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| {{about|the mobility for electrons and holes in metals and semiconductors|the general concept|Electrical mobility}}
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| In [[solid-state physics]], the '''electron mobility''' characterizes how quickly an [[electron]] can move through a [[metal]] or [[semiconductor]], when pulled by an [[electric field]]. In semiconductors, there is an analogous quantity for [[Electron hole|holes]], called '''hole mobility'''. The term '''carrier mobility''' refers in general to both electron and hole mobility in semiconductors.
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| Electron and hole mobility are special cases of [[electrical mobility]] of charged particles in a fluid under an applied electric field.
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| When an electric field ''E'' is applied across a piece of material, the electrons respond by moving with an average velocity called the [[drift velocity]], <math>\, v_d</math>. Then the electron mobility μ is defined as
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| :<math>\,v_d = \mu E</math>.
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| Electron mobility is almost always specified in units of [[square centimetre|cm<sup>2</sup>]]/([[volt|V]]·[[second|s]]). This is different from the [[SI]] unit of mobility, [[square metre|m<sup>2</sup>]]/([[volt|V]]·[[second|s]]). They are related by 1m<sup>2</sup>/(V·s) = 10<sup>4</sup>cm<sup>2</sup>/(V·s).
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| [[Electrical conductivity|Conductivity]] is proportional to the product of mobility and carrier concentration. For example, the same conductivity could come from a small number of electrons with high mobility for each, or a large number of electrons with a small mobility for each. For metals, it would not typically matter which of these is the case, since most metal electrical behavior depends on conductivity alone. Therefore mobility is relatively unimportant in metal physics. On the other hand, for semiconductors, the behavior of [[transistor]]s and other devices can be very different depending on whether there are many electrons with low mobility or few electrons with high mobility. Therefore mobility is a very important parameter for semiconductor materials. Almost always, higher mobility leads to better device performance, with other things equal.
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| Semiconductor mobility depends on the impurity concentrations (including donor and acceptor concentrations), defect concentration, temperature, and electron and hole concentrations. It also depends on the electric field, particularly at high fields when [[velocity saturation]] occurs. It can be determined by the [[Hall effect]], or inferred from transistor behavior.
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| ==Introduction==
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| ===Drift velocity in an electric field===
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| {{main|Drift velocity}}
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| Without any applied electric field, in a solid, [[electrons]] (or, in the case of [[semiconductors]], both [[electrons]] and [[electron hole|holes]]) [[Brownian motion|move around randomly]]. Therefore, on average there will be no overall motion of charge carriers in any particular direction over time.
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| However, when an electric field is applied, each electron is accelerated by the electric field. If the electron were in a vacuum, it would be accelerated to faster and faster velocities (called [[ballistic transport]]). However, in a solid, the electron repeatedly scatters off [[Crystallographic defect|crystal defects]], [[phonons]], impurities, etc. Therefore, it does not accelerate faster and faster; instead it moves with a finite average velocity, called the [[drift velocity]]. This net electron motion is usually much slower than the normally occurring random motion.
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| In a semiconductor the two charge carriers, electrons and [[Electron hole|holes]], will typically have different drift velocities for the same electric field.
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| Quasi-[[ballistic transport]] is possible in solids if the electrons are accelerated across a very small distance (as small as the [[mean free path]]), or for a very short time (as short as the [[mean free time]]). In these cases, drift velocity and mobility are not meaningful.
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| ===Definition and units===
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| {{see also|Electrical mobility}}
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| The electron mobility is defined by the equation:
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| :<math>\,v_d = \mu E</math>.
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| where:
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| :''E'' is the [[Euclidean vector|magnitude]] of the [[electric field]] applied to a material,
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| :''v<sub>d</sub>'' is the [[Euclidean vector|magnitude]] of the electron drift velocity (in other words, the electron drift [[speed]]) caused by the electric field, and
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| :µ is the electron mobility.
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| The hole mobility is defined by the same equation. Both electron and hole mobilities are positive by definition.
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| Usually, the electron drift velocity in a material is directly proportional to the electric field, which means that the electron mobility is a constant (independent of electric field). When this is not true (for example, in very large electric fields), the mobility depends on the electric field.
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| The SI unit of velocity is [[Metre per second|m/s]], and the SI unit of electric field is [[volt|V]]/[[metre|m]]. Therefore the SI unit of mobility is (m/s)/(V/m) = [[square metre|m<sup>2</sup>]]/([[volt|V]]·[[second|s]]). However, mobility is much more commonly expressed in cm<sup>2</sup>/(V·s) = 10<sup>−4</sup> m<sup>2</sup>/(V·s).
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| Mobility is usually a strong function of material impurities and temperature, and is determined empirically. Mobility values are typically presented in table or chart form. Mobility is also different for electrons and holes in a given [[semiconductor]].
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| ===Relation to conductivity===
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| There is a simple relation between mobility and [[electrical conductivity]]. Let ''n'' be the [[number density]] of electrons, and let μ<sub>e</sub> be their mobility. In the electric field '''E''', each of these electrons will move with the velocity vector <math>-\mu_e \mathbf{E}</math>, for a total current density of <math>n e \mu_e \mathbf{E}</math> (where ''e'' is the [[elementary charge]]). Therefore, the electrical conductivity σ satisfies:<ref name=BVZ>[http://ece-www.colorado.edu/~bart/book/book/chapter2/ch2_7.htm Chapter 2: Semiconductor Fundamentals]. Online textbook by B. Van Zeghbroeck]</ref>
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| :<math>\sigma=ne\mu_e</math>.
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| This formula is valid when the conductivity is due entirely to electrons. In a [[p-type semiconductor]], the conductivity is due to holes instead, but the formula is essentially the same: If ''p'' is the density of holes and μ<sub>h</sub> is the hole mobility, then the conductivity is
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| :<math>\sigma=pe\mu_h</math>.
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| If a semiconductor has both electrons and [[electron hole|holes]], the total conductivity is<ref name=BVZ/>
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| :<math>\sigma=e(n\mu_e+p\mu_h).</math>
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| ==Examples==
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| Typical electron mobility for [[Silicon|Si]] at room temperature (300 K) is 1400 cm<sup>2</sup>/ (V·s) and the hole mobility is around 450 cm<sup>2</sup>/ (V·s).<ref>[http://www.ioffe.rssi.ru/SVA/NSM/Semicond/Si/electric.html Electrical properties of silicon], Ioffe Institute Database</ref>
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| Very high mobility has been found in several low-dimensional systems, such as two-dimensional electron gases ([[2DEG]]) (35,000,000 cm<sup>2</sup>/(V·s) at low temperature),<ref>{{cite journal|last1=Umansky|first1=V.|last2=Heiblum|first2=M.|last3=Levinson|first3=Y.|last4=Smet|first4=J.|last5=Nübler|first5=J.|last6=Dolev|first6=M.|title=MBE growth of ultra-low disorder 2DEG with mobility exceeding 35×106 cm2 V−1 s−1|journal=Journal of Crystal Growth|volume=311|pages=1658–1661|year=2009|bibcode = 2009JCrGr.311.1658U |doi = 10.1016/j.jcrysgro.2008.09.151 }}</ref> [[carbon nanotubes]] (100,000 cm<sup>2</sup>/(V·s) at room temperature) <ref>{{cite journal|last1=Dürkop|first1=T.|last2=Getty|first2=S. A.|last3=Cobas|first3=Enrique|last4=Fuhrer|first4=M. S.|title=Extraordinary Mobility in Semiconducting Carbon Nanotubes|journal=Nano Letters|volume=4|pages=35|year=2004|doi=10.1021/nl034841q|bibcode = 2004NanoL...4...35D }}</ref> and more recently, [[graphene]] (200,000 cm<sup>2</sup>/ V·s at low temperature).<ref>{{cite journal|last1=Bolotin|first1=K|last2=Sikes|first2=K|last3=Jiang|first3=Z|last4=Klima|first4=M|last5=Fudenberg|first5=G|last6=Hone|first6=J|last7=Kim|first7=P|last8=Stormer|first8=H|title=Ultrahigh electron mobility in suspended graphene|journal=Solid State Communications|volume=146|pages=351|year=2008|doi=10.1016/j.ssc.2008.02.024|arxiv=0802.2389|bibcode = 2008SSCom.146..351B }}</ref>
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| Organic semiconductors ([[polymer]], [[oligomer]]) developed thus far have carrier mobilities below 10 cm<sup>2</sup>/(V·s), and usually much lower.
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| ==Electric field dependence and velocity saturation==
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| {{main|Velocity saturation}}
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| At low fields, the drift velocity ''v''<sub>''d''</sub> is proportional to the electric field ''E'', so mobility ''μ'' is constant. This value of ''μ'' is called the ''low-field mobility''.
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| As the electric field is increased, however, the carrier velocity increases sublinearly and asymptotically towards a maximum possible value, called the ''saturation velocity'' ''v''<sub>sat</sub>. For example, the value of ''v''<sub>sat</sub> is on the order of 1×10<sup>7</sup> cm/s for both electrons and holes in Si. It is on the order of 6×10<sup>6</sup> cm/s for Ge. This velocity is a characteristic of the material and a strong function of [[Doping (semiconductor)|doping]] or impurity levels and temperature. It is one of the key material and semiconductor device properties that determine a device such as a transistor's ultimate limit of speed of response and frequency.
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| This velocity saturation phenomenon results from a process called ''[[optical phonon]] scattering''. At high fields, carriers are accelerated enough to gain sufficient [[kinetic energy]] between collisions to emit an optical phonon, and they do so very quickly, before being accelerated once again. The velocity that the electron reaches before emitting a phonon is:
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| :::<math>\frac{m^*v_{emit}^2}{2} \approx \hbar \omega_{phonon (opt.)}</math>
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| where ''ω<sub>phonon(opt.)</sub>'' is the optical-phonon angular frequency and m* the carrier effective mass in the direction of the electric field. The value of ''E''<sub>phonon (opt.)</sub> is 0.063 eV for Si and 0.034 eV for GaAs and Ge. The saturation velocity is only one-half of ''v''<sub>emit</sub>, because the electron starts at zero velocity and accelerates up to ''v''<sub>emit</sub> in each cycle.<ref name=Mitin>{{cite book|author1=Vladimir Vasilʹevich Mitin|author2=Vi︠a︡cheslav Aleksandrovich Kochelap|author3=Michael A. Stroscio|title=Quantum heterostructures: microelectronics and optoelectronics|url=http://books.google.com/books?id=Wzo4IdxS48oC&pg=PA308|accessdate=2 March 2011|year=1999|publisher=Cambridge University Press|isbn=978-0-521-63635-3|pages=307–9}}</ref> (This is a somewhat oversimplified description.<ref name=Mitin/>)
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| Velocity saturation is not the only possible high-field behavior. Another is the [[Gunn effect]], where a sufficiently high electric field can cause intervalley electron transfer, which reduces drift velocity. This is unusual; increasing the electric field almost always ''increases'' the drift velocity, or else leaves it unchanged. The result is [[negative differential resistance]].
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| In the regime of velocity saturation (or other high-field effects), mobility is a strong function of electric field. This means that mobility is a somewhat less useful concept, compared to simply discussing drift velocity directly.
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| ==Relation between scattering and mobility==
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| Recall that by definition, mobility is dependent on the drift velocity. The main factor determining drift velocity (other than [[effective mass (solid-state physics)|effective mass]]) is [[scattering]] time, i.e. how long the carrier is [[ballistic transport|ballistically accelerated]] by the electric field until it scatters (collides) with something that changes its direction and/or energy. The most important sources of scattering in typical semiconductor materials, discussed below, are ionized impurity scattering and acoustic phonon scattering (also called lattice scattering). In some cases other sources of scattering may be important, such as neutral impurity scattering, optical phonon scattering, surface scattering, and [[crystallographic defect|defect]] scattering.<ref name=Singh>{{cite book|author=Singh|title=Electronic Devices And Integrated Circuits|url=http://books.google.com/books?id=2aqtlybkFE0C&pg=PA77|accessdate=1 March 2011|publisher=PHI Learning Pvt. Ltd.|isbn=978-81-203-3192-1|pages=77–}}</ref>
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| Elastic scattering means that energy is (almost) conserved during the scattering event. Some elastic scattering processes are scattering from acoustic phonons, impurity scattering, piezoelectric scattering, etc. In acoustic phonon scattering, electrons scatter from state '''k''' to''' k'''', while emitting or absorbing a phonon of wave vector '''q'''. This phenomenon is usually modeled by assuming that lattice vibrations cause small shifts in energy bands. The additional potential causing the scattering process is generated by the deviations of bands due to these small transitions from frozen lattice positions.<ref name="sct">Ferry, David K. Semiconductor transport. London: Taylor & Francis, 2000. ISBN 0-7484-0865-7 (hbk.), ISBN 0-7484-0866-5 (pbk.)</ref>
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| ===Ionized impurity scattering===
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| Semiconductors are doped with donors and/or acceptors, which are typically ionized, and are thus charged. The Coulombic forces will deflect an electron or hole approaching the ionized impurity. This is known as ''[[ionized impurity scattering]]''. The amount of deflection depends on the speed of the carrier and its proximity to the ion. The more heavily a material is doped, the higher the probability that a carrier will collide with an ion in a given time, and the smaller the [[mean free time]] between collisions, and the smaller the mobility. When determining the strength of these interactions due to the long-range nature of the Coulomb potential, other impurities and free carriers cause the range of interaction with the carriers to reduce significantly compared to bare Coulomb interaction.
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| If these scatterers are near the interface, the complexity of the problem increases due to the existence of crystal defects and disorders. Charge trapping centers that scatter free carriers form in many cases due to defects associated with dangling bonds. Scattering happens because after trapping a charge, the defect becomes charged and therefore starts interacting with free carriers. If scattered carriers are in the inversion layer at the interface, the reduced dimensionality of the carriers makes the case differ from the case of bulk impurity scattering as carriers move only in two dimensions. Interfacial roughness also causes short-range scattering limiting the mobility of quasi two-dimensional electrons at the interface.<ref name=sct />
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| ===Lattice (phonon) scattering===
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| At any temperature above [[absolute zero]], the vibrating atoms create pressure (acoustic) waves in the crystal, which are termed [[phonon]]s. Like electrons, phonons can be considered to be particles. A phonon can interact (collide) with an electron (or hole) and scatter it. At higher temperature, there are more phonons, therefore increased phonon scattering which tends to reduce mobility.
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| ===Piezoelectric scattering===
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| Piezoelectric effect can occur only in compound semiconductor due to their polar nature. It is small in most semiconductors but may lead to local electric fields that cause scattering of carriers by deflecting them, this effect is important mainly at low temperatures where other scattering mechanisms are weak. These electric fields arise from the distortion of the basic unit cell as strain is applied in certain directions in the lattice.<ref name=sct />
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| ===Surface roughness scattering===
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| Surface roughness scattering caused by interfacial disorder is short range scattering limiting the mobility of quasi two-dimensional electrons at the interface. From high-resolution transmission electron micrographs, it has been determined that the interface is not abrupt on the atomic level, but actual position of the interfacial plane varies one or two atomic layers along the surface. These variations are random and cause fluctuations of the energy levels at the interface, which then causes scattering.<ref name=sct />
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| ===Alloy scattering===
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| In compound (alloy) semiconductors, which many thermoelectric materials are, scattering caused by the perturbation of crystal potential due to the random positioning of substituting atom species in a relevant sublattice is known as alloy scattering. This can only happen in ternary or higher alloys as their crystal structure forms by randomly replacing some atoms in one of the cubic lattices (sublattice) that form zinc-blende structure. Generally, this phenomenon is quite weak but in certain materials or circumstances, it can become dominant effect limiting conductivity. In bulk materials, interface scattering is usually ignored.<ref name=sct /><ref name="ssp">Ibach, Harald. ; Luth, Hans. Solid-state physics : an introduction to principles of materials science / Harald Ibach, Hans Luth. New York: Springer, 2009. -(Advanced texts in physics) ISBN 978-3-540-93803-3</ref><ref name="bulusu">A. Bulusu, D.G. Walker. Review of electronic transport models for thermoelectric materials, Superlattices and Microstructures, Volume 44, Issue 1, July 2008, Pages 1-36, ISSN 0749-6036, {{doi|10.1016/j.spmi.2008.02.008}}.</ref><ref name="pallab">Bhattacharya, Pallab. Semiconductor optoelectronic devices / Pallab Bhattacharya. Upper Saddle River (NJ): Prentice-Hall, 1997. ISBN 0-13-495656-7 (nid.)</ref><ref name="Takeda">Y. Takeda and T.P. Pearsall, "Failure of Mattheissen's Rule in the Calculation of Carrier Mobility and Alloy Scattering Effects in Ga0.47In0.53As", Electronics Lett. 17, 573-574 (1981).</ref>
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| ===Inelastic scattering===
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| During inelastic scattering processes, significant energy exchange happens. As with elastic phonon scattering also in the inelastic case, the potential arouses from energy band deformations caused by atomic vibrations. Optical phonons causing inelastic scattering usually have the energy in the range 30-50 meV, for comparison energies of acoustic phonon are typically less than 1 meV but some might have energy in order of 10 meV. There is significant change in carrier energy during the scattering process. Optical or high-energy acoustic phonons can also cause intervalley or interband scattering, which means that scattering is not limited within single valley.<ref name=sct />
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| ===Electron–electron scattering===
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| Due to the Pauli exclusion principle, electrons can be considered as non-interacting if their density does not exceed the value 10<sup>17</sup> cm<sup>−3</sup> or electric field value 10<sup>3</sup> V/cm. However, significantly above these limits electron–electron scattering starts to dominate. Long range and nonlinearity of the Coulomb potential governing interactions between electrons make these interactions difficult to deal with.<ref name=sct /><ref name=ssp /><ref name=bulusu />
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| ===Relation between mobility and scattering time===
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| A simple model gives the approximate relation between scattering time (average time between scattering events) and mobility. It is assumed that after each scattering event, the carrier's motion is randomized, so it has zero average velocity. After that, it accelerates uniformly in the electric field, until it scatters again. The resulting average drift mobility is:<ref>{{cite book|author1=Peter Y. Yu|author2=Manuel Cardona|title=Fundamentals of Semiconductors: Physics and Materials Properties|url=http://books.google.com/books?id=5aBuKYBT_hsC&pg=PA205+|accessdate=1 March 2011|date=30 May 2010|publisher=Springer|isbn=978-3-642-00709-5|pages=205–}}</ref>
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| :<math>\mu = \frac{q}{m^*}\overline{\tau}</math>
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| where ''q'' is the [[elementary charge]], m* is the carrier [[effective mass (solid-state physics)|effective mass]], and {{overline|τ}} is the average scattering time.
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| If the effective mass is anisotropic (direction-dependent), m* is the effective mass in the direction of the electric field.
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| ===Matthiessen's rule===
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| Normally, more than one source of scattering is present, for example both impurities and lattice phonons. It is normally a very good approximation to combine their influences using "Matthiessen's Rule" (developed from work by [[Augustus Matthiessen]] in 1864):
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| :<math>\frac{1}{\mu} = \frac{1}{\mu_{\rm impurities}} + \frac{1}{\mu_{\rm lattice}}</math>.
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| where µ is the actual mobility, <math>\mu_{\rm impurities}</math> is the mobility that the material would have if there was impurity scattering but no other source of scattering, and <math>\mu_{\rm lattice}</math> is the mobility that the material would have if there was lattice phonon scattering but no other source of scattering. Other terms may be added for other scattering sources, for example
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| :<math>\frac{1}{\mu} = \frac{1}{\mu_{\rm impurities}} + \frac{1}{\mu_{\rm lattice}} + \frac{1}{\mu_{\rm defects}} + \cdots</math>.
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| Matthiessen's rule can also be stated in terms of the scattering time:
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| :<math>\frac{1}{\tau} = \frac{1}{\tau_{\rm impurities}} + \frac{1}{\tau_{\rm lattice}} + \frac{1}{\tau_{\rm defects}} + \cdots</math>.
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| where ''τ'' is the true average scattering time and τ<sub>impurities</sub> is the scattering time if there was impurity scattering but no other source of scattering, etc.
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| Matthiessen's rule is an approximation and is not universally valid. This rule is not valid if the factors affecting the mobility depend on each other, because individual scattering probabilities cannot be summed unless they are independent of each other.<ref name=Takeda/> The average free time of flight of a carrier and therefore the relaxation time is inversely proportional to the scattering probability.<ref name=sct /><ref name=ssp /><ref name=pallab /> For example, lattice scattering alters the average electron velocity (in the electric-field direction), which in turn alters the tendency to scatter off impurities. There are more complicated formulas that attempt to take these effects into account.<ref>{{cite book|author1=Antonio Luque|author2=Steven Hegedus|title=Handbook of photovoltaic science and engineering|url=http://books.google.com/books?id=u-bCMhl_JjQC|accessdate=2 March 2011|date=9 June 2003|publisher=John Wiley and Sons|isbn=978-0-471-49196-5|page=79, eq. 3.58}} [http://www.knovel.com/web/portal/browse/display?_EXT_KNOVEL_DISPLAY_bookid=1081 weblink (subscription only)]</ref>
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| ===Temperature dependence of mobility=== | |
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| {| class="wikitable" style="float:right; text-align:center;"
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| |+ Typical temperature dependence of mobility<ref name=BVZ/>
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| !
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| ! Si
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| !Ge
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| !GaAs
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| |-
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| ! Electrons
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| | ∝T <sup>−2.4</sup>
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| | ∝T <sup>−1.7</sup>
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| | ∝T <sup>−1.0</sup>
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| |-
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| ! Holes
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| | ∝T <sup>−2.2</sup>
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| | ∝T <sup>−2.3</sup>
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| | ∝T <sup>−2.1</sup>
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| |}
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| With increasing temperature, phonon concentration increases and causes increased scattering. Thus lattice scattering lowers the carrier mobility more and more at higher temperature. Theoretical calculations reveal that the mobility in [[Chemical polarity|non-polar]] semiconductors, such as silicon and germanium, is dominated by [[Phonon|acoustic phonon]] interaction. The resulting mobility is expected to be proportional to ''T'' <sup>−3/2</sup>, while the mobility due to optical phonon scattering only is expected to be proportional to ''T'' <sup>−1/2</sup>. Experimentally, values of the temperature dependence of the mobility in Si, Ge and GaAs are listed in table.<ref name=BVZ/>
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| As <math>\frac{1}{\tau }\propto \left \langle v\right \rangle\Sigma </math>, where <math>\Sigma </math> is the scattering cross section for electrons and holes at a scattering center and <math>\left \langle v\right \rangle</math> is a thermal average (Boltzmann statistics) over all electron or hole velocities in the lower conduction band or upper valence band, temperature dependence of the mobility can be determined. In here, the following definition for the scattering cross section is used: number of particles scattered into solid angle dΩ per unit time divided by number of particles per area per time (incident intensity), which comes from classical mechanics. As Boltzmann statistics are valid for semiconductors
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| <math>\left \langle v\right \rangle\sim\sqrt{T}</math>.
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| For scattering from acoustic phonons, for temperatures well above Debye temperature, the estimated cross section Σ<sub>ph</sub> is determined from the square of the average vibrational amplitude of a phonon to be proportional to T. The scattering from charged defects (ionized donors or acceptors) leads to the cross section <math>{\Sigma }_{def}\propto {\left \langle v\right \rangle}^{-4}</math>. This formula is the scattering cross section for "Rutherford scattering", where a point charge (carrier) moves past another point charge (defect) experiencing Coulomb interaction.
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| The temperature dependencies of these two scattering mechanism in semiconductors can be determined by combining formulas for τ, Σ and <math>\left \langle v\right \rangle</math>, to be for scattering from acoustic phonons
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| <math>{\mu }_{ph}\sim T^{-3/2}</math> and from charged defects <math>{\mu }_{def}\sim T^{3/2}</math>.<ref name=ssp /><ref name=pallab />
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| The effect of ionized impurity scattering, however, ''decreases'' with increasing temperature because the average thermal speeds of the carriers are increase.<ref name=Singh/> Thus, the carriers spend less time near an ionized impurity as they pass and the scattering effect of the ions is thus reduced.
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| These two effects operate simultaneously on the carriers through Matthiessen's rule. At lower temperatures, ionized impurity scattering dominates, while at higher temperatures, phonon scattering dominates, and the actual mobility reaches a maximum at an intermediate temperature.
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| ==Measurement of semiconductor mobility==
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| ===Hall mobility===
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| {{Main|Hall effect}}
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| [[File:Hall Effect Measurement Setup for Holes.png|right|frame|Hall effect measurement setup for holes]]
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| [[File:Hall Effect Measurement Setup for Electrons.png|right|frame|Hall effect measurement setup for electrons]]
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| Carrier mobility is most commonly measured using the [[Hall effect]]. The result of the measurement is called the "Hall mobility" (meaning "mobility inferred from a Hall-effect measurement").
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| Consider a semiconductor sample with a rectangular cross section as shown in the figures, a current is flowing in the ''x''-direction and a [[magnetic field]] is applied in the ''z''-direction. The resulting Lorentz force will accelerate the electrons (''n''-type materials) or holes (''p''-type materials) in the (−''y'') direction, according to the right hand rule and set up an electric field ''ξ<sub>y</sub>''. As a result there is a voltage across the sample, which can be measured with a [[High impedance|high-impedance]] voltmeter. This voltage, ''V<sub>H</sub>'', is called the [[Hall effect|Hall voltage]]. ''V<sub>H</sub>'' is negative for ''n''-type material and positive for ''p''-type material.
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| Mathematically, the [[Lorentz force]] acting on a charge ''q'' is given by
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| For electrons:
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| :::<math>\overrightarrow{F}_{Hn} = -q(\overrightarrow{v}_n \times \overrightarrow{B}_z)</math>
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| For holes:
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| :::<math>\overrightarrow{F}_{Hp} = +q(\overrightarrow{v}_p \times \overrightarrow{B}_z)</math>
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| In steady state this force is balanced by the force set up by the Hall voltage, so that there is no [[net force]] on the carriers in the ''y'' direction. For electron,
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| :::<math>\overrightarrow{F}_y = (-q)\overrightarrow{\xi}_y + (-q)[\overrightarrow{v}_n \times \overrightarrow{B}_z] = 0</math>
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| :::<math>\Rightarrow -q\xi_y + qv_xB_z = 0</math>
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| :::<math> \xi_y = v_xB_z</math>
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| For electrons, the field points in the -''y'' direction, and for holes, it points in the +''y'' direction.
