Semiconductor
Semiconductors are solids that are dependent on their condition electric conductors or non-conductors. Semiconductors may have different chemical structures. A distinction is made between elemental semiconductors, which are composed of a single element and compound semiconductors, in this case specifically the organic semiconductors.
The electrical conductivity of semiconductors is strongly temperature dependent. In the near absolute zero temperature point semiconductors are insulators. At room temperature they are conductive or non-conductive material, depending on the specific distance of conduction and valence bands. The electrical conductivity of semiconductors increases with increasing temperature, so that they belong to the thermistors. Furthermore, the conductivity can be influenced specifically within wide limits by the introduction of foreign atoms ( doping ) from another major chemical group.
Semiconductors are used in monocrystalline, polycrystalline and amorphous form. Significance of semiconductors for electrical engineering and especially for electronics, this can be their conductivity by applying a control voltage or current control, such as the transistor to the appropriate structures are changed or they have a direction-dependent conductivity ( diode rectifier ). Other applications include thermistors, varistors, radiation sensors ( photoconductor, photoresistors, photodiodes or solar cells), thermoelectric generators, Peltier elements and radiation or light sources (laser diode, light emitting diode).
History
Stephen Gray discovered in 1727 the difference between conductors and non-conductors. After Georg Simon Ohm 1821 Ohm's law aufstellte, so that the proportionality between current and voltage will be described in an electrical conductor, the conductivity of an object could be determined.
The Nobel Prize winner Ferdinand Braun discovered the rectifying effect of semiconductors in 1874, he wrote: . " [ ... ] In a large number of natural and artificial sulphides [ ... ] the resistance of them was different with the direction, intensity and duration of the current. The differences of up to 30 % of the whole value. " He described the first time that the ohmic resistance can be variable.
Greenleaf Whittier Pickard received in 1906 the first patent for a silicon-based tip diode for demodulating the carrier signal in a detector receiver. Initially, in the same receiver ( " Pickard Crystal Radio Kit" ) used mostly galena as semiconductors, which emerged in the 1920s, more robust and more powerful diodes based on copper sulfide -copper contacts. The operation of the based on a semiconductor -metal transition rectifier effect remained unexplained despite technical application over decades. Only Walter Schottky in 1939 could lay the theoretical foundations for the description of named after him Schottky diode.
The first patent for the transistor principle was filed in 1925 by Julius Edgar Lilienfeld ( U.S. physicist Austro- Hungarian origin ). Lilienfeld describes in his work an electronic component that with today's field-effect transistors is comparable in its broadest sense, he lacked the time the technologies necessary to implement field-effect transistors practical.
Been able When in 1947 at Bell Laboratories scientists John Bardeen, William Bradford Shockley and Walter Houser Brattain put two metal wire tips on a germanium plate and thus the p- type region with the second wire tip control with an electrical voltage, the point-contact transistor ( bipolar ) was realized. This earned them the Nobel Prize in Physics in 1956 and founded the Microelectronics.
The production of high purity silicon succeeded in 1954 Eberhard Spenke and his team in the Siemens & Halske AG with the zone melting process. This brought the mid-1950s along with the availability of an insulating material ( silica) with favorable properties ( not water soluble as germanium, easy to manufacture, etc. ) the breakthrough of silicon as a semiconductor material for the electronics industry and about 30 years later for the first products of microsystems technology. Today ( 2009) is almost exclusively used with the Czochralski method, cheaper silicon produced in the production of integrated circuits.
Alan Heeger, Alan MacDiarmid and Hideki Shirakawa, 1976 showed that with a doping of polyacetylene, - a polymer, which is an insulator in the undoped state - with oxidizing agents, the electrical resistivity of up to 10-5 Ω · m ( silver: ≈ 10-8 OMEGA.m ) may decrease. In 2000, they received the Nobel Prize in Chemistry (see Section organic semiconductors ).
Classification
Can be classified into two groups in the semiconductor microelectronics used, the elemental semiconductors and compound semiconductors. The semiconductor element include elements having four valence electrons, such as silicon (Si) and germanium (Ge). The group of the compound semiconductor comprises chemical compounds that have four valence electrons in the agent. These include compounds of elements of III. with the main group V of the Periodic Table (III -V semiconductors ), such as gallium arsenide (GaAs ) or indium antimonide ( InSb ), and the II side with the VI. Main group of (II -VI semiconductors ) such as zinc selenide ( ZnSe) and cadmium sulfide ( CdS).
