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height="130" class="mw-file-element" data-file-width="220" data-file-height="130" /></a><figcaption>Diagram showing Drift Velocity</figcaption></figure> <p><b>Saturation velocity</b> is the maximum velocity a charge carrier in a <a href="/wiki/Semiconductor" title="Semiconductor">semiconductor</a>, generally an <a href="/wiki/Electron" title="Electron">electron</a>, attains in the presence of very high <a href="/wiki/Electric_field" title="Electric field">electric fields</a>.<sup id="cite_ref-1" class="reference"><a href="#cite_note-1"><span class="cite-bracket">[</span>1<span class="cite-bracket">]</span></a></sup> When this happens, the semiconductor is said to be in a state of <b>velocity saturation</b>. <sup id="cite_ref-e2_2-0" class="reference"><a href="#cite_note-e2-2"><span class="cite-bracket">[</span>2<span class="cite-bracket">]</span></a></sup> <a href="/wiki/Charge_carrier" title="Charge carrier">Charge carriers</a> normally move at an average <a href="/wiki/Drift_speed" class="mw-redirect" title="Drift speed">drift speed</a> proportional to the electric <a href="/wiki/Field_strength" title="Field strength">field strength</a> they experience temporally. The proportionality constant is known as <a href="/wiki/Electrical_mobility" title="Electrical mobility">mobility</a> of the carrier, which is a material property. A good <a href="/wiki/Electrical_conductor" title="Electrical conductor">conductor</a> would have a high mobility value for its charge carrier, which means higher velocity, and consequently higher current values for a given electric field strength. There is a limit though to this process and at some high field value, a charge carrier can not move any faster, having reached its saturation velocity, due to mechanisms that eventually limit the movement of the carriers in the material.<sup id="cite_ref-3" class="reference"><a href="#cite_note-3"><span class="cite-bracket">[</span>3<span class="cite-bracket">]</span></a></sup> </p><p>As the applied electric field increases from that point, the carrier velocity no longer increases because the carriers lose energy through increased levels of interaction with the lattice, by emitting <a href="/wiki/Phonons" class="mw-redirect" title="Phonons">phonons</a> and even <a href="/wiki/Photon" title="Photon">photons</a> as soon as the carrier energy is large enough to do so.<sup id="cite_ref-co_4-0" class="reference"><a href="#cite_note-co-4"><span class="cite-bracket">[</span>4<span class="cite-bracket">]</span></a></sup> </p> <meta property="mw:PageProp/toc" /> <div class="mw-heading mw-heading2"><h2 id="Field_effect_transistors">Field effect transistors</h2><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Saturation_velocity&action=edit&section=1" title="Edit section: Field effect transistors"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <p>Saturation velocity is a very important parameter in the design of semiconductor devices, especially <a href="/wiki/Field_effect_transistors" class="mw-redirect" title="Field effect transistors">field effect transistors</a>, which are basic building blocks of almost all modern <a href="/wiki/Integrated_circuits" class="mw-redirect" title="Integrated circuits">integrated circuits</a>. Typical values of saturation velocity may vary greatly for different materials, for example for <a href="/wiki/Silicon" title="Silicon">Si</a> it is in the order of 1×10<sup>7</sup> cm/s, for <a href="/wiki/GaAs" class="mw-redirect" title="GaAs">GaAs</a> 1.2×10<sup>7</sup> cm/s, while for <a href="/wiki/SiC#Structure_and_properties" class="mw-redirect" title="SiC">6H-SiC</a>, it is near 2×10<sup>7</sup> cm/s. Typical electric field strengths at which carrier velocity saturates is usually on the order of 10-100 kV/cm. Both saturation field and the saturation velocity of a semiconductor material are typically strong function of impurities, <a href="/wiki/Crystal_defect" class="mw-redirect" title="Crystal defect">crystal defects</a> and temperature. </p> <div class="mw-heading mw-heading2"><h2 id="Small_scale_devices">Small scale devices</h2><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Saturation_velocity&action=edit&section=2" title="Edit section: Small scale devices"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <p>For extremely small scale devices, where the high-field regions may be comparable or smaller than the average <a href="/wiki/Mean_free_path" title="Mean free path">mean free path</a> of the charge carrier, one can observe <a href="/wiki/Velocity_overshoot" title="Velocity overshoot">velocity overshoot</a>, or <a href="/wiki/Hot_electron" class="mw-redirect" title="Hot electron">hot electron</a> effects which has become more important as the transistor geometries continually decrease to enable design of faster, larger and more dense integrated circuits.