semiconductor

1. Any of various solid crystalline substances, such as germanium or silicon, having electrical conductivity greater than insulators but less than good conductors, and used especially as a base material for computer chips and other electronic devices.
2. An integrated circuit or other electronic component containing a semiconductor as a base material.

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A solid crystalline material whose electrical conductivity is intermediate between that of a metal and an insulator. Semiconductors exhibit conduction properties that may be temperature-dependent, permitting their use as thermistors (temperature-dependent resistors), or voltage-dependent, as in varistors. By making suitable contacts to a semiconductor or by making the material suitably inhomogeneous, electrical rectification and amplification can be obtained. Semiconductor devices, rectifiers, and transistors have replaced vacuum tubes almost completely in low-power electronics, making it possible to save volume and power consumption by orders of magnitude. In the form of integrated circuits, they are vital for complicated systems. The optical properties of a semiconductor are important for the understanding and the application of the material. Photodiodes, photoconductive detectors of radiation, injection lasers, light-emitting diodes, solar-energy conversion cells, and so forth are examples of the wide variety of optoelectronic devices. See also Integrated circuits; Laser; Light-emitting diode; Photodiode; Photoelectric devices; Semiconductor diode; Semiconductor rectifier; Transistor; Varistor.

Conduction in semiconductors

The electrical conductivity of semiconductors ranges from about 10³ to 10⁻⁹ ohm⁻¹ cm⁻¹, as compared with a maximum conductivity of 10⁷ for good conductors and a minimum conductivity of 10⁻¹⁷ ohm⁻¹ cm⁻¹ for good insulators. See also Electric insulator.

The electric current is usually due only to the motion of electrons, although under some conditions, such as very high temperatures, the motion of ions may be important. The basic distinction between conduction in metals and in semiconductors is made by considering the energy bands occupied by the conduction electrons. See also Band theory of solids; Ionic crystals.

At absolute zero temperature, the electrons occupy the lowest possible energy levels, with the restriction that at most two electrons with opposite spin may be in the same energy level. In semiconductors and insulators, there are just enough electrons to fill completely a number of energy bands, leaving the rest of the energy bands empty. The highest filled energy band is called the valence band. The next higher band, which is empty at absolute zero temperature, is called the conduction band. The conduction band is separated from the valence band by an energy gap, which is an important characteristic of the semiconductor. In metals, the highest energy band that is occupied by the electrons is only partially filled. This condition exists either because the number of electrons is not just right to fill an integral number of energy bands or because the highest occupied energy band overlaps the next higher band without an intervening energy gap. The electrons in a partially filled band may acquire a small amount of energy from an applied electric field by going to the higher levels in the same band. The electrons are accelerated in a direction opposite to the field and thereby constitute an electric current. In semiconductors and insulators, the electrons are found only in completely filled bands, at low temperatures. In order to increase the energy of the electrons, it is necessary to raise electrons from the valence band to the conduction band across the energy gap. The electric fields normally encountered are not large enough to accomplish this with appreciable probability. At sufficiently high temperatures, depending on the magnitude of the energy gap, a significant number of valence electrons gain enough energy thermally to be raised to the conduction band. These electrons in an unfilled band can easily participate in conduction. Furthermore, there is now a corresponding number of vacancies in the electron population of the valence band. These vacancies, or holes as they are called, have the effect of carriers of positive charge, by means of which the valence band makes a contribution to the conduction of the crystal. See also Hole states in solids.

The type of charge carrier, electron or hole, that is in largest concentration in a material is sometimes called the majority carrier and the type in smallest concentration the minority carrier. The majority carriers are primarily responsible for the conduction properties of the material. Although the minority carriers play a minor role in electrical conductivity, they can be important in rectification and transistor actions in a semiconductor.

Intrinsic semiconductors

A semiconductor in which the concentration of charge carriers is characteristic of the material itself rather than of the content of impurities and structural defects of the crystal is called an intrinsic semiconductor. Electrons in the conduction band and holes in the valence band are created by thermal excitation of electrons from the valence to the conduction band. Thus an intrinsic semiconductor has equal concentrations of electrons and holes. The carrier concentration, and hence the conductivity, is very sensitive to temperature and depends strongly on the energy gap. The energy gap ranges from a fraction of 1 eV to several electronvolts. A material must have a large energy gap to be an insulator.

Extrinsic semiconductors

Typical semiconductor crystals such as germanium and silicon are formed by an ordered bonding of the individual atoms to form the crystal structure. The bonding is attributed to the valence electrons which pair up with valence electrons of adjacent atoms to form so-called shared pair or covalent bonds. These materials are all of the quadrivalent type; that is, each atom contains four valence electrons, all of which are used in forming the crystal bonds. See also Crystal structure.