In addition to these commonly used semiconductors, there is the I-VII semiconductors, such as copper ( I) chloride. And materials which do not have four valence electrons, on average, may be referred to as a semiconductor when Ω have a resistivity in the range of greater than · 10-4 m and less than 106 Ω · m. A group of promising new semiconductor for example, the organic semiconductors, which have already been used in organic solar cells or organic field effect transistors. Another group are carbon nanotubes, which can be used, for example, carbon nanotube field effect transistor.
Crystalline Semiconductors
Physical Basics
Zinc blende structure ( unit cell)
The semiconductor properties of substances back to their chemical bonds and thus their atomic structure. Semiconductors can crystallize in different structures, silicon and germanium crystallize in the diamond structure (pure covalent bond ), III-V and II -VI compound semiconductor, however, mostly in the zinc blende structure ( mixed covalent- ionic bonding ).
The basic properties of crystalline semiconductors can be explained on the basis of the band model: the electrons in solids interact over many atomic distances away together. This leads in fact to a widening of the (still present as discrete levels in the single-atom ) potential energy values to extensive areas of energy, the so-called energy bands. Since the energy bands are different from each other depending on the expansion and atomic species, bands may overlap or exist through energy areas in which, according to quantum mechanics no allowed states ( energy or band gap), be separated.
In semiconductors, the highest occupied energy band ( valence band ) and the next higher band ( conduction band ) by a band gap are now separated. The Fermi level is located exactly in the band gap. At a temperature near absolute zero the valence band is full and the conduction band completely free of charge carriers. Since unoccupied bands lack of mobile charge carriers do not conduct electric current and charge carriers in crowded bands lack of accessible free states can not absorb energy, which leads to limited mobility, guided semiconductor electrical current not at a temperature close to absolute zero.
Partially occupied bands are required for the conduction process, which can be found in metals by an overlap of the outer bands at each temperature. This is - as mentioned above - not given in semiconductors and insulators. The band gap ( " forbidden band " or "forbidden zone ") of that semiconductors, however (> 4 eV EC) is in contrast to insulators relatively small ( InAs: ≈ 0.4 eV, Ge: ≈ 0.7 eV, Si: ≈ 1.1 eV, GaAs: ≈ 1.4 eV, SiC: ≈ 2.39 to 3.33 eV, diamond: ≈ 5.45 eV ), so that, for example, by the energy of thermal vibrations at room temperature or by absorption of light electrons can be excited from the valence band to the conduction band fully occupied. Therefore have an intrinsic semiconductor, increasing with temperature electrical conductivity. Therefore semiconductors are also counted among the hot conductors. The transition from semiconductors to insulators is fluid. For example, gallium nitride (GaN, used in blue LEDs ) is also counted with a band gap energy of ≈ 3.2 eV to the semiconductors. Semiconductors are significantly larger than 1 eV, with a band gap referred to as a semiconductor with a large band gap (English wide bandgap semiconductor).
, As described above, an electron in a semiconductor excited from the valence band into the conduction band, it leaves in its original location a defective electron, referred to as " hole". Bound valence electrons in the neighborhood of such holes can change place in a hole "jump", this moves the hole. It can therefore be considered as moving positive charge. Both the excited electron and the hole- thus contribute to electrical conduction.
Electrons from the conduction band electrons can recombine with the defect ( electron-hole recombination). This transition between the involved levels can be done with the release of electromagnetic recombination radiation ( photon) and / or the delivery of a pulse to the crystal lattice ( phonon ).
Direct and indirect semiconductors
Semiconductors are classified into two groups, the direct and indirect semiconductors. Their different properties can be understood only by considering the band structure in the so-called momentum space: the charge carriers in semiconductors can be understood as matter waves with a quasi-momentum. Within a band the energy from the quasi-momentum dependent (often expressed as a wave vector ) from.
The extreme values of energy within the bands, so the band edges are at different wave vectors - where exactly, depends on the material and the structure. When an electron is excited from the valence band into the conduction band, it is energetically more preferred (and thus most likely ) when it is excited by the maximum of the valence band to the conduction band minimum.
These extremes lie almost at the same quasi- pulse excitation by a photon, for example, it is readily possible, since only the electron 's energy, but not need to change his pulse. One speaks of a direct semiconductor. However, the extremes are in different quasi pulses, the electron must also change to its power and its momentum, to be excited into the conduction band. This impulse can not by a photon ( having a very small pulse) are, but must be of a lattice vibration ( phonon also ) be contributed.
