InternationalJournalofNanoelectronicsandMaterialsVolume 11, No. 1, Jan 2018 [109-118]

FETwithUnderlapStructureforBiosensingApplications

ClarisC.J.W1,2,M.K.MdArshad1,2,C.Ibau1,R.MatAyub1,M.F.MFathil1andNorhaimiW.M.W2

1InstituteofNanoElectronicEngineering,2SchoolofMicroelectronicEngineeringUniversitiMalaysiaPerlis,Kangar,Malaysia

ABSTRACT

Thispaperpresentsthenumericalsimulationofanunderlapfieldeffecttransistor(FET)device architectureon silicon‐on‐insulator (SOI) substrate forbiosensing applications.ByusingtheSilvacoATLASdevicesimulator,thisworkisaimedtoelucidatetheeffectsof thedifferentgate lengths, thepresenceof interfacechargeon theunderlapsensingregion,andalsotheeffectsofdifferentgatebiases(i.e.singlegatebiasing,synchronousdoublegate biasing and asynchronous doublegate biasing) on the magnitude of draincurrent(ID)ofthesimulateddevice.Itisfoundthatshortergatelengthwiththepositivecharges (on the n‐p‐n structure), at the sensing channel area increased the electronconcentration at the channel and substrate/buried oxide interface. In asynchronousdoublegatewitha+3Vofback‐gatesupplyandsynchronousdouble‐gate,bothincreasedtheIDatdifferentmagnitudelevelandoff‐current.Thus,dependingonthebiomoleculecharges,thesubstratebiasingcanbealteredtoimprovethedevice’ssensitivity.Keywords: Underlap field‐effect transistor; biosensors; singlegate; synchronousdoublegate;asynchronousdouble‐gatebiasing.

1. INTRODUCTION

A biosensor has an ability to transform physical or chemical changes as it accompanyingbiological reaction intoanelectrical signal.Typically, abiosensorhas twoelements,whicharebio‐receptor and transducer. Bio‐receptor is an immobilized sensitive biological element thatrecognizes the analyte such as an enzyme, antibody and deoxyribonucleic acid (DNA). On theotherhand,atransducerisusedtoconvertthebiochemicalsignalfromtheresultofinteractionbetweentheanalyteandbio‐receptor,intoanotherformofreadablesignal.Thesevariabilityoftransduced signals are based on the concentration of analytes that is being detected in thesolution[1],[2].Thesemiconductor‐basedbiosensorconfigurationbasedonfield‐effecttransistor(FET)deviceisoftenusedduetoitspowerfulplatformforthedirectdetectionofbiologicalandchemicalspecies[3], [4]. FET‐based biosensors is claimed to have high scalability, mass reproducibility, andcompatibility with the on‐chip integration on both the sensor and measurement system [5].Variousdevicearchitectureshavebeendemonstratedatthedevice level[6],[7], [8]. Intheso‐calledion‐sensitiveFET(ISFET)basedbiosensor,themetalgateoftheFETisreplacedbyabio‐filmlayermaterial.Draincurrent ischangingdueto thebio‐modulatedgatesurface,knownasthe‘gatingeffect’thatinfluencesthechannelconductivityofthedevice.ISFETbiosensoroffersanCorrespondingAuthor:mohd.khairuddin@unimap.edu.my

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outstandingarchitectureascomparedtootherdevicesintermofforuseinhighsensitivemeasurements, portable instrumentation, easy operationwith a small amount of sample, lowcostwithmassreproducibleandhighspeed[9].In order to overcome the problems of low binding probability in a carved nanogap and lowstructural stability which is stemming from the mechanically suspended nanogap structure,thereforetheFETwithunderlapstructureisused.Biomoleculesinunderlapstructureshowedahigherprobabilitytobindwithdesignedarea[10].Besidesthat,thechannelpotentialisknowntobeverysensitivetoexternalchargesintheunderlapstructure.Therefore,theobservationoncurrent change can be done more precisely. The targeted molecules can be easily detectedelectrically by detecting the changes in the drain current [11]. Moreover, the combination ofhighly sensitive underlap FET structurewith the presence of back‐gate biasing is expected tofurtherenhancethesensitivityofthedevicesinceitcanboostalowlimit‐of‐detection(LOD)oftheanalytemolecules.Withtheimplementationofdoublegate[12],[13],[14]itisarguedthatthesensitivitycanbeenhancedasdemonstratedondevicelevel[15].Inthispaper,wesimulatetheunderlapFETstructurewithanoveltyofhavinganunderlapasthetransducingregion for thebio‐molecularrecognitionsite.Thepaper isexpected toexplain thesignificanceofdifferentgatelengthonthedevice’selectricalresponses(I‐V),besidesanalyzingontheimpactofsinglegate,synchronousdoublegateandasynchronousdoublegatetowardstheelectricalpropertiesoftheproposeddevicestructureforbiosensingapplications.2. METHODOLOGYONDEVICESIMULATION