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| The [[Electric current|electron current]] ''I'' is given by <math>I = -qnv_xtW</math>. Sub ''v''<sub>''x''</sub> into the expression for ''ξ''<sub>''y''</sub>,
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| :::<math>\xi_y = -\frac{IB}{nqtW} = +\frac{R_{Hn}IB}{tW}</math>
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| where ''R<sub>Hn</sub>'' is the Hall coefficient for electron, and is defined as
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| :::<math>R_{Hn} = -\frac{1}{nq}</math>
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| Since <math>\xi_y = \frac{V_H}{W}</math>
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| :::<math>R_{Hn} = -\frac{1}{nq} = \frac{V_{Hn}t}{IB}</math>
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| Similarly, for holes
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| :::<math>R_{Hp} = \frac{1}{pq} = \frac{V_{Hp}t}{IB}</math>
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| From the Hall coefficient, we can obtain the carrier mobility as follows:
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| :::<math>\mu_n = (-nq)\mu_n(-\frac{1}{nq}) = -\sigma_n R_{Hn}</math>
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| ::::<math>= -\frac{\sigma_n V_{Hn}t}{IB}</math>
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| Similarly,
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| :::<math>\mu_p = \frac{\sigma_p V_{Hp}t}{IB}</math>
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| Here the value of ''V<sub>Hp</sub> (Hall voltage), t (sample thickness), I (current) and B (magnetic field)'' can be measured directly, and the conductivities σ<sub>n</sub> or σ<sub>p</sub> are either known or can be obtained from measuring the resistivity.
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| ===Field-effect mobility===
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| {{See also|MOSFET}}
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| {{Distinguish|Wien effect}}
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| The mobility can also be measured using a [[field-effect transistor]] (FET). The result of the measurement is called the "field-effect mobility" (meaning "mobility inferred from a field-effect measurement").
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| The measurement can work in two ways: From saturation-mode measurements, or linear-region measurements.<ref name=Rost>{{cite book|author=Constance Rost-Bietsch|title=Ambipolar and Light-Emitting Organic Field-Effect Transistors|url=http://books.google.com/books?id=Xxvt0CkVKaIC&pg=PA17|accessdate=1 March 2011|date=August 2005|publisher=Cuvillier Verlag|isbn=978-3-86537-535-3|pages=17–}}. This reference mistakenly leaves out a factor of 1/V<sub>DS</sub> in eqn (2.11). The correct version of that equation can be found, e.g., in {{cite journal|last1=Stassen|first1=A. F.|last2=De Boer|first2=R. W. I.|last3=Iosad|first3=N. N.|last4=Morpurgo|first4=A. F.|title=Influence of the gate dielectric on the mobility of rubrene single-crystal field-effect transistors|journal=Applied Physics Letters|volume=85|pages=3899|year=2004|doi=10.1063/1.1812368|arxiv = cond-mat/0407293 |bibcode = 2004ApPhL..85.3899S }}</ref> (See [[MOSFET]] for a description of the different modes or regions of operation.)
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| ====Using saturation mode====
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| In this technique,<ref name=Rost/> for each fixed gate voltage V<sub>GS</sub>, the drain-source voltage V<sub>DS</sub> is increased until the current I<sub>D</sub> saturates. Next, the square root of this saturated current is plotted against the gate voltage, and the slope m<sub>sat</sub> is measured. Then the mobility is:
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| :<math>\mu = m_{sat}^2 \frac{2L}{W} \frac{1}{C_i}</math>
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| where ''L'' and ''W'' are the length and width of the channel and ''C''<sub>''i''</sub> is the gate insulator capacitance per unit area. This equation comes from the approximate equation for a MOSFET in saturation mode:
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| :<math>I_D = \frac{\mu C_i}{2}\frac{W}{L}(V_{GS}-V_{th})^2.</math>
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| where V<sub>th</sub> is the threshold voltage. This approximation ignores the [[Early effect]] (channel length modulation), among other things. In practice, this technique may underestimate the true mobility.<ref name=Rost2>{{cite book|author=Constance Rost-Bietsch|title=Ambipolar and Light-Emitting Organic Field-Effect Transistors|url=http://books.google.com/books?id=Xxvt0CkVKaIC&pg=PA19|accessdate=20 April 2011|date=August 2005|publisher=Cuvillier Verlag|isbn=978-3-86537-535-3|pages=19–}} "Extracting the field-effect mobility directly from the linear region of the output characteristic may yield larger values for the field-effect mobility than the actual one, since the drain current is linear only for very small VDS and large VG. In contrast, extracting the field-effect mobility from the saturated region might yield rather conservative values for the field-effect mobility, since the drain-current dependence from the gate-voltage becomes sub-quadratic for large VG as well as for small VDS."</ref>
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| ====Using the linear region====
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| In this technique,<ref name=Rost/> the transistor is operated in the linear region (or "ohmic mode"), where V<sub>DS</sub> is small and <math>I_D \propto V_{GS}</math> with slope m<sub>lin</sub>. Then the mobility is:
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| :<math>\mu = m_{lin} \frac{L}{W} \frac{1}{V_{DS}} \frac{1}{C_i}</math>.