In the recombination of electrons -hole pairs in the principle is the same. In a direct semiconductor can be emitted during the recombination a photon. In an indirect semiconductor contrast to the photon, the energy would still have a phonon for momentum generated (or absorbed) and are the radiative recombination is less likely. It then often dominate other non-radiative recombination, for example, impurities. It follows that only direct semiconductors can be used for the effective generation of radiation. Direct and indirect semiconductors are distinguished from each other by means of absorption experiments. In general, elemental semiconductors are ( silicon, germanium) and compound semiconductors of group IV indirect and compound semiconductors from different major groups (III / V: GaAs, InP, GaN) directly.
Wherein a band structure in momentum space are possible in the vicinity of the valence band line or number of points, there may be the so-called Gunn effect.
Intrinsic semiconductor and extrinsic semiconductor
The density of free electrons and holes in pure, i.e. undoped semiconductors, intrinsic charge carrier density or intrinsic density mentioned - a i-type semiconductor is therefore also called intrinsic semiconductor, is the dominant conduction mechanism is intrinsic. The charge carrier density in the undoped semiconductor is heavily dependent on the temperature and increasing with it on. However, the concentration of the charge carriers in the conduction band (electrons) in the valence band, respectively (holes ) is determined by the dopant, one speaks of a extrinsic semiconductor or extrinsic semiconductors - here the dominant conduction mechanism is extrinsic.
Doping and extrinsic
Donors and acceptors
By introducing impurities in a semiconductor crystal, the electrical properties of the ( pure ) semiconductor can be influenced. Impurities are impurities which differ for example in their valence of the atoms of the host material, examples are boron or phosphorus in a silicon crystal. The process is commonly referred to as a dopant or as " doping ". Furthermore, by localized doping of the combination of differently doped regions different components, for example, a bipolar transistor can be manufactured.
The introduction of impurities generates additional, locally -bound energy levels in the band diagram of the crystal. The levels are generally in the otherwise present for the host material energy gap ( band gap) between the valence and conduction band. Due to the lower compared to undoped semiconductors energy differences of the " intermediate levels " for the valence or conduction band, these levels can be more easily stimulated, thus providing mobile charge carriers available. The chemical potential is shifted from the center of the band gap in the vicinity of the additional levels. Therefore, there are more charge carriers for conduction of electricity available, which manifests itself in an over pure semiconductors increased conductivity. This is called conduction mechanism therefore extrinsic. There are two main ways of doing impurities: donors and acceptors.
As (electron ) donors (Latin donare = give ) impurity atoms are called, the one more electron in the valence band than the pure semiconductor is referred to such areas as n- doped semiconductors. If such impurities incorporated into the semiconductor ( substituted ) so brings each of these impurities (in the case of phosphorus-doped silicon ), with an electron that is not required for binding and can be easily peeled. It forms a trap level in the vicinity of the lower energy of the conduction band.
Be ( assume Latin accipere = ) as ( electron ) acceptor impurities, respectively, having one less electron in the valence band analog. This electron is missing for the bond to the neighboring atom. They act as an additional defect electron ( hole ) with ( p- doping), which can be easily occupied by valence band electrons - therefore can also be found in some considerations of the term holes donors. In the band diagram of such a defect level lies just above the valence band edge.
In an intrinsic semiconductor, the carrier concentration of electrons and holes is equal to ( electron-hole pairs). Therefore, both types of charge carriers are involved in approximately equal parts on the charge transport. Through the introduction of donors and acceptors can specifically influence this balance. When doping with donors mainly provide the electrons in the conduction band, when doped with acceptors who thought positively charged holes in the valence band for electrical conductivity. In the first case we speak of electron conduction or n-type conductivity ( n → negative), in the other case of hole conduction or p-type conductivity (p → positive). Semiconductor regions with an excess of electrons is called ( as mentioned above) as n-doped, those with a shortage, so with "holes surplus ", as a p- doped. In the n-type conductor, the electrons are called majority carriers (mostly known carriers ), the holes as minority carriers in the p-type conductor, the corresponding inverse applies. Through a clever combination of n -and p- doped regions (see pn junction ), you can build individual, so-called discrete semiconductor devices such as diodes and transistors and complex, made up of many components constructed in a single crystal integrated circuits.
