Semiconductor devices can display a range of different useful properties, such as passing current more easily in one direction than the other, showing variable resistance, and having sensitivity to light or heat. Because the electrical properties of a semiconductor material can be modified by doping and by the application of electrical fields or light, devices made from semiconductors can be used for amplification, switching, andenergy conversion. The term semiconductor is also used to describe materials used in high capacity, medium- tohigh-voltage cables as part of their insulation, and these materials are often plastic XLPE (cross-linked polyethylene) with carbon black.[4]
The conductivity of silicon can be increased by adding a small amount (of the order of 1 in 108) of pentavalent (antimony,phosphorus, orarsenic) or trivalent (boron,gallium,indium) atoms.[5] This process is known as doping, and the resulting semiconductors are known as doped orextrinsic semiconductors. Apart from doping, the conductivity of a semiconductor can be improved by increasing its temperature. This is contrary to the behavior of a metal, in which conductivity decreases with an increase in temperature.[6]
The modern understanding of the properties of a semiconductor relies onquantum physics to explain the movement of charge carriers in acrystal lattice.[7] Doping greatly increases the number of charge carriers within the crystal. When a semiconductor is doped by Group V elements, they will behave likedonors creating freeelectrons, known as "n-type" doping. When a semiconductor is doped by Group III elements, they will behave likeacceptors creating free holes, known as "p-type" doping. The semiconductor materials used in electronic devices are doped under precise conditions to control the concentration and regions of p- and n-type dopants. A single semiconductor devicecrystal can have many p- and n-type regions; thep–n junctions between these regions are responsible for the useful electronic behavior. Using ahot-point probe, one can determine quickly whether a semiconductor sample is p- or n-type.[8]
A few of the properties of semiconductor materials were observed throughout the mid-19th and first decades of the 20th century. The first practical application of semiconductors in electronics was the 1904 development of thecat's-whisker detector, a primitive semiconductor diode used in earlyradio receivers. Developments in quantum physics led in turn to the invention of thetransistor in 1947[9] and theintegrated circuit in 1958.
Semiconductors in their natural state are poor conductors because acurrent requires flow of electrons, and semiconductors have theirvalence bands filled, preventing the entire flow of new electrons. Several developed techniques allow semiconducting materials to behave like conducting materials, such asdoping orgating. These modifications have two outcomes:n-type andp-type. These refer to the excess or shortage of electrons, respectively. A balanced number of electrons would cause a current to flow throughout the material.[10]
Homojunctions occur when two differently doped semiconducting materials are joined. For example, a configuration could consist of p-doped and n-dopedgermanium. This results in an exchange of electrons and holes between the differently doped semiconducting materials. The n-doped germanium would have an excess of electrons, and the p-doped germanium would have an excess of holes. The transfer occurs until an equilibrium is reached by a process calledrecombination, which causes the migrating electrons from the n-type to come in contact with the migrating holes from the p-type.[11] The result of this process is a narrow strip of immobileions, which causes anelectric field across the junction.[7][10]
A difference in the electric potential on a semiconducting material would cause it to leave thermal equilibrium and create a non-equilibrium situation. This introduces electrons and holes to the system, which interact via a process calledambipolar diffusion. Whenever thermal equilibrium is disturbed in a semiconducting material, the number of holes and electrons changes. Such disruptions can occur as a result of a temperature difference orphotons, which can enter the system and create electrons and holes. The processes that create or annihilate electrons and holes are calledgeneration and recombination, respectively.[10]
In certain semiconductors, excited electrons can relax by emitting light instead of producing heat.[12] Controlling the semiconductor composition andelectrical current allows for the manipulation of the emitted light's properties.[13] These semiconductors are used in the construction oflight-emitting diodes and fluorescentquantum dots.
A large number of elements and compounds have semiconducting properties, including:[18]
Certain pure elements are found ingroup 14 of theperiodic table; the most commercially important of these elements aresilicon andgermanium. Silicon and germanium are used here effectively because they have 4 valence electrons in their outermost shell, which gives them the ability to gain or lose electrons equally at the same time.
Binary compounds, particularly between elements in groups 13 and 15, such asgallium arsenide, groups 12 and 16, groups 14 and 16, and between different group-14 elements, e.g.silicon carbide.