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| This equation comes from the approximate equation for a MOSFET in the linear region:
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| :<math>I_D= \mu C_i \frac{W}{L} \left( (V_{GS}-V_{th})V_{DS}-\frac{V_{DS}^2}{2} \right)</math>
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| In practice, this technique may overestimate the true mobility, because if V<sub>DS</sub> is not small enough and V<sub>G</sub> is not large enough, the MOSFET may not stay in the linear region.<ref name=Rost2/>
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| ==Doping concentration dependence in heavily-doped silicon==
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| The [[charge carrier]]s in semiconductors are electrons and holes. Their numbers are controlled by the concentrations of impurity elements, i.e. doping concentration. Thus doping concentration has great influence on carrier mobility.
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| While there is considerable scatter in the [[experimental data]], for noncompensated material (no counter doping) for heavily doped substrates (i.e. <math>10^{18}cm^{-3} </math> and up), the mobility in silicon is often characterized by the [[empirical relationship]]:<ref>B. L. Anderson and R. L. Anderson, "Fundamentals of Semiconductor Devices, " Mc Graw Hill, 2005</ref>
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| :::<math>\mu = \mu_o + \frac{\mu_1}{1+(\frac{N}{N_\text{ref}})^\alpha}</math>
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| where ''N'' is the doping concentration (either ''N<sub>D</sub>'' or ''N<sub>A</sub>''), and ''N<sub>ref</sub>'' and α are fitting parameters. At [[room temperature]], the above equation becomes:
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| Majority carriers:<ref>{{cite journal|last1=Caughey|first1=D.M.|last2=Thomas|first2=R.E.|title=Carrier mobilities in silicon empirically related to doping and field|journal=Proceedings of the IEEE|volume=55|pages=2192|year=1967|doi=10.1109/PROC.1967.6123}}</ref>
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| :::<math>\mu_n(N_D) = 65 + \frac{1265}{1+(\frac{N_D}{8.5\times10^{16}})^{0.72}}</math>
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| :::<math>\mu_p(N_A) = 48 + \frac{447}{1+(\frac{N_A}{6.3\times10^{16}})^{0.76}}</math>
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| Minority carriers:<ref>{{cite journal|last1=Del Alamo|first1=J|title=Measuring and modeling minority carrier transport in heavily doped silicon|journal=Solid-State Electronics|volume=28|pages=47|year=1985|doi=10.1016/0038-1101(85)90209-6 |bibcode = 1985SSEle..28...47D }}</ref>
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| :::<math>\mu_n(N_A) = 232 + \frac{1180}{1+(\frac{N_A}{8\times10^{16}})^{0.9}}</math>
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| :::<math>\mu_p(N_D) = 130 + \frac{370}{1+(\frac{N_D}{8\times10^{17}})^{1.25}}</math>
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| These equations apply only to silicon, and only under low field.
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| ==See also==
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| *[[Speed of electricity]]
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| ==References==
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| {{reflist|colwidth=30em}}
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| ==External links==
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| *[http://semiconductorglossary.com/default.asp?searchterm=electron+mobility semiconductor glossary entry for electron mobility]
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| *[http://www.ee.byu.edu/cleanroom/ResistivityCal.phtml Resistivity and Mobility Calculator from the BYU Cleanroom]
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| *Online lecture- [http://nanohub.org/resources/6151 Mobility] from an atomistic point of view
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| [[Category:Physical quantities]]
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| [[Category:Condensed matter physics]]
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| [[Category:Materials science]]
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| [[Category:Semiconductors]]
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| [[Category:Electric and magnetic fields in matter]]
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