameters.



Although other junction types have been invented (including p-n-p and n-p-n),
p-n junctions remain fundamental to semiconductor devices. For further details
on applications of these basic semiconductor properties, see transistor and
integrated circuit.

The Editors of Encyclopaedia Britannica
This article was most recently revised and updated by Adam Augustyn.


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   Point-Contact Rectifier Effect is Discovered
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