Conduction mechanisms in doped semiconductors
At absolute zero (T = 0 K) is doped and undoped semiconductors differ in the charge carrier density is not - there is not enough energy available to excite electrons into the conduction band or impurity levels. If the temperature increases (so that the available energy increases by thermal excitation ), the situation changes. Since the energy differences of the impurity to the valence or conduction bands are very much less than the band gap, electrons can be excited to the conduction band or holes in the valence band of the acceptor from the donor level. There are a function of the temperature of free charge carriers available, the conductivity of doped semiconductors increases. Since not all impurity levels are ionized or occupied, referred to this area as Störstellenreserve. If the temperature is further increased until all impurity levels are ionized or occupied, one speaks of impurity exhaustion. The carrier density and thus the conductivity in this area is dependent essentially only on the doping concentration. In a still further increase in the temperature then is enough energy available to electrons directly from the valence band to increase in the conduction band. Since typical doping concentrations are significantly lower than the number of semiconductor atoms (at least 6 orders of magnitude) outweighs the carrier -generation of electron- hole pairs, this area is referred to as intrisisch or the intrinsic semiconductor.
Interfacial
The combination of a p-doped and an n- doped semiconductor is formed at the interface, a pn junction. The combination of a doped semiconductor, a metal (e.g., Schottky diode ) or a non-conductor is also of interest, and, when two semiconductor such as gallium arsenide and aluminum gallium arsenide, one above the other, creating a heterojunction. Here are not only the pn junctions of importance but also pp- transitions and nn - transitions, the so-called isotype hetero -junctions, which are used for example in a quantum well.
Recently there are efforts, semiconductors, superconductors, and silicon and III -V semiconductors on a chip to merge. Since the crystal structures are not compatible, arise in the interface breaks and lattice defects if it is not possible to find suitable materials for a few atomic layers thick intermediate layer in which the lattice spacings can adjust.
Semi Magnetic Semiconductors
Semi Magnetic semiconductors belong to the major group of the compound semiconductor (English compound semiconductors ). There are compounds such as gallium nitride (GaN), in which an ion has been replaced by, for example, manganese. A characteristic feature of these semi- magnetic semiconductor is the large Zeeman effect. In English, it is called semi- magnetic semiconductors diluted magnetic semiconductors, since they are diluted magnetic.
Organic Semiconductors
In general, organic materials are electrically insulating. Molecules or polymers have a conjugated bond system comprising double bonds, triple bonds and aromatic rings, these can also be electrically conductive and are used as the organic semiconductor. This was first observed in 1976 polyacetylene. Polyacetylene is an unbranched polymer with alternating double bond and single bond (-C = C- C = C-). Will this plastic still an acceptor such as chlorine, bromine or iodine added (oxidative doping), are additional holes. By the addition of a donor such as sodium (reductive dopant ) of the plastic receives additional electrons. Through this chemical change, the double bonds break, and there is a continuous conduction band: The original non-conductive polymer is electrically conductive. Own molecules or polymers in the undoped state semiconducting properties, is referred to as inorganic semiconductors of the intrinsic conductivity (proper conductivity), such as pentacene or poly (3- hexylthiophene ). When the plastic material is produced in the form of a thin layer ( about 5 nm to 1 micron in thickness), it is arranged enough to form an electrically continuous layer.
Applications
Semiconductors are used in electronics in a variety of forms. The corresponding subregion is referred to as semiconductor electronics. This mainly includes the semiconductor-based integrated circuits (ICs such as microprocessors, microcontrollers, etc. ) and various power devices (eg IGBT ). Other areas of application of increasing importance are the photovoltaic ( solar cells) and detectors and radiation sources in optics and optoelectronics (eg photodetectors and light emitting diodes). The Department, which is dealt with the fabrication of semiconductor- based microelectronic components and assemblies referred to as semiconductor technology. This requires knowledge of how the semiconductor has to be machined to achieve the desired electrical characteristics. These include the doping of the semiconductor and the shapes of the interface between the semiconductor and another material.
Economy
The polysilicon market is currently (2010) in transition. After polysilicon was in great demand due to the high requirements from the solar market in 2008/2009, the price rose sharply. This has a number of companies prompted to begin the construction of new production facilities. The established manufacturers also expanded capacity. In addition push new providers - mainly from Asia - to the market. Which of these manufacturers will be able to take his assets as announced in operation and yet profitable to act in a sharp decline in prices is uncertain.
The world's largest maker of wafers is the Japanese company Shin- Etsu Handotai ( SEH ) with a wafer sales of $ 4 billion in 2007., The world's second largest, also Japanese manufacturer Sumitomo Mitsubishi Silicon Corp.. ( Sumco ) had a turnover of $ 2.7 billion in the same year. This is followed by the German Siltronic AG ( Wacker) by $ 1.8 billion and the U.S. company MEMC Electronic Materials with 1.2 billion dollars. These four companies share about 75 % of the entire wafer market of 12.5 billion dollars.
During the global financial crisis, sales almost halved, 2009, only silicon for $ 6.7 billion was implemented. By 2010, sales had recovered again to $ 9.7 billion.