Doping (semiconductor)
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/* Doping in organic conductors */
In [[semiconductor]] production, '''doping''' is the process of intentionally introducing impurities into an extremely pure (also referred to as ''intrinsic'') semiconductor in order to change its electrical properties. The impurities are dependent upon the type of semiconductor. Lightly and moderately doped semiconductors are referred to as [[Extrinsic semiconductor|''extrinsic'']]. A semiconductor which is doped to such high levels that it acts more like a [[Conductor (material)|conductor]] than a semiconductor is called ''degenerate''.
Some [[dopant]]s are generally added as the (usually [[silicon]]) [[boule (crystal)|boule]] is grown, giving each [[wafer (electronics)|wafer]] an almost uniform initial doping.<ref>{{cite book |title=Microelectronic Materials and Processes |last=Levy |first=Roland Albert |year=1989 |pages=6-7 |isbn=0792301544 |url=http://books.google.com/books?id=wZPRPU6ne7UC&pg=PA248&sig=8JXOn_Dc0fPFnyEEkWh53lKWCOk#PPA1,M1 |accessdate=2008-02-23}}</ref>
To define circuit elements, selected areas (typically controlled by [[photolithography]])<ref>[http://www.computerhistory.org/semiconductor/timeline/1955-Photolithography.html Computer History Museum - The Silicon Engine | 1955 - Photolithography Techniques Are Used to Make Silicon Devices<!-- Bot generated title -->]</ref> are further doped by such processes as [[diffusion]]<ref>[http://www.computerhistory.org/semiconductor/timeline/1954-Diffusion.html Computer History Museum - The Silicon Engine | 1954 - Diffusion Process Developed for Transistors<!-- Bot generated title -->]</ref> and [[ion implantation]], the latter method being more popular in large production runs due to its better controllability.
The number of dopant atoms needed to create a difference in the ability of a semiconductor to conduct is very small. Where a comparatively small number of dopant atoms are added (of the order of 1 every 100,000,000 atoms) then the doping is said to be low, or light. Where many more are added (of the order of 1 in 10,000) then the doping is referred to as heavy, or high. This is often shown as n+ for n-type dopant or p+ for p-type doping. A more detailed description of the mechanism of doping can be found in the article on [[semiconductors]].
==Dopant elements==
===Group IV semiconductors===
For the group IV semiconductors such as [[silicon]], [[germanium]], and [[silicon carbide]], the most common dopants are [[Acceptor (semiconductors) | acceptors]] from [[boron group|group III]] or [[Donor (semiconductors) | donors]] from [[nitrogen group|group]] [[group 15 element|V]] elements (Group number refers to the [[Roman numerals]] of the columns in the [[periodic table]] of the [[Chemical element|elements]]). [[Boron]], [[arsenic]], [[phosphorus]] and occasionally [[gallium]] are used to dope silicon. Boron is the [[Acceptor (semiconductors) | p-type dopant]] of choice for silicon integrated circuit production, since it diffuses at a rate which makes junction depths easily controllable. Phosphorus is typically used for bulk doping of silicon wafers, while arsenic is used to diffuse junctions, since it diffuses more slowly than phosphorus and is thus more controllable.
By doping pure silicon with group V elements such as phosphorus, extra [[valence electrons]] are added which become unbonded from individual atoms and allow the compound to be electrically conductive, [[n-type semiconductor]]. Doping with group III elements, such as boron, which are missing the fourth valence electron creates "broken bonds", or holes, in the silicon lattice that are free to move. This is electrically conductive, [[p-type semiconductor]]. In this context then, a group V element is said to behave as an electron '''donor''', and a group III element as an '''acceptor'''.
==Compensation==
In most cases, many types of impurity will be present. 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 as ''compensation'', and occurs at the [[p-n junction]] in the vast majority of semiconductor devices. Partial compensation, where donors outnumber acceptors or vice-versa, allows device makers to repeatedly reverse the type of a given portion of the material by applying successively higher doses of dopants.
Although ''compensation'' can be used to increase or decrease the number of donors or acceptors, the electron and hole [[Electron mobility|mobility]] is always decreased by compensation because mobility is affected by the sum of the donor and acceptor ions.
==Doping in organic conductors==
{{main|Conductive polymer}}
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[[Conductive polymer]]s can be 'doped' by adding chemical reactants to oxidise (or sometimes reduce) the system to push electrons into the conducting orbitals within the already (potentially) conducting system. (In a silicon lattice, the system is far from conducting to begin with!) There are two primary methods of doping a conductive polymer, both through an oxidation-reduction ([[redox]]) process. The first method, chemical doping, involves exposing a polymer, such as [[melanin]] (typically a [[thin film]]), to an [[oxidant]] (typically [[iodine]] or [[bromine]]) or [[Reducing agent|reductant]] (far less common, but typically involves [[alkali metals]]). The second is [[electrochemical]] doping in which a polymer-coated, working
[[electrode]] is suspended in an [[electrolyte]] solution in which the polymer is [[insoluble]] along with separate counter and reference electrodes. An electric [[potential difference]] is created between the electrodes which causes a charge (and the appropriate counter [[ion]] from the [[electrolyte]]) to enter the polymer in the form of electron addition (n doping) or removal (p doping).
The reason n doping is so much less common is that [[Earth's atmosphere]] is [[oxygen]]-rich, which creates an [[oxidizing]] environment. An electron-rich n doped polymer will react immediately with elemental oxygen to de-dope (re-oxidize to the neutral state) the polymer. Thus, chemical n doping has to be done in an environment of [[inert]] gas (e.g., [[argon]]). Electrochemical n doping is far more common in research, because it is easier to exclude oxygen from a [[solvent]] in a sealed [[flask]]; however, there are likely no commercialized n doped conductive polymers.
==History==
Doping was originally developed by [[John Robert Woodyard]] working at [[Sperry Gyroscope Company]] during [[World War II]].<ref>US Patent No.2,530,110, filed, 1944, granted 1950</ref> The demands of his war work on [[radar]] denied Woodyard the opportunity to pursue this line of research but, post-war, his [[patent]] proved the grounds of extensive litigation by [[Sperry Rand]].<ref>{{ cite web | author=Morton, P. L. ''et al.'' | url=http://content.cdlib.org/xtf/view?docId=hb4d5nb20m&doc.view=frames&chunk.id=div00182&toc.depth=1&toc.id=&brand=oac | year=1985 | work=University of California: In Memoriam | title=John Robert Woodyard, Electrical Engineering: Berkeley | accessdate=2007-08-12 }}
</ref>. Related work was done at [[Bell Labs]] by Teal and Sparks.<ref>Sparks, Morgan and Teal, Gordon K. “Method of Making P-N Junctions in Semiconductor Materials,” U. S. Patent 2,631,356 (Filed June 15, 1950. Issued March 17, 1953)</ref>
==See also==
* [[Intrinsic semiconductor]]
* [[Extrinsic semiconductor]]
==References==
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[[Category:Condensed matter physics]]
[[Category:Semiconductor device fabrication]]
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