Atoms having a valence of +3 or +5 can be added to a pure or intrinsic semiconductor material with the result that the +3 atoms will give rise to an unsatisfied bond with one of the valence electrons of the semiconductor atoms, and +5 atoms will result in an extra or free electron that is not required in the bond structure. Electrically, the +3 impurities add holes and the +5 impurities add electrons. They are called acceptor and donor impurities, respectively. Typical valence +3 impurities used are boron, aluminum, indium, and gallium. Valence +5 impurities used are arsenic, antimony, and phosphorus.

Semiconductor material “doped” or “poisoned” by valence +3 acceptor impurities is termed p‐type, whereas material doped by valence +5 donor material is termed n-type. The names are derived from the fact that the holes introduced are considered to carry positive charges and the electrons negative charges. The number of electrons in the energy bands of the crystal is increased by the presence of donor impurities and decreased by the presence of acceptor impurities. See also Acceptor atom; Donor atom.

At sufficiently high temperatures, the intrinsic carrier concentration becomes so large that the effect of a fixed amount of impurity atoms in the crystal is comparatively small and the semiconductor becomes intrinsic. When the carrier concentration is predominantly determined by the impurity content, the conduction of the material is said to be extrinsic. Physical defects in the crystal structure may have similar effects as donor or acceptor impurities. They can also give rise to extrinsic conductivity.

Materials

The group of chemical elements which are semiconductors includes germanium, silicon, gray (crystalline) tin, selenium, tellurium, and boron. Germanium, silicon, and gray tin belong to group 14 of the periodic table and have crystal structures similar to that of diamond. Germanium and silicon are two of the best-known semiconductors. They are used extensively in devices such as rectifiers and transistors.

A large number of compounds are known to be semiconductors. A group of semiconducting compounds of the simple type AB consists of elements from columns symmetrically placed with respect to column 14 of the periodic table. Indium antimonide (InSb), cadmium telluride (CdTe), and silver iodide (AgI) are examples of III–V, II–IV, and I–VI compounds, respectively. The various III–V compounds are being studied extensively, and many practical applications have been found for these materials. Some of these compounds have the highest carrier mobilities known for semiconductors. The compounds have zincblende crystal structure which is geometrically similar to the diamond structure possessed by the elemental semiconductors, germanium and silicon, of column 14, except that the four nearest neighbors of each atom are atoms of the other kind. The II–VI compounds, zinc sulfide (ZnS) and cadmium sulfide (CdS), are used in photoconductive devices. Zinc sulfide is also used as a luminescent material. See also Luminescence; Photoconductivity.

The properties of semiconductors are extremely sensitive to the presence of impurities. It is therefore desirable to start with the purest available materials and to introduce a controlled amount of the desired impurity. The zone-refining method is often used for further purification of obtainable materials. The floating zone technique can be used, if feasible, to prevent any contamination of molten material by contact with the crucible. See also Zone refining.

For basic studies as well as for many practical applications, it is desirable to use single crystals. Various methods are used for growing crystals of different materials. For many semiconductors, including germanium, silicon, and the III–V compounds, the Czochralski method is commonly used. The method of condensation from the vapor phase is used to grow crystals of a number of semiconductors, for instance, selenium and zinc sulfide. See also Crystal growth.

The introduction of impurities, or doping, can be accomplished by simply adding the desired quantity to the melt from which the crystal is grown. When the amount to be added is very small, a preliminary ingot is often made with a larger content of the doping agent; a small slice of the ingot is then used to dope the next melt accurately. Impurities which have large diffusion constants in the material can be introduced directly by holding the solid material at an elevated temperature while this material is in contact with the doping agent in the solid or the vapor phase.

A doping technique, ion implantation, has been developed and used extensively. The impurity is introduced into a layer of semiconductor by causing a controlled dose of highly accelerated impurity ions to impinge on the semiconductor. See also Ion implantation.

An important subject of scientific and technological interest is amorphous semiconductors. In an amorphous substance the atomic arrangement has some short-range but no long-range order. The representative amorphous semiconductors are selenium, germanium, and silicon in their amorphous states, and arsenic and germanium chalcogenides, including such ternary systems as Ge-As-Te. Some amorphous semiconductors can be prepared by a suitable quenching procedure from the melt. Amorphous films can be obtained by vapor deposition.

Rectification in semiconductors

In semiconductors, narrow layers can be produced which have abnormally high resistances. The resistance of such a layer is nonohmic; it may depend on the direction of current, thus giving rise to rectification. Rectification can also be obtained by putting a thin layer of semiconductor or insulator material between two conductors of different material.