The most common semiconducting materials are crystalline solids, butamorphous and liquid semiconductors are also known. These includehydrogenated amorphous silicon and mixtures ofarsenic,selenium, andtellurium in a variety of proportions. These compounds share with better-known semiconductors the properties of intermediate conductivity and a rapid variation of conductivity with temperature, as well as occasionalnegative resistance. Such disordered materials lack the rigid crystalline structure of conventional semiconductors such as silicon. They are generally used inthin film structures, which do not require material of higher electronic quality, being relatively insensitive to impurities and radiation damage.[citation needed]
Almost all of today's electronic technology involves the use of semiconductors, with the most important aspect being theintegrated circuit (IC), which are found indesktops,laptops, scanners,cell-phones, and other electronic devices. Semiconductors for ICs are mass-produced. To create an ideal semiconducting material, chemical purity is paramount. Any small imperfection can have a drastic effect on how the semiconducting material behaves due to the scale at which the materials are used.[10]
A high degree of crystalline perfection is also required, since faults in the crystal structure (such asdislocations,twins, andstacking 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 crystalingots between 100 and 300 mm (3.9 and 11.8 in) in diameter, grown as cylinders and sliced intowafers. The round shape characteristic of these wafers comes fromsingle-crystal ingots usually produced using theCzochralski method. Silicon wafers were first introduced in the 1940s.[21][22]
There is a combination of processes that are used to prepare semiconducting materials for ICs. One process is calledthermal oxidation, which formssilicon dioxide on the surface of thesilicon. This is used as agate insulator andfield oxide. Other processes are calledphotomasks andphotolithography. This process is what creates the patterns on the circuit in the integrated circuit.Ultraviolet light is used along with aphotoresist layer to create a chemical change that generates the patterns for the circuit.[10]
The etching is the next process that is required. The part of the silicon that was not covered by thephotoresist layer from the previous step can now be etched. The main process typically used today is calledplasma etching. Plasma etching usually involves anetch gas pumped in a low-pressure chamber to createplasma. A common etch gas ischlorofluorocarbon, or more commonly knownFreon. A highradio-frequencyvoltage between thecathode andanode is what creates the plasma in the chamber. Thesilicon wafer is located on the cathode, which causes it to be hit by the positively charged ions that are released from the plasma. The result is silicon that is etchedanisotropically.[7][10]
The last process is calleddiffusion. This is the process that gives the semiconducting material its desired semiconducting properties. It is also known asdoping. The process introduces an impure atom to the system, which creates thep–n junction. To get the impure atoms embedded in the silicon wafer, the wafer is first put in a 1,100 degree Celsius chamber. The atoms are injected in and eventually diffuse with the silicon. After the process is completed and the silicon has reached room temperature, the doping process is done and the semiconductingwafer is almost prepared.[7][10]
Semiconductors are defined by their unique electric conductive behavior, somewhere between that of a conductor and an insulator.[23] The differences between these materials can be understood in terms of thequantum states for electrons, each of which may contain zero or one electron (by thePauli exclusion principle). These states are associated with theelectronic band structure of the material.Electrical conductivity arises due to the presence of electrons in states that aredelocalized (extending through the material), however in order to transport electrons a state must bepartially filled, containing an electron only part of the time.[24] If the state is always occupied with an electron, then it is inert, blocking the passage of other electrons via that state. The energies of these quantum states are critical since a state is partially filled only if its energy is near theFermi level[citation needed] (seeFermi–Dirac statistics).
High conductivity in material comes from it having many partially filled states and much state delocalization.Metals are goodelectrical conductors and have many partially filled states with energies near their Fermi level.Insulators, by contrast, have few partially filled states, their Fermi levels sit withinband gaps with few energy states to occupy. Importantly, an insulator can be made to conduct by increasing its temperature: heating provides energy to promote some electrons across the band gap, inducing partially filled states in both the band of states beneath the band gap (valence band) and the band of states above the band gap (conduction band). An (intrinsic) semiconductor has a band gap that is smaller than that of an insulator and at room temperature, significant numbers of electrons can be excited to cross the band gap.[25]
A pure semiconductor, however, is not very useful, as it is neither a very good insulator nor a very good conductor.However, one important feature of semiconductors (and some insulators, known assemi-insulators) is that their conductivity can be increased and controlled bydoping with impurities andgating with electric fields. Doping and gating move either the conduction or valence band much closer to the Fermi level and greatly increase the number of partially filled states.[citation needed]
Somewider-bandgap semiconductor materials are sometimes referred to assemi-insulators. When undoped, these have electrical conductivity nearer to that of electrical insulators, however they can be doped (making them as useful as semiconductors). Semi-insulators find niche applications in micro-electronics, such as substrates forHEMT. An example of a common semi-insulator isgallium arsenide.[26] Some materials, such astitanium dioxide, can even be used as insulating materials for some applications, while being treated as wide-gap semiconductors for other applications.[citation needed]
The partial filling of the states at the bottom of the conduction band can be understood as adding electrons to that band. The electrons do not stay indefinitely (due to the natural thermalrecombination) but they can move around for some time. The actual concentration of electrons is typically very dilute, and so (unlike in metals) it is possible to think of the electrons in the conduction band of a semiconductor as a sort of classicalideal gas, where the electrons fly around freely without being subject to thePauli exclusion principle. In most semiconductors, the conduction bands have a parabolicdispersion relation, and so these electrons respond to forces (electric field, magnetic field, etc.) much as they would in a vacuum, though with a differenteffective mass.[25] Because the electrons behave like an ideal gas, one may also think about conduction in very simplistic terms such as theDrude model, and introduce concepts such aselectron mobility.
For partial filling at the top of the valence band, it is helpful to introduce the concept of anelectron hole. Although the electrons in the valence band are always moving around, a completely full valence band is inert, not conducting any current. If an electron is taken out of the valence band, then the trajectory that the electron would normally have taken is now missing its charge. For the purposes of electric current, this combination of the full valence band, minus the electron, can be converted into a picture of a completely empty band containing a positively charged particle that moves in the same way as the electron. Combined with thenegative effective mass of the electrons at the top of the valence band, we arrive at a picture of a positively charged particle that responds to electric and magnetic fields just as a normal positively charged particle would do in a vacuum, again with some positive effective mass.[25] This particle is called a hole, and the collection of holes in the valence band can again be understood in simple classical terms (as with the electrons in the conduction band).
Whenionizing radiation strikes a semiconductor, it may excite an electron out of its energy level and consequently leave a hole. This process is known aselectron-hole pair generation. Electron-hole pairs are constantly generated fromthermal 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 ofenergy larger than theband gap, be accompanied by the emission of thermal energy (in the form ofphonons) or radiation (in the form ofphotons).
As the probability that electrons and holes meet together is proportional to the product of their numbers, the product is in the 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 neighbor 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 approximatelyexp(−EG/kT), wherek is theBoltzmann constant,T is the absolute temperature andEG is bandgap.
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.[27]
Doping of a puresilicon array. Silicon based intrinsic semiconductor becomes extrinsic when impurities such asBoron andAntimony are introduced.
The conductivity of semiconductors may easily be modified by introducing impurities into theircrystal lattice. The process of adding controlled impurities to a semiconductor is known asdoping. The amount of impurity, or dopant, added to anintrinsic (pure) semiconductor varies its level of conductivity.[28] Doped semiconductors are referred to asextrinsic.[29] By adding impurity to the pure semiconductors, the electrical conductivity may be varied by factors of thousands or millions.[30]
A 1 cm3 specimen of a metal or semiconductor has the order of 1022 atoms.[31] In a metal, every atom donates at least one free electron for conduction, thus 1 cm3 of metal contains on the order of 1022 free electrons,[32] whereas a 1 cm3 sample of pure germanium at 20°C contains about4.2×1022 atoms, but only2.5×1013 free electrons and2.5×1013 holes. The addition of 0.001% ofarsenic (an impurity) donates an extra 1017 free electrons in the same volume and the electrical conductivity is increased by a factor of 10,000.[33][34]
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 electronacceptors ordonors. Semiconductors doped withdonor impurities are calledn-type, while those doped withacceptor impurities are known asp-type. The n and p type designations indicate which charge carrier acts as the material'smajority carrier. The opposite carrier is called theminority carrier, which exists due to thermal excitation at a much lower concentration compared to the majority carrier.[35]
For example, the pure semiconductorsilicon has four valence electrons that bond each silicon atom to its neighbors.[36] In silicon, the most common dopants aregroup III andgroup V elements. Group III elements all contain three valence electrons, causing them to function as acceptors when used to dope silicon. When an acceptor atom replaces a silicon atom in the crystal, a vacant state (an electron "hole") is created, which can move around the lattice and function as a charge carrier. Group V elements have five valence electrons, which allows them to act as a donor; substitution of these atoms for silicon creates an extra free electron. Therefore, a silicon crystal doped withboron creates a p-type semiconductor whereas one doped withphosphorus results in an n-type material.[37]
Duringmanufacture, dopants can be diffused into the semiconductor body by contact with gaseous compounds of the desired element, orion implantation can be used to accurately position the doped regions.
Some materials, when rapidly cooled to a glassy amorphous state, have semiconducting properties. These include B,Si, Ge, Se, and Te, and there are multiple theories to explain them.[38][39]
The history of the understanding of semiconductors begins with experiments on the electrical properties of materials. The properties of the time-temperature coefficient of resistance, rectification, and light-sensitivity were observed starting in the early 19th century.
