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Ohmicresistancebehavior465●OOOcclusion—Atypeofcoprecipitationinwhichacon-taminantspeciesistrappedwithinthegrowingcrystal.Theamountofoccludedmaterialisgreatestinthepartofacrystalthatformsfirst[i].Occlusionoforganicaddi-tivescanoccurinelectrodeposition,e.g.,incorporationofcumarininnickel[ii].Refs.:[i]SkoogD,WestD,HollerF()Fundamentalsofanalyti-calchemistry.SaundersCollegePublishing,NewYork;[ii]PauvonicM,SchlesingerM()Fundamentalsofelectrochemicaldeposition,ndedn.Wiley,NewYork,pFGOhm,GeorgSimon(CourtesyofGeorg-Simon-Ohm-FachhochschuleNürnberg)(Mar.,,Erlangen,Germany–July,,Mu-nich,Germany)OhmwasaGermanphysicistwhostud-iedfundamentalprinciplesofelectricityandelectro-magnetism[i,ii].Hediscoveredthelaw,namedafterhimOhm’slaw,whichstatesthatthecurrent(I)flow-ingthroughaconductorisdirectlyproportionaltothepotentialdifference(voltage,U)andinverselypro-portionaltotheresistance(R):I=U/R(Ohmequa-tion)[iii].TheresistanceunitOhm(SI)wasnamedafterhim.Refs.:[i]PourprixB()ArchIntHistSci:–;[ii]JungnickelC,McCormmachR()Intellectualmasteryofnature:theoreticalphysicsfromOhmtoEinstein.UniversityofChicagoPress,Chicago;[iii]OhmGS()DiegalvanischeKette:mathematischbearbeitet(TheGalvaniccircuitinvestigatedmathematically).Riemann,BerlinEKOhmiccontact—Insemiconductorsciencetheterm‘ohmiccontact’designatesametalorsilicidecontacttoasemiconductorwithasmallinterfacialresistance.Inthestrictsense,ohmiccharacteristicappliesonlytotheobservationofalineardependencebetweencur-rentIandappliedvoltageU.Butinpractice,con-tactsareusuallydenominatedohmicifthecontactresis-tanceislowcomparedtotheresistanceofthedevicetobecontacted,eveniftheI(U)characteristicsarenotlin-ear.Asatisfactoryohmiccontactshouldnotsignificantlyperturbthedeviceperformance.Animportantfigureofmeritforohmiccontactsisitscontactresistanceatzerobias.Thecontactresistanceisdefinedasthereciprocalofthederivativeofthecurrentdensitywithrespecttoappliedvoltage,Rc∂J∂U−dU=Refs.:[i]NgKK()Completeguidetosemiconductordevices.Wiley,NewYork;[ii]SzeSM()Physicsofsemiconductordevices.Wiley,SingaporeIHOhmicdropdistortion—Distortionofanelectrochem-icalresponsecausedbyuncompensatedohmicresis-tance(see:IRu(ohmicpotential)drop).AMBOhmicpolarizationohmicoverpotentialOhmicresistancebehavior—AsystemobeyingOhm’slaw(thevoltageequalstheproductofcur-rentandresistance,U=IR)issaidtoexhibitohmicresistancebehavior.TheelectricalresistanceRofanohmicallybehavingdeviceorelementisconstantanddoesnotdependonthemagnitudeofthecurrentorappliedvoltageU,i.e.,thedifferentialorincrementalresistancedefinedasR=dUdIhastobeconstant.TheelectricalresistanceRisequivalenttothedifferentialresistanceonlyfortrueohmicresistancebehavior.IftheU−Ifunctionisnotmonotonic(i.e.,ithasamaximumorminimum),thedifferentialresistancewillbenegativeforsomevaluesofvoltageandcurrent.Thispropertyiscallednegativedifferentialresistance.Norealdevicedemonstratesidealohmicresistancebehavior,butforawidevarietyofmaterialsandconditions,manydevicesapproximateitclosely,includingresistors,piecesofmetalwire,lightbulbs,andelectrolytesolutions,whereaschargetransferresistanceisnon-ohmic.Insemiconductordevices,highlydopedregions(ohmiccontacts)approximateohmicresponseseventhoughmoderatelydopedregionsarestronglydepen-dentonvoltage.Theelectricresistanceofatypicalmetalconductorincreaseslinearlywiththetemperature:466Ohmicoverpotential●OR=R+aT,whereasthatofatypicalsemiconduc-tordecreasesexponentiallywiththetemperature:R=R+e(aT).Ref.:[i]CutnellJD,JohnsonKW()Physics.Wiley,NewYorkMHerOhmicoverpotential—Thedeviationoftheappliedpotentialorthevoltagemeasuredinanelectro-chemicalsystemunderconditionsofcurrentpassageduetosolutionresistance.Whenevercurrentpassesthroughanelectrochemicalsystem,thevoltagemea-suredorappliedsuffersdeviationfromtheequilibriumvalueduetoseveralfactors.Themainfactorsarecon-centrationpolarization,activationpolarization,andohmicoverpotential,whichisalsocalled“ohmicpolar-ization”.Inthecaseofohmicoverpotential,thevoltagedifferenceamountstoΔU=IR,inwhichIandRde-notethecurrentandthesolutionresistance,respectively.Regardlessofthesystem,(abattery,anelectrolysiscell,orathree-electrodeelectroanalyticalsystem)ohmicoverpotentialalwayslowerstheactualbiasedormea-suredpotential(itsabsolutevalue).Incaseswhereverylargecurrentsareemployed,ohmicoverpotentialscanalsobecausedduetotheconductors(wires,contacts)re-sistance.Moreover,electrodesmadeofmaterialsoflowelectronicconductivity,e.g.,compositeelectrodes,canalsodevelopohmicoverpotentialsduetotheelectronicresistancewithintheelectrodemass.Thesecomeinad-ditiontothesolutionohmicoverpotential.Occasionally,initiallyelectronicallyconductingelectrodesdevelopin-sulatingsurfacefilmsasaresultofchemicalorelectro-chemicalreaction,orduetoadsorption.Insuchcasesanohmicoverpotentialwouldalsobedeveloped.OhmicoverpotentialisalsonamedIRdrop,resis-tanceoverpotential,orohmicdrop.Refs.:[i]BroadheadJ,KuoHC()Electrochemicalprinciplesandreactions.In:LindenD(ed)Handbookofbatteries,ndedn.McGraw