<sup id="cite_ref-5" class="reference"><a href="#cite_note-5"><span class="cite-bracket">[</span>5<span class="cite-bracket">]</span></a></sup> The regime where the two terminals between which the electron moves is much smaller than the mean free path, is sometimes referred as <a href="/wiki/Ballistic_transport" class="mw-redirect" title="Ballistic transport">ballistic transport</a>. There have been numerous attempts in the past to build <a href="/wiki/Transistors" class="mw-redirect" title="Transistors">transistors</a> based on this principle without much success. Nevertheless, developing field of <a href="/wiki/Nanotechnology" title="Nanotechnology">nanotechnology</a>, and new materials such as <a href="/wiki/Carbon" title="Carbon">Carbon</a> <a href="/wiki/Carbon_nanotube" title="Carbon nanotube">nanotubes</a> and <a href="/wiki/Graphene" title="Graphene">graphene</a>, offers new hope. </p> <div class="mw-heading mw-heading2"><h2 id="Negative_differential_resistivity">Negative differential resistivity</h2><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Saturation_velocity&action=edit&section=3" title="Edit section: Negative differential resistivity"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <p>Though in a semiconductor such as Si saturation velocity of a carrier is same as the peak velocity of the carrier, for some other materials with more complex <a href="/wiki/Energy_band" class="mw-redirect" title="Energy band">energy band</a> structures, this is not true. In <a href="/wiki/GaAs" class="mw-redirect" title="GaAs">GaAs</a> or <a href="/wiki/InP" class="mw-redirect" title="InP">InP</a> for example the carrier drift velocity reaches to a maximum as a function of field and then it begins to actually decrease as the electric field applied is increased further. Carriers which have gained enough energy are kicked up to a different <a href="/wiki/Conduction_band" class="mw-redirect" title="Conduction band">conduction band</a> which presents a lower drift velocity and eventually a lower saturation velocity in these materials. This results in an overall decrease of current for higher voltage until all electrons are in the "slow" band and this is the principle behind operation of a <a href="/wiki/Gunn_diode" title="Gunn diode">Gunn diode</a>, which can display negative differential resistivity. Due to the transfer of electrons to a different conduction band involved, such devices, usually single terminal, are referred to as <a href="/wiki/Transferred_electron_device" class="mw-redirect" title="Transferred electron device">Transferred electron devices</a>, or TEDs. </p> <div class="mw-heading mw-heading2"><h2 id="Design_considerations">Design considerations</h2><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Saturation_velocity&action=edit&section=4" title="Edit section: Design considerations"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <p>When designing <a href="/wiki/Semiconductor_device" title="Semiconductor device">semiconductor devices</a>, especially on a sub-micrometre scale as used in modern <a href="/wiki/Microprocessor" title="Microprocessor">microprocessors</a>, velocity saturation is an important design characteristic. Velocity saturation greatly affects the voltage transfer characteristics of a <a href="/wiki/Field-effect_transistor" title="Field-effect transistor">field-effect transistor</a>, which is the basic device used in most <a href="/wiki/Integrated_circuits" class="mw-redirect" title="Integrated circuits">integrated circuits</a>. If a semiconductor device enters velocity saturation, an increase in voltage applied to the device will not