Thomas Johann Seebeck was the first to notice that semiconductors exhibit special feature such that experiment concerning anSeebeck effect emerged with much stronger result when applying semiconductors, in 1821.[40] In 1833,Michael Faraday reported that the resistance of specimens ofsilver sulfide decreases when they are heated. This is contrary to the behavior of metallic substances such as copper. In 1839,Alexandre Edmond Becquerel reported observation of a voltage between a solid and a liquid electrolyte, when struck by light, thephotovoltaic effect. In 1873,Willoughby Smith observed thatseleniumresistors exhibit decreasing resistance when light falls on them. In 1874,Karl Ferdinand Braun observed conduction andrectification in metallicsulfides, although this effect had been discovered earlier by Peter Munck af Rosenschöld (sv) writing for theAnnalen der Physik und Chemie in 1835; Rosenschöld's findings were ignored.[41]Simon Sze stated that Braun's research was the earliest systematic study of semiconductor devices.[42] Also in 1874,Arthur Schuster found that a copper oxide layer on wires had rectification properties that ceased when the wires are cleaned.William Grylls Adams and Richard Evans Day observed the photovoltaic effect in selenium in 1876.[43]
A unified explanation of these phenomena required a theory ofsolid-state physics, which developed greatly in the first half of the 20th century. In 1878Edwin Herbert Hall demonstrated the deflection of flowing charge carriers by an applied magnetic field, theHall effect. The discovery of theelectron byJ.J. Thomson in 1897 prompted theories of electron-based conduction in solids.Karl Baedeker, by observing a Hall effect with the reverse sign to that in metals, theorized that copper iodide had positive charge carriers. Johan Koenigsberger classified solid materials like metals, insulators, and "variable conductors" in 1914 although his student Josef Weiss already introduced the termHalbleiter (a semiconductor in modern meaning) in his Ph.D. thesis in 1910.[44][45]Felix Bloch published a theory of the movement of electrons through atomic lattices in 1928. In 1930, Bernhard Gudden stated that conductivity in semiconductors was due to minor concentrations of impurities. By 1931, the band theory of conduction had been established byAlan Herries Wilson and the concept of band gaps had been developed.Walter H. Schottky andNevill Francis Mott developed models of the potential barrier and of the characteristics of ametal–semiconductor junction. By 1938, Boris Davydov had developed a theory of the copper-oxide rectifier, identifying the effect of thep–n junction and the importance of minority carriers and surface states.[41]
Agreement between theoretical predictions (based on developing quantum mechanics) and experimental results was sometimes poor. This was later explained byJohn Bardeen as due to the extreme "structure sensitive" behavior of semiconductors, whose properties change dramatically based on tiny amounts of impurities.[41] Commercially pure materials of the 1920s containing varying proportions of trace contaminants produced differing experimental results. This spurred the development of improved material refining techniques, culminating in modern semiconductor refineries producing materials with parts-per-trillion purity.
Devices using semiconductors were at first constructed based on empirical knowledge before semiconductor theory provided a guide to the construction of more capable and reliable devices.
Alexander Graham Bell used the light-sensitive property of selenium totransmit sound over a beam of light in 1880. A working solar cell, of low efficiency, was constructed byCharles Fritts in 1883, using a metal plate coated with selenium and a thin layer of gold; the device became commercially useful in photographic light meters in the 1930s.[41] Point-contact microwave detector rectifiers made of lead sulfide were used byJagadish Chandra Bose in 1904; thecat's-whisker detector using natural galena or other materials became a common device in thedevelopment of radio. However, it was somewhat unpredictable in operation and required manual adjustment for best performance. In 1906,H.J. Round observed light emission when electric current passed throughsilicon carbide crystals, the principle behind thelight-emitting diode.Oleg Losev observed similar light emission in 1922, but at the time the effect had no practical use. Power rectifiers, using copper oxide and selenium, were developed in the 1920s and became commercially important as an alternative tovacuum tube rectifiers.[43][41]
In the years preceding World War II, infrared detection and communications devices prompted research into lead-sulfide and lead-selenide materials. These devices were used for detecting ships and aircraft, for infrared rangefinders, and for voice communication systems. The point-contact crystal detector became vital for microwave radio systems since available vacuum tube devices could not serve as detectors above about 4000 MHz; advanced radar systems relied on the fast response of crystal detectors. Considerable research and development ofsilicon materials occurred during the war to develop detectors of consistent quality.[41]
Detector and power rectifiers could not amplify a signal. Many efforts were made to develop a solid-state amplifier and were successful in developing a device called thepoint contact transistor which could amplify 20 dB or more.[48] In 1922,Oleg Losev developed two-terminal,negative resistance amplifiers for radio, but he died in theSiege of Leningrad after successful completion. In 1926,Julius Edgar Lilienfeld patented a device resembling afield-effect transistor, but it was not practical. Rudolf Hilsch andR. W. Pohl [de] in 1938 demonstrated a solid-state amplifier using a structure resembling the control grid of a vacuum tube; although the device displayed power gain, it had acut-off frequency of one cycle per second, too low for any practical applications, but an effective application of the available theory.[41] AtBell Labs,William Shockley and A. Holden started investigating solid-state amplifiers in 1938. The first p–n junction in silicon was observed byRussell Ohl about 1941 when a specimen was found to be light-sensitive, with a sharp boundary between p-type impurity at one end and n-type at the other. A slice cut from the specimen at the p–n boundary developed a voltage when exposed to light.
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