2.1Device’sSubstrateThe simulations are carried out by utilizing ATLAS device simulation software. The devicemodellingstructureisbasedonsilicon‐on‐insulator(SOI)substratewiththethicknessofthetopp‐typesiliconlayerof70nm,thethicknessoftheburiedoxide(BOx) layerof145nm,andthebottombulksiliconsubstratewith100μmthickrespectively[16].ThisstudyisgravitatedtouseSOI as substrate because it is known as good electrostatic control for the transistor devicechannel.Addingtothis,SOIhastheabilitytoco‐integratedeviceswithotherdevicesonsiliconbulkbyetchingthetopsiliconlayerandtheunderlyingBOxlayer.Fig.1illustratesthetopviewandsideviewofsimulationdevicemodelwiththepresenceofunderlapregionbetweenthegate‐drainterminals.

Figure1.TopviewandsideviewofdevicemodelstructurewithanunderlapregiononanSOI.

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2.2 DeviceModel’sDesigning

Thetop70nmsiliconlayerandbottom100µmsiliconbulkweredopinguniformlywithp‐typedopantataconcentrationof1x1015cm‐3.Ontheotherhand,thesourceanddrainregionsweredopinguniformlywithn‐typedopantataconcentrationof1x1020cm‐3.Thecontactsweremadefromaluminumwithwork‐functionsetat4.6electron‐volt(eV) layeredonthecontactsurface,formingaSchottkycontactoneachoftheterminals.Thethicknessofgateoxidewas10nmwiththeunderlaplengthat2µmwerepositioningbetweenthegate‐drainterminals.Followingtothis,the length of source and drain terminals were fixed at 2 µm and 5 µm respectively. Theseparameterswerekeptconstantthroughoutthedevicemodelingandsimulations.Table1showsthefixedparametersonthemodeleddeviceforsimulation.

Table1Simulationdevice’sfixedparameters

ModelledDevice’sDimensions

Parameters Dimensionvalue

Silicon‐on‐insulator(SOI)

TopSibulk:70nmBox:145nm

BottomSi‐bulk:100µmGateoxidethickness 10nm

Underlaplength 2µm

Sourcelength 2µm

Drainlength 5µm

Back‐gatelength 2.5µm

Source/draindoping n‐typeat1x1020cm‐3

Undopedregion p‐typeat1x1015cm‐3

Interfacecharge 6x1010cm‐2

Thesimulatedgatelengthswere1μm,500nmand100nmandhavedifferentinterfacecharges,i.e.positive,negativeandwithoutinterfacechargeontheunderlapregion.Theinterfacechargewere representing the biomolecule charges. For example,DNA always carries negative chargeduetoitsnegativelychargedphosphatebackbone,meanwhile,proteinscanbeeitherpositivelyornegativelychargeddependingonitsisoelectricpoint(pI).Later,thesedesignparametersweresimulating under the effect of singlegate biasing, synchronous doublegate biasing andasynchronousdoublegatebiasingtoanalyzetheinteractionsbetweenthem.Table2showsthelistsofsimulationvariablesundertestonthemodeleddevice.

Table2Device’sSimulationVariables

ManipulativeVariablesDevice’sdesignand

architecturesTestedvariables

Interface charge on theunderlapsurface

Positive,negativeandwithoutinterface

charge

Gatelength1µm,500nmand100

nm

TypeofgatebiasesSingle‐gate,synchronousdoublegateandasynchronousdoublegatewith+3V

back‐gatesupply

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2.3Device’sSimulationParametersFor single‐gate, synchronous doublegate and asynchronous doublegate were biasing with thegate voltage at +1.5 V under voltage step of +0.02 V. However, the back gate terminal of theasynchronousdoublegatewasbiasingat+3Vundervoltagestepof+1.0V.Thedrain forall thethreetypesofbiasingwasdeterminedat+1.0Vunder+0.1Vofvoltagestep.3. RESULTSANDDISCUSSION

Fig.2showsthecurrent–voltage(I‐V)characteristicsfordifferentgatelengthwhenthedeviceisinsinglebiased(onlytop‐gateisbiasedfrom0to1.6V).Asclaimedpreviously,the500nmgate length has a better performance with better trade‐off between on‐state and off‐statecurrent,comparingtohigher1μmand100nmgatelength.Theanalysisindicatesthatalongergatehasacapabilitytointroduceparasiticresistancebetweenthesourceanddrainterminals.This phenomenon has restricted the formation of inversion layerwhen the gate voltagewasapplied,thusreducingtheID.Whilefor100nmlength,thegateislosingcontrolofthechannel,thusincreasetheoff‐statecurrent.Atthisgatelength,itformsaweakinversionregionbetweenthe source and drain. As the gate length is reducing, the capacitive coupling of the channelpotentialtothesourceanddrainincreasesrelativetothegate,leadingtosignificantlydegradedshort‐channel effects (SCE)which causedby theDrain‐Inducedbarrier lowering (DIBL). Thishappeneddueto impactofdrