Insemiconductor production,doping is the intentional introduction of impurities into anintrinsic (undoped) semiconductor for the purpose of modulating its electrical, optical and structural properties. The doped material is referred to as anextrinsic semiconductor.
Small numbers of dopantatoms can change the ability of a semiconductor to conduct electricity. When on the order of one dopant atom is added per 100 million atoms, the doping is said to below orlight. When many more dopant atoms are added, on the order of one per ten thousand atoms, the doping is referred to ashigh orheavy. This is often shown asn+ forn-type doping orp+ forp-type doping. (See the article onsemiconductors for a more detailed description of the doping mechanism.) A semiconductor doped to such high levels that it acts more like aconductor than a semiconductor is referred to as adegenerate semiconductor. A semiconductor can be consideredi-type semiconductor if it has been doped in equal quantities of p and n.
In the context ofphosphors andscintillators, doping is better known asactivation; this is not to be confused withdopant activation in semiconductors. Doping is also used to control the color in some pigments.
The effects of impurities in semiconductors (doping) were long known empirically in such devices ascrystal radiodetectors andselenium rectifiers. For instance, in 1885Shelford Bidwell, and in 1930 the German scientist Bernhard Gudden, each independently reported that the properties of semiconductors were due to the impurities they contained.[1][2] A doping process was formally developed byJohn Robert Woodyard working atSperry Gyroscope Company duringWorld War II. Though the worddoping is not used in it, his US Patent issued in 1950 describes methods for adding tiny amounts of solid elements from the nitrogen column of the periodic table to germanium to produce rectifying devices.[3] The demands of his work onradar prevented Woodyard from pursuing further research on semiconductor doping.
Similar work was performed atBell Labs byGordon K. Teal andMorgan Sparks, with a US Patent issued in 1953.[4]
Woodyard's priorpatent proved to be the grounds of extensive litigation bySperry Rand.[5]
The concentration of the dopant used affects many electrical properties. Most important is the material'scharge carrier concentration. In an intrinsic semiconductor underthermal equilibrium, the concentrations ofelectrons andholes are equivalent. That is,
In a non-intrinsic semiconductor under thermal equilibrium, the relation becomes (for low doping):
wheren0 is the concentration of conducting electrons,p0 is the conducting hole concentration, andni is the material's intrinsic carrier concentration. The intrinsic carrier concentration varies between materials and is dependent on temperature.Silicon'sni, for example, is roughly 1.08×1010 cm−3 at 300kelvins, aboutroom temperature.[6]
In general, increased doping leads to increased conductivity due to the higher concentration of carriers. Degenerate (very highly doped) semiconductors have conductivity levels comparable tometals and are often used inintegrated 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. Even degenerate levels of doping imply low concentrations of impurities with respect to the base semiconductor. In intrinsiccrystalline 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 on 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.
Doping a semiconductor in a good crystal introduces allowed energy states within theband gap, but very close to the energy band that corresponds to the dopant type. In other words,electron donor impurities create states near theconduction band whileelectron acceptor impurities create states near the valence band. The gap between these energy states and the nearest energy band is usually referred to as dopant-sitebonding energy orEB and is relatively small. For example, theEB forboron in silicon bulk is 0.045 eV, compared with silicon's band gap of about 1.12 eV. BecauseEB is so small, room temperature is hot enough tothermally ionize practically all of the dopant atoms and create freecharge carriers in the conduction or valence bands.
Dopants also have the important effect of shifting the energy bands relative to theFermi level. The energy band that corresponds with the dopant with the greatest concentration ends up closer to the Fermi level. Since the Fermi level must remain constant in a system inthermodynamic equilibrium, stacking layers of materials with different properties leads to many useful electrical properties induced byband bending, if the interfaces can be made cleanly enough. For example, thep-n junction's properties are due to the band bending that happens as a result of the necessity to line up the bands in contacting regions of p-type and n-type material.This effect is shown in aband diagram. The band diagram typically indicates the variation in the valence band and conduction band edges versus some spatial dimension, often denotedx. The Fermi level is also usually indicated in the diagram. Sometimes theintrinsic Fermi level,Ei, which is the Fermi level in the absence of doping, is shown. These diagrams are useful in explaining the operation of many kinds ofsemiconductor devices.
For low levels of doping, the relevant energy states are populated sparsely by electrons (conduction band) or holes (valence band). It is possible to write simple expressions for the electron and hole carrier concentrations, by ignoring Pauli exclusion (viaMaxwell–Boltzmann statistics):
whereEF is theFermi level,EC is the minimum energy of the conduction band, andEV is the maximum energy of the valence band. These are related to the value of the intrinsic concentration via[7]
an expression which is independent of the doping level, sinceEC –EV (theband gap) does not change with doping.
The concentration factorsNC(T) andNV(T) are given by
whereme* andmh* are thedensity of states effective masses of electrons and holes, respectively, quantities that are roughly constant over temperature.[7]
Somedopants are added as the (usuallysilicon)boule is grown byCzochralski method, giving eachwafer an almost uniform initial doping.[8]
Alternately, synthesis of semiconductor devices may involve the use ofvapor-phase epitaxy. In vapor-phase epitaxy, a gas containing the dopant precursor can be introduced into the reactor. For example, in the case of n-type gas doping ofgallium arsenide,hydrogen sulfide is added, and sulfur is incorporated into the structure.[9] This process is characterized by a constant concentration of sulfur on the surface.[10] In the case of semiconductors in general, only a very thin layer of the wafer needs to be doped in order to obtain the desired electronic properties.[11]
To define circuit elements, selected areas — typically controlled byphotolithography[12] — are further doped by such processes asdiffusion[13] andion implantation, the latter method being more popular in large production runs because of increased controllability.
Spin-on glass or spin-on dopant doping is a two-step process. First, a mixture of SiO2 and dopants (in a solvent) is applied to a wafer surface byspin-coating. Then it is stripping and baked at a certain temperature in a furnace with constant nitrogen+oxygen flow.[14]
Neutrontransmutation doping (NTD) is an unusual doping method for special applications. Most commonly, it is used to dope silicon n-type in high-power electronics andsemiconductor detectors. It is based on the conversion of the Si-30 isotope intophosphorus atom by neutron absorption as follows:
In practice, the silicon is typically placed near anuclear reactor to receive the neutrons. As neutrons continue to pass through the silicon, more and more phosphorus atoms are produced by transmutation, and therefore the doping becomes more and more strongly n-type. NTD is a far less common doping method than diffusion or ion implantation, but it has the advantage of creating an extremely uniform dopant distribution.[15][16]
(Note: When discussingperiodic table groups, semiconductor physicists always use an older notation, not the currentIUPAC group notation. For example, thecarbon group is called "Group IV", not "Group 14".)
For theGroup IV semiconductors such asdiamond,silicon,germanium,silicon carbide, andsilicon–germanium, the most common dopants areacceptors fromGroup III ordonors fromGroup V elements.Boron,arsenic,phosphorus, and occasionallygallium are used to dope silicon. Boron is thep-type dopant of choice for silicon integrated circuit production because it diffuses at a rate that makes junction depths easily controllable. Phosphorus is typically used for bulk-doping of silicon wafers, while arsenic is used to diffuse junctions, because it diffuses more slowly than phosphorus and is thus more controllable.
By doping pure silicon withGroup V elements such as phosphorus, extravalence electrons are added that become unbounded from individual atoms and allow the compound to be an electrically conductiven-type semiconductor. Doping withGroup III elements, which are missing the fourth valence electron, creates "broken bonds" (holes) in the silicon lattice that are free to move. The result is an electrically conductivep-type semiconductor. In this context, aGroup V element is said to behave as an electrondonor, and aGroup III element as anacceptor. This is a key concept in the physics of adiode.
A very heavily doped semiconductor behaves more like a good conductor (metal) and thus exhibits more linear positive thermal coefficient. Such effect is used for instance insensistors.[17] Lower dosage of doping is used in other types (NTC or PTC)thermistors.
[25]In the following list the "(substituting X)" refers to all of the materials preceding said parenthesis.
In most cases many types of impurities will be present in the resultant doped semiconductor. If an equal number of donors and acceptors are present in the semiconductor, the extra core electrons provided by the former will be used to satisfy the broken bonds due to the latter, so that doping produces no free carriers of either type. This phenomenon is known ascompensation, and occurs at thep-n junction in the vast majority of semiconductor devices.
Partial compensation, where donors outnumber acceptors or vice versa, allows device makers to repeatedly reverse (invert) the type of a certain layer under the surface of a bulk semiconductor by diffusing or implanting successively higher doses of dopants, so-calledcounterdoping. Most modern semiconductor devices are made by successive selective counterdoping steps to create the necessary P and N type areas under the surface of bulk silicon.[26] This is an alternative to successively growing such layers by epitaxy.
Although compensation can be used to increase or decrease the number of donors or acceptors, the electron and holemobility is always decreased by compensation because mobility is affected by the sum of the donor and acceptor ions.
Conductive polymers can be doped by adding chemical reactants tooxidize, or sometimes reduce, the system so that electrons are pushed into the conductingorbitals within the already potentially conducting system. There are two primary methods of doping a conductive polymer, both of which use an oxidation-reduction (i.e.,redox) process.
N-doping is much less common because theEarth's atmosphere isoxygen-rich, thus creating anoxidizing environment. An electron-rich, n-doped polymer will react immediately with elemental oxygen tode-dope (i.e., reoxidize to the neutral state) the polymer. Thus, chemical n-doping must be performed in an environment ofinert gas (e.g.,argon). Electrochemical n-doping is far more common in research, because it is easier to exclude oxygen from asolvent in a sealedflask. However, it is unlikely that n-doped conductive polymers are available commercially.
Molecular dopants are preferred in doping molecular semiconductors due to their compatibilities of processing with the host, that is, similar evaporation temperatures or controllable solubility.[27] Additionally, the relatively large sizes of molecular dopants compared with those of metal ion dopants (such as Li+ and Mo6+) are generally beneficial, yielding excellent spatial confinement for use in multilayer structures, such asOLEDs andOrganic solar cells. Typical p-type dopants include F4-TCNQ[28] and Mo(tfd)3.[29] However, similar to the problem encountered in doping conductive polymers, air-stable n-dopants suitable for materials with lowelectron affinity (EA) are still elusive. Recently, photoactivation with a combination of cleavable dimeric dopants, such as [RuCp∗Mes]2, suggests a new path to realize effective n-doping in low-EA materials.[27]
Research on magnetic doping has shown that considerable alteration of certain properties such as specific heat may be affected by small concentrations of an impurity; for example, dopant impurities in semiconductingferromagnetic alloys can generate different properties as first predicted by White, Hogan, Suhl and Nakamura.[30][31]The inclusion of dopant elements to impart dilute magnetism is of growing significance in the field ofmagnetic semiconductors. The presence of disperse ferromagnetic species is key to the functionality of emergingspintronics, a class of systems that utilise electron spin in addition to charge. Usingdensity functional theory (DFT) the temperature dependent magnetic behaviour of dopants within a given lattice can be modeled to identify candidate semiconductor systems.[32]
The sensitive dependence of a semiconductor's properties on dopants has provided an extensive range of tunable phenomena to explore and apply to devices. It is possible to identify the effects of a solitary dopant on commercial device performance as well as on the fundamental properties of a semiconductor material. New applications have become available that require the discrete character of a single dopant, such as single-spin devices in the area of quantum information or single-dopant transistors. Dramatic advances in the past decade towards observing, controllably creating and manipulating single dopants, as well as their application in novel devices have allowed opening the new field of solotronics (solitary dopant optoelectronics).[33]
Electrons or holes introduced by doping are mobile, and can be spatially separated from dopant atoms they have dissociated from. Ionized donors and acceptors however attract electrons and holes, respectively, so this spatial separation requires abrupt changes of dopant levels, of band gap (e.g. aquantum well), or built-in electric fields (e.g. in case ofnoncentrosymmetric crystals). This technique is calledmodulation doping and is advantageous owing to suppressedcarrier-donor scattering, allowing very highmobility to be attained.
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