A narrow region in a semiconductor which has an abnormally high resistance is called a barrier layer. A barrier may exist at the contact of the semiconductor with another material, at a crystal boundary in the semiconductor, or at a free surface of the semiconductor. In the bulk of a semiconductor, even in a single crystal, barriers may be found as the result of a nonuniform distribution of impurities. The thickness of a barrier layer is small, usually 10⁻³ to 10⁻⁵ cm.

A barrier is usually associated with the existence of a space charge. In an intrinsic semiconductor, a region is electrically neutral if the concentration n of conduction electrons is equal to the concentration p of holes. Any deviation in the balance gives a space charge equal to e(p − n), where e is the charge on an electron. In an extrinsic semiconductor, ionized donor atoms give a positive space charge and ionized acceptor atoms give a negative space charge.

Surface electronics

The surface of a semiconductor plays an important role technologically, for example, in field-effect transistors and charge-coupled devices. Also, it presents an interesting case of two-dimensional systems where the electric field in the surface layer is strong enough to produce a potential wall which is narrower than the wavelengths of charge carriers. In such a case, the electronic energy levels are grouped into subbands, each of which corresponds to a quantized motion normal to the surface, with a continuum for motion parallel to the surface. Consequently, various properties cannot be trivially deduced from those of the bulk semiconductor. See also Charge-coupled devices; Surface physics.

A material that conducts (see conduction) electricity, but very poorly. Silicon is the most common and familiar semiconductor. Devices made from semiconductors, such as the transistor, are the basis of the modern microelectric industry.

A solid state material that can be electrically altered. Certain elements in nature, such as silicon, perform like semiconductors when chemically combined with other elements. Various optical materials can also change their state (see phase change disc).

When electricity or light is applied to semiconductors, they change their state between conductive and non-conductive or reflective and non-reflective. The most significant semiconductor is the transistor, which in digital circuits works like an on/off switch. For analog applications, it may be an on/off switch as well, but is more likely used as an amplifier, taking in a low-voltage signal and outputting a higher voltage. See n-type silicon, doping and chip.

Conceptual View of a Transistor

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Another word for "chip." A semiconductor is a material such as silicon, which conducts electrical charges but not as well as metals such as copper and aluminum.

Investopedia Says:
To recap: chip = semiconductor = integrated circuit. Semiconductors are used in computers, DVD players, cell phones, household appliances, and video games, along with many other products.

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Material, such as silicon, that is neither a good conductor nor a good insulator. Semiconductor devices, such as diodes, transistors, and integrated circuits, are the essential parts that make it possible to build computers and other small, inexpensive electronic machines.

A solid crystalline substance the electrical conductivity of which is intermediate between that of a conductor and an insulator.

Semiconductors are solid materials with a level of electrical conductivity between that of insulators and conductors. Beginning with semiconducting elements found in nature, such as silicon or germanium, scientists learned to enhance and manipulate conductivity by changing the configuration of electrons in the material through the combination of materials and the precise introduction of impurities. Because of their ability to control electrical currents, semiconductors have been used in the manufacture of a wide range of electronic devices—including computers—that changed American life during the second half of the twentieth century.

Although the scientific study of semiconductors began in the nineteenth century, concentrated investigation of their use did not begin until the 1930s. The development of quantum physics during the first third of the twentieth century gave scientists the theoretical tools necessary to understand the behavior of atoms in solids, including semiconductors. But it was a commercial need that really stimulated semiconductor research in the United States. The rapid growth of the national telephone network had by 1930 made the replacement of mechanical switches—too large and too slow for the expanding system—highly desirable. Vacuum tubes, used in radios and other devices, were too expensive and fragile for use in the telephone network, so researchers turned their focus to solid crystals. Radar research during World War II, through efforts to make reliable and sensitive transmitters and receivers, advanced understanding of the relative merits of different crystal substances. Germanium and silicon showed the most promise. Scientists at Bell Laboratories, the research arm of the AT&T Corporation, built upon wartime investigations done there and elsewhere to design the first transistor using the semiconductor germanium. A prototype was produced in 1947, and innovation followed rapidly. William Shockley, Walter Brattain, and John Bardeen, all Bell Labs researchers, were awarded the 1956 Nobel Prize in physics for their research on semi-conductors and the design of the transistor.

Transistors replaced vacuum tubes in electronic devices slowly at first. Hearingaids were the first technology to use the new, small transistors, but it was inexpensive portable radios that created the first large commercial market for the device. Initially manufactured by the Texas Instruments Company, the first large semiconductor company, "transistor" radios soon became a specialty of manufacturers in the Far East. Not limiting themselves to these consumer products and military signal devices, American researchers and manufacturers sought ways to use germanium-based transistors in computing machines. The more versatile silicon, however, ultimately replaced germanium to satisfy the needs of evolving computer technology.