cause a linear increase in current as would be expected by <a href="/wiki/Ohm%27s_law" title="Ohm's law">Ohm's law</a>. Instead, the current may only increase by a small amount, or not at all. It is possible to take advantage of this result when trying to design a device that will pass a constant current regardless of the voltage applied, a <a href="/wiki/Current_limiter" class="mw-redirect" title="Current limiter">current limiter</a> in effect. </p> <div class="mw-heading mw-heading2"><h2 id="References">References</h2><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Saturation_velocity&action=edit&section=5" title="Edit section: References"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <style data-mw-deduplicate="TemplateStyles:r1239543626">.mw-parser-output .reflist{margin-bottom:0.5em;list-style-type:decimal}@media screen{.mw-parser-output .reflist{font-size:90%}}.mw-parser-output .reflist .references{font-size:100%;margin-bottom:0;list-style-type:inherit}.mw-parser-output .reflist-columns-2{column-width:30em}.mw-parser-output .reflist-columns-3{column-width:25em}.mw-parser-output .reflist-columns{margin-top:0.3em}.mw-parser-output .reflist-columns ol{margin-top:0}.mw-parser-output .reflist-columns li{page-break-inside:avoid;break-inside:avoid-column}.mw-parser-output .reflist-upper-alpha{list-style-type:upper-alpha}.mw-parser-output .reflist-upper-roman{list-style-type:upper-roman}.mw-parser-output .reflist-lower-alpha{list-style-type:lower-alpha}.mw-parser-output .reflist-lower-greek{list-style-type:lower-greek}.mw-parser-output .reflist-lower-roman{list-style-type:lower-roman}</style><div class="reflist reflist-columns references-column-width reflist-columns-2"> <ol class="references"> <li id="cite_note-1"><span class="mw-cite-backlink"><b><a href="#cite_ref-1">^</a></b></span> <span class="reference-text"><i>Fundamentals of Semiconductors: Physics and Materials Properties</i>, Peter Y. Yu, Manuel Cardona, pp. 227-228, Springer, New York 2005, <style data-mw-deduplicate="TemplateStyles:r1238218222">.mw-parser-output cite.citation{font-style:inherit;word-wrap:break-word}.mw-parser-output .citation q{quotes:"\"""\"""'""'"}.mw-parser-output .citation:target{background-color:rgba(0,127,255,0.133)}.mw-parser-output .id-lock-free.id-lock-free a{background:url("//upload.wikimedia.org/wikipedia/commons/6/65/Lock-green.svg")right 0.1em center/9px no-repeat}.mw-parser-output .id-lock-limited.id-lock-limited a,.mw-parser-output .id-lock-registration.id-lock-registration a{background:url("//upload.wikimedia.org/wikipedia/commons/d/d6/Lock-gray-alt-2.svg")right 0.1em center/9px no-repeat}.mw-parser-output .id-lock-subscription.id-lock-subscription a{background:url("//upload.wikimedia.org/wikipedia/commons/a/aa/Lock-red-alt-2.svg")right 0.1em center/9px no-repeat}.mw-parser-output .cs1-ws-icon a{background:url("//upload.wikimedia.org/wikipedia/commons/4/4c/Wikisource-logo.svg")right 0.1em center/12px no-repeat}body:not(.skin-timeless):not(.skin-minerva) .mw-parser-output .id-lock-free a,body:not(.skin-timeless):not(.skin-minerva) .mw-parser-output .id-lock-limited a,body:not(.skin-timeless):not(.skin-minerva) .mw-parser-output .id-lock-registration a,body:not(.skin-timeless):not(.skin-minerva) .mw-parser-output .id-lock-subscription a,body:not(.skin-timeless):not(.skin-minerva) .mw-parser-output .cs1-ws-icon a{background-size:contain;padding:0 1em 0 0}.mw-parser-output .cs1-code{color:inherit;background:inherit;border:none;padding:inherit}.mw-parser-output .cs1-hidden-error{display:none;color:var(--color-error,#d33)}.mw-parser-output .cs1-visible-error{color:var(--color-error,#d33)}.mw-parser-output .cs1-maint{display:none;color:#085;margin-left:0.3em}.mw-parser-output .cs1-kern-left{padding-left:0.2em}.mw-parser-output .cs1-kern-right{padding-right:0.2em}.mw-parser-output .citation .mw-selflink{font-weight:inherit}@media screen{.mw-parser-output .cs1-format{font-size:95%}html.skin-theme-clientpref-night .mw-parser-output .cs1-maint{color:#18911f}}@media screen and (prefers-color-scheme:dark){html.skin-theme-clientpref-os .mw-parser-output .cs1-maint{color:#18911f}}</style><a href="/wiki/ISBN_(identifier)" class="mw-redirect" title="ISBN (identifier)">ISBN</a> <a href="/wiki/Special:BookSources/3-540-25470-6" title="Special:BookSources/3-540-25470-6">3-540-25470-6</a></span> </li> <li id="cite_note-e2-2"><span class="mw-cite-backlink"><b><a href="#cite_ref-e2_2-0">^</a></b></span> <span class="reference-text"><link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r1238218222" /><cite class="citation web cs1"><a rel="nofollow" class="external text" href="http://everything2.com/index.pl?node_id=1321513">"Velocity Saturation"</a><span class="reference-accessdate">. Retrieved <span class="nowrap">2006-10-23</span></span>.