The semiconductor silicon gave its name to a region—an area between San Jose and San Francisco, California, that became known as Silicon Valley—and fomented revolutions in technology, business, and culture. Silicon Valley grew outward from Palo Alto, home to Stanford University and host to a number of electronic pioneers beginning in the 1920s with the vacuum tube researcher Lee De Forest. Once scientists had determined that silicon had the necessary properties for applications in computing, practical concerns took center stage. Although silicon is one of the most common elements on earth—sand is made of silicon and oxygen—isolating and purifying it is notoriously difficult. But interest in silicon-based devices was very strong, and by the late 1950s a diversified semiconductor industry was developing, centered in California but serving government and commercial clients throughout the country.

The electronics industry initially turned to semiconducting materials to replace large, slow, electromechanical switches and fragile, unreliable vacuum tubes. But the new technology proved to be far more than an incremental improvement. Semiconductors showed promise for miniaturization and acceleration that previously seemed fanciful. An insatiable desire for faster, smaller devices became the driving force for the semiconductor industry. An impressive stream of innovations in theory, design, and manufacturing led the semiconductor industry to make ever-smaller, ever-faster devices for the next half century. Improvements in semiconductor devices led to faster, cheaper electronics of all kinds, and to the spread of the semiconductor and its dependent industries throughout the world.

Although the semiconductor industry was born and developed in the United States, manufacturing of silicon-based devices—including the "memory chips" that are most essential to computer and other electronic technologies—began to move overseas in the 1970s. Japan was a particularly strong participant in the manufacture of high-quality chips. While American companies were eager to buy from Japanese manufacturers, American semiconductor manufacturers turned to the government for support and market intervention. Struggles in the industry continued throughout the 1970s and 1980s but the great expansion of the market for computers and continuing innovation kept semiconductor-based businesses flourishing both in the United States and abroad.

Silicon, the premier semiconductor, belongs among a small number of other substances that have changed the course of history. Unlike earlier, comparably influential materials—salt and gold, for example—mastering the use of silicon required an enormous amount of research. In fact, silicon is the most studied substance in history. Semi-conductor science, and the industry it spawned, drew upon uniquely American elements in their development. Industrial research labs such as those at AT&T, IBM, and the entrepreneurial companies of Silicon Valley were vital to the development of the semiconductor industry, as was the government support of research during and after World War II. The military also influenced development as an important customer to the industry. The future will likely bring a replacement for silicon in the ongoing search for smaller, faster electronic devices, but silicon has earned a most valuable place in the history of technology and twentieth-century culture.

Bibliography

Bassett, Ross. To the Digital Age: Research Labs, Startup Companies, and the Rise of MOS Technology. Baltimore: Johns Hopkins University Press, 2002.

Misa, Thomas J. "Military Needs, Commercial Realities, and the Development of the Transistor." In Military Enterprise and Technological Change: Perspectives on the American Experience. Edited by Merritt Roe Smith. Cambridge, Mass.: MIT Press, 1987.

Queisser, Hans. The Conquest of the Microchip. Cambridge, Mass.: Harvard University Press, 1990.

For information on devices using semiconductors and their history, see semiconductor device.

This article is about the material. For the artists, see Semiconductor (artists).

A semiconductor is a material that has an electrical conductivity between that of a conductor and an insulator, that is, generally in the range 10³ Siemens/cm to 10⁻⁸ S/cm. Devices made from semiconductor materials are the foundation of modern electronics, including radio, computers, telephones, and many other devices. Semiconductor devices include the various types of transistor, solar cells, many kinds of diodes including the light-emitting diode, the silicon controlled rectifier, and digital and analog integrated circuits. Solar photovoltaic panels are large semiconductor devices that directly convert light energy into electrical energy. An external electrical field may change a semiconductor's resistivity. In a metallic conductor, current is carried by the flow of electrons. In semiconductors, current can be carried either by the flow of electrons or by the flow of positively-charged "holes" in the electron structure of the material.

Common semiconducting materials are crystalline solids but amorphous and liquid semiconductors are known, such as mixtures of arsenic, selenium and tellurium in a variety of proportions. They share with better known semiconductors intermediate conductivity and a rapid variation of conductivity with temperature but lack the rigid crystalline structure of conventional semiconductors such as silicon and so are relatively insensitive to impurities and radiation damage.

Silicon is used to create most semiconductors commercially. Dozens of other materials are used, including germanium, gallium arsenide, and silicon carbide. A pure semiconductor is often called an “intrinsic” semiconductor. The conductivity, or ability to conduct, of common semiconductor materials can be drastically changed by adding other elements, called “impurities” to the melted intrinsic material and then allowing the melt to solidify into a new and different crystal. This process is called "doping".^[1]

Contents 1 Explaining semiconductor energy bands 2 Energy bands and electrical conduction 3 Holes: electron absence as a charge carrier 4 Energy–momentum dispersion 5 Carrier generation and recombination 6 Semi-insulators 7 Doping 7.1 Dopants 7.2 Carrier concentration 7.3 Effect on band structure 8 Preparation of semiconductor materials 9 See also 10 References 11 External links

Explaining semiconductor energy bands

There are three popular ways to classify the electronic structure of a crystal.

• Band structure

For more details on this topic, see Electronic band structure.

atoms - crystal - vacuum

In a single H-atom an electron resides in well known orbits. Note that the orbits are called s,p,d in order of increasing circular current.	Putting two atoms together leads to delocalized orbits across two atoms, yielding a covalent bond. Due to the Pauli exclusion principle, every state can contain only one electron.	This can be continued with more atoms. Note: This picture shows a metal, not an actual semiconductor.	Continuing to add creates a crystal, which may then be cut into a tape and fused together at the ends to allow circular currents.	For this regular solid the band structure can be calculated or measured.
Integrating over the k axis gives the bands of a semiconductor showing a full valence band and an empty conduction band. Generally stopping at the vacuum level is undesirable, because some people want to calculate: photoemission, inverse photoemission	After the band structure is determined states can be combined to generate wave packets. As this is analogous to wave packages in free space, the results are similar.	An alternative description, which does not really appreciate the strong Coulomb interaction, shoots free electrons into the crystal and looks at the scattering.	A third alternative description uses strongly localized unpaired electrons in chemical bonds, which looks almost like a Mott insulator.

Energy bands and electrical conduction

Like in other solids, the electrons in semiconductors can have energies only within certain bands (ie. ranges of levels of energy) between the energy of the ground state, corresponding to electrons tightly bound to the atomic nuclei of the material, and the free electron energy, which is the energy required for an electron to escape entirely from the material. The energy bands each correspond to a large number of discrete quantum states of the electrons, and most of the states with low energy (closer to the nucleus) are full, up to a particular band called the valence band. Semiconductors and insulators are distinguished from metals because the valence band in the semiconductor materials is very nearly full under usual operating conditions, thus causing more electrons to be available in the "conduction band," which is the band immediately above the valence band.

The ease with which electrons in a semiconductor can be excited from the valence band to the conduction band depends on the band gap between the bands, and it is the size of this energy bandgap that serves as an arbitrary dividing line (roughly 4 eV) between semiconductors and insulators.

In the picture of covalent bonds, an electron moves by hopping to a neighboring bond. Because of the Pauli exclusion principle it has to be lifted into the higher anti-bonding state of that bond. In the picture of delocalized states, for example in one dimension - that is in a nanowire, for every energy there is a state with electrons flowing in one direction and one state for the electrons flowing in the other. For a net current to flow some more states for one direction than for the other direction have to be occupied and for this energy is needed. For a metal this can be a very small energy in the semiconductor the next higher states lie above the band gap. Often this is stated as: full bands do not contribute to the electrical conductivity. However, as the temperature of a semiconductor rises above absolute zero, there is more energy in the semiconductor to spend on lattice vibration and — more importantly for us — on lifting some electrons into an energy states of the conduction band. The current-carrying electrons in the conduction band are known as "free electrons", although they are often simply called "electrons" if context allows this usage to be clear.

Electrons excited to the conduction band also leave behind electron holes, or unoccupied states in the valence band. Both the conduction band electrons and the valence band holes contribute to electrical conductivity. The holes themselves don't actually move, but a neighboring electron can move to fill the hole, leaving a hole at the place it has just come from, and in this way the holes appear to move, and the holes behave as if they were actual positively charged particles.

One covalent bond between neighboring atoms in the solid is ten times stronger than the binding of the single electron to the atom, so freeing the electron does not imply destruction of the crystal structure.

Holes: electron absence as a charge carrier

The motion of holes, which was introduced for semiconductors, can also be applied to metals, where the Fermi level lies within the conduction band. With most metals the Hall effect reveals electrons to be the charge carriers, but some metals have a mostly filled conduction band, and the Hall effect reveals positive charge carriers, which are not the ion-cores, but holes. Contrast this to some conductors like solutions of salts, or plasma. In the case of a metal, only a small amount of energy is needed for the electrons to find other unoccupied states to move into, and hence for current to flow. Sometimes even in this case it may be said that a hole was left behind, to explain why the electron does not fall back to lower energies: It cannot find a hole. In the end in both materials electron-phonon scattering and defects are the dominant causes for resistance.

Fermi-Dirac distribution. States with energy ε below the Fermi energy, here µ, have higher probability n to be occupied, and those above are less likely to be occupied. Smearing of the distribution increases with temperature.

The energy distribution of the electrons determines which of the states are filled and which are empty. This distribution is described by Fermi-Dirac statistics. The distribution is characterized by the temperature of the electrons, and the Fermi energy or Fermi level. Under absolute zero conditions the Fermi energy can be thought of as the energy up to which available electron states are occupied. At higher temperatures, the Fermi energy is the energy at which the probability of a state being occupied has fallen to 0.5.

The dependence of the electron energy distribution on temperature also explains why the conductivity of a semiconductor has a strong temperature dependency, as a semiconductor operating at lower temperatures will have fewer available free electrons and holes able to do the work.

Energy–momentum dispersion

In the preceding description an important fact is ignored for the sake of simplicity: the dispersion of the energy. The reason that the energies of the states are broadened into a band is that the energy depends on the value of the wave vector, or k-vector, of the electron. The k-vector, in quantum mechanics, is the representation of the momentum of a particle.

The dispersion relationship determines the effective mass, m^*, of electrons or holes in the semiconductor, according to the formula:

The effective mass is important as it affects many of the electrical properties of the semiconductor, such as the electron or hole mobility, which in turn influences the diffusivity of the charge carriers and the electrical conductivity of the semiconductor.

Typically the effective mass of electrons and holes are different. This affects the relative performance of p-channel and n-channel IGFETs.^[2]

The top of the valence band and the bottom of the conduction band might not occur at that same value of k. Materials with this situation, such as silicon and germanium, are known as indirect bandgap materials. Materials in which the band extrema are aligned in k, for example gallium arsenide, are called direct bandgap semiconductors. Direct gap semiconductors are particularly important in optoelectronics because they are much more efficient as light emitters than indirect gap materials.

Carrier generation and recombination

For more details on this topic, see Carrier generation and recombination.

When ionizing radiation strikes a semiconductor, it may excite an electron out of its energy level and consequently leave a hole. This process is known as electron–hole pair generation. Electron-hole pairs are constantly generated from thermal energy as well, in the absence of any external energy source.

Electron-hole pairs are also apt to recombine. Conservation of energy demands that these recombination events, in which an electron loses an amount of energy larger than the band gap, be accompanied by the emission of thermal energy (in the form of phonons) or radiation (in the form of photons).

In some states, the generation and recombination of electron–hole pairs are in equipoise. The number of electron-hole pairs in the steady state at a given temperature is determined by quantum statistical mechanics. The precise quantum mechanical mechanisms of generation and recombination are governed by conservation of energy and conservation of momentum.

As the probability that electrons and holes meet together is proportional to the product of their amounts, the product is in steady state nearly constant at a given temperature, providing that there is no significant electric field (which might "flush" carriers of both types, or move them from neighbour regions containing more of them to meet together) or externally driven pair generation. The product is a function of the temperature, as the probability of getting enough thermal energy to produce a pair increases with temperature, being approximately exp(−E_G / kT), where k is Boltzmann's constant, T is absolute temperature and E_G is band gap.

The probability of meeting is increased by carrier traps – impurities or dislocations which can trap an electron or hole and hold it until a pair is completed. Such carrier traps are sometimes purposely added to reduce the time needed to reach the steady state.

Semi-insulators

Some materials are classified as semi-insulators. These have electrical conductivity nearer to that of electrical insulators. Semi-insulators find niche applications in micro-electronics, such as substrates for HEMT. An example of a common semi-insulator is gallium arsenide.^[3]

Doping

For more details on this topic, see Doping (semiconductor).

The property of semiconductors that makes them most useful for constructing electronic devices is that their conductivity may easily be modified by introducing impurities into their crystal lattice. The process of adding controlled impurities to a semiconductor is known as doping. The amount of impurity, or dopant, added to an intrinsic (pure) semiconductor varies its level of conductivity. Doped semiconductors are often referred to as extrinsic. By adding impurity to pure semiconductors, the electrical conductivity may be varied not only by the number of impurity atoms but also, by the type of impurity atom and the changes may be thousand folds and million folds. For example, 1 cm³ of a metal or semiconductor specimen has a number of atoms on the order of 10²². Since every atom in metal donates at least one free electron for conduction in metal, 1 cm³ of metal contains free electrons on the order of 10²². At the temperature close to 20 °C , 1 cm³ of pure germanium contains about 4.2×10²² atoms and 2.5×10¹³ free electrons and 2.5×10¹³ holes (empty spaces in crystal lattice having positive charge) The addition of 0.001% of arsenic (an impurity) donates an extra 10¹⁷ free electrons in the same volume and the electrical conductivity increases about 10,000 times."

Dopants

The materials chosen as suitable dopants depend on the atomic properties of both the dopant and the material to be doped. In general, dopants that produce the desired controlled changes are classified as either electron acceptors or donors. A donor atom that activates (that is, becomes incorporated into the crystal lattice) donates weakly-bound valence electrons to the material, creating excess negative charge carriers. These weakly-bound electrons can move about in the crystal lattice relatively freely and can facilitate conduction in the presence of an electric field. (The donor atoms introduce some states under, but very close to the conduction band edge. Electrons at these states can be easily excited to the conduction band, becoming free electrons, at room temperature.) Conversely, an activated acceptor produces a hole. Semiconductors doped with donor impurities are called n-type, while those doped with acceptor impurities are known as p-type. The n and p type designations indicate which charge carrier acts as the material's majority carrier. The opposite carrier is called the minority carrier, which exists due to thermal excitation at a much lower concentration compared to the majority carrier.

For example, the pure semiconductor silicon has four valence electrons. In silicon, the most common dopants are IUPAC group 13 (commonly known as group III) and group 15 (commonly known as group V) elements. Group 13 elements all contain three valence electrons, causing them to function as acceptors when used to dope silicon. Group 15 elements have five valence electrons, which allows them to act as a donor. Therefore, a silicon crystal doped with boron creates a p-type semiconductor whereas one doped with phosphorus results in an n-type material.

Carrier concentration

The concentration of dopant introduced to an intrinsic semiconductor determines its concentration and indirectly affects many of its electrical properties. The most important factor that doping directly affects is the material's carrier concentration. In an intrinsic semiconductor under thermal equilibrium, the concentration of electrons and holes is equivalent. That is,

n = p = n_i

If we have a non-intrinsic semiconductor in thermal equilibrium the relation becomes:

n₀·p₀ = (n_i)²

Where n₀ is the concentration of conducting electrons, p₀ is the electron hole concentration, and n_i is the material's intrinsic carrier concentration. Intrinsic carrier concentration varies between materials and is dependent on temperature. Silicon's n_i, for example, is roughly 1.0×10¹⁰ cm⁻³ at 300 kelvin (room temperature).

In general, an increase in doping concentration affords an increase in conductivity due to the higher concentration of carriers available for conduction. Degenerately (very highly) doped semiconductors have conductivity levels comparable to metals and are often used in modern integrated circuits as a replacement for metal. Often superscript plus and minus symbols are used to denote relative doping concentration in semiconductors. For example, n⁺ denotes an n-type semiconductor with a high, often degenerate, doping concentration. Similarly, p^- would indicate a very lightly doped p-type material. It is useful to note that even degenerate levels of doping imply low concentrations of impurities with respect to the base semiconductor. In crystalline intrinsic silicon, there are approximately 5×10²² atoms/cm³. Doping concentration for silicon semiconductors may range anywhere from 10¹³ cm⁻³ to 10¹⁸ cm⁻³. Doping concentration above about 10¹⁸ cm⁻³ is considered degenerate at room temperature. Degenerately doped silicon contains a proportion of impurity to silicon in the order of parts per thousand. This proportion may be reduced to parts per billion in very lightly doped silicon. Typical concentration values fall somewhere in this range and are tailored to produce the desired properties in the device that the semiconductor is intended for.

Effect on band structure

Band diagram of a p⁺n junction. The band bending is a result of the positioning of the Fermi levels in the p⁺ and n sides.

Doping a semiconductor crystal introduces allowed energy states within the band gap but very close to the energy band that corresponds to the dopant type. In other words, donor impurities create states near the conduction band while acceptors create states near the valence band. The gap between these energy states and the nearest energy band is usually referred to as dopant-site bonding energy or E_B and is relatively small. For example, the E_B for boron in silicon bulk is 0.045 eV, compared with silicon's band gap of about 1.12 eV. Because E_B is so small, it takes little energy to ionize the dopant atoms and create free carriers in the conduction or valence bands. Usually the thermal energy available at room temperature is sufficient to ionize most of the dopant.

Dopants also have the important effect of shifting the material's Fermi level towards the energy band that corresponds with the dopant with the greatest concentration. Since the Fermi level must remain constant in a system in thermodynamic equilibrium, stacking layers of materials with different properties leads to many useful electrical properties. For example, the p-n junction's properties are due to the energy band bending that happens as a result of lining up the Fermi levels in contacting regions of p-type and n-type material.

This effect is shown in a band diagram. The band diagram typically indicates the variation in the valence band and conduction band edges versus some spatial dimension, often denoted x. The Fermi energy is also usually indicated in the diagram. Sometimes the intrinsic Fermi energy, E_i, which is the Fermi level in the absence of doping, is shown. These diagrams are useful in explaining the operation of many kinds of semiconductor devices.

Preparation of semiconductor materials

Semiconductors with predictable, reliable electronic properties are necessary for mass production. The level of chemical purity needed is extremely high because the presence of impurities even in very small proportions can have large effects on the properties of the material. A high degree of crystalline perfection is also required, since faults in crystal structure (such as dislocations, twins, and stacking faults) interfere with the semiconducting properties of the material. Crystalline faults are a major cause of defective semiconductor devices. The larger the crystal, the more difficult it is to achieve the necessary perfection. Current mass production processes use crystal ingots between 100 mm and 300 mm (4-12 inches) in diameter which are grown as cylinders and sliced into wafers.

Because of the required level of chemical purity and the perfection of the crystal structure which are needed to make semiconductor devices, special methods have been developed to produce the initial semiconductor material. A technique for achieving high purity includes growing the crystal using the Czochralski process. An additional step that can be used to further increase purity is known as zone refining. In zone refining, part of a solid crystal is melted. The impurities tend to concentrate in the melted region, while the desired material recrystalizes leaving the solid material more pure and with fewer crystalline faults.

In manufacturing semiconductor devices involving heterojunctions between different semiconductor materials, the lattice constant, which is the length of the repeating element of the crystal structure, is important for determining the compatibility of materials.

See also

Electronics portal

• Effective mass
• Electronic band structure
• Bloch waves
• Tight-binding model
• Electron mobility
• Exciton
• Luttinger parameter
• Semiconductor device fabrication
• List of semiconductor materials
• Semiconductor industry
• Quantum tunneling
• Semiconductor chip
• Thin film

References

1. ^ International Union of Pure and Applied Chemistry. "semiconductor". Compendium of Chemical Terminology Internet edition.
2. ^ Muller, Richard S.; Theodore I. Kamins (1986). Device Electronics for Integrated Circuits (2d ed.). New York: Wiley. p. 427. ISBN 0-471-88758-7. 
3. ^ J. W. Allen (1960). "Gallium Arsenide as a semi-insulator". Nature 187: 403-405. doi:10.1038/187403b0. 

Notes

• A. A. Balandin and K. L. Wang (2006), Handbook of Semiconductor Nanostructures and Nanodevices (5-Volume Set), American Scientific Publishers. ISBN 1-58883-073-X
• Sze, Simon M. (1981). Physics of Semiconductor Devices (2nd ed.). John Wiley and Sons (WIE). ISBN 0-471-05661-8. 
• Turley, Jim (2002). The Essential Guide to Semiconductors. Prentice Hall PTR. ISBN 0-13-046404-X. 
• Yu, Peter Y.; Cardona, Manuel (2004). Fundamentals of Semiconductors : Physics and Materials Properties. Springer. ISBN 3-540-41323-5. 

• Ben G. Steetman and Sanjay Kumar Banerjee (2006), Solid State Electronic Devices, Pearson Education Inc. ISBN 0-13-149726-X

External links

• Howstuffworks' semiconductor page
• Semiconductor Concepts at Hyperphysics
• Semiconductor OneSource Hall of Fame, Glossary
• Principles of Semiconductor Devices by Bart Van Zeghbroeck, University of Colorado. An online textbook]
• US Navy Electrical Engineering Training Series
• NSM-Archive Physical Properties of Semiconductors]
• Amkor Technology Semiconductor Assembly & Test
• Semiconductor Experts

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Dansk (Danish)
n. - halvleder

Nederlands (Dutch)
halfgeleider

Français (French)
n. - semi-conducteur

Deutsch (German)
n. - Halbleiter

Ελληνική (Greek)
n. - (φυσ.) ημιαγωγός

Italiano (Italian)
semiconduttore

Português (Portuguese)
n. - semicondutor (m)

Русский (Russian)
полупроводник

Español (Spanish)
n. - semiconductor

Svenska (Swedish)
n. - halvledare

中文（简体）(Chinese (Simplified))
半导体

中文（繁體）(Chinese (Traditional))
n. - 半導體

한국어 (Korean)
n. - 반도체, 반도체를 이용한 장치(트랜지스터, IC등)

日本語 (Japanese)
n. - 半導体

العربيه (Arabic)
‏(الاسم) شبه موصل‏

עברית (Hebrew)
n. - ‮חומר מוצק שאינו מוליך חשמל במצבו הטהור ובחום נמוך אך נעשה למוליך כשהתנאים משתנים, מוליך-למחצה‬

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