</cite><span title="ctx_ver=Z39.88-2004&rft_val_fmt=info%3Aofi%2Ffmt%3Akev%3Amtx%3Abook&rft.genre=unknown&rft.btitle=Velocity+Saturation&rft_id=http%3A%2F%2Feverything2.com%2Findex.pl%3Fnode_id%3D1321513&rfr_id=info%3Asid%2Fen.wikipedia.org%3ASaturation+velocity" class="Z3988"></span></span> </li> <li id="cite_note-3"><span class="mw-cite-backlink"><b><a href="#cite_ref-3">^</a></b></span> <span class="reference-text"><i>GaAs Devices and Circuits</i>, Michael Shur, pp. 310-324, Plenum Press, NY 1987, <link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r1238218222" /><a href="/wiki/ISBN_(identifier)" class="mw-redirect" title="ISBN (identifier)">ISBN</a> <a href="/wiki/Special:BookSources/0-306-42192-5" title="Special:BookSources/0-306-42192-5">0-306-42192-5</a></span> </li> <li id="cite_note-co-4"><span class="mw-cite-backlink"><b><a href="#cite_ref-co_4-0">^</a></b></span> <span class="reference-text"><link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r1238218222" /><cite class="citation web cs1"><a rel="nofollow" class="external text" href="http://ece-www.colorado.edu/~bart/book/book/chapter7/ch7_7.htm#7_7_7">"Advanced MOSFET issues"</a><span class="reference-accessdate">. Retrieved <span class="nowrap">2006-10-23</span></span>.</cite><span title="ctx_ver=Z39.88-2004&rft_val_fmt=info%3Aofi%2Ffmt%3Akev%3Amtx%3Abook&rft.genre=unknown&rft.btitle=Advanced+MOSFET+issues&rft_id=http%3A%2F%2Fece-www.colorado.edu%2F~bart%2Fbook%2Fbook%2Fchapter7%2Fch7_7.htm%237_7_7&rfr_id=info%3Asid%2Fen.wikipedia.org%3ASaturation+velocity" class="Z3988"></span></span> </li> <li id="cite_note-5"><span class="mw-cite-backlink"><b><a href="#cite_ref-5">^</a></b></span> <span class="reference-text">High Field Hole Velocity and Velocity Overshoot in Silicon Inversion Layers, D. Sinitsky, F. Assaderaghi, C. Hu, and J. Bokor, <a href="/wiki/IEEE_Electron_Device_Letters" title="IEEE Electron Device Letters">IEEE Electron Device Letters</a>, vol. 18, no. 2, February 1997</span> </li> </ol></div>    </div><noscript><img src="https://login.wikimedia.org/wiki/Special:CentralAutoLogin/start?useformat=desktop&type=1x1&usesul3=0" alt="" width="1" height="1" style="border: none; position: absolute;"></noscript> <div class="printfooter" data-nosnippet="">Retrieved from "<a dir="ltr" href="https://en.wikipedia.org/w/index.php?title=Saturation_velocity&oldid=1258480123">https://en.wikipedia.org/w/index.php?title=Saturation_velocity&oldid=1258480123</a>"</div></div> <div id="catlinks" class="catlinks" data-mw="interface"><div id="mw-normal-catlinks" class="mw-normal-catlinks"><a href="/wiki/Help:Category" title="Help:Category">Categories</a>: <ul><li><a href="/wiki/Category:Transistors" title="Category:Transistors">Transistors</a></li><li><a href="/wiki/Category:Charge_carriers" title="Category:Charge carriers">Charge carriers</a></li><li><a href="/wiki/Category:Physical_quantities" title="Category:Physical quantities">Physical quantities</a></li><li><a href="/wiki/Category:Semiconductors" title="Category:Semiconductors">Semiconductors</a></li></ul></div></div> </div> </main> </div> <div class="mw-footer-container"> <footer id="footer" class="mw-footer" > <ul id="footer-info"> <li id="footer-info-lastmod"> This page was last edited on 19 November 2024, at 23:52<span class="anonymous-show"> (UTC)</span>.</li> <li id="footer-info-copyright">Text is available under the <a href="/wiki/Wikipedia:Text_of_the_Creative_Commons_Attribution-ShareAlike_4.0_International_License" title="Wikipedia:Text of the Creative Commons Attribution-ShareAlike 4.0 International License">Creative Commons Attribution-ShareAlike 4.0 License</a>; additional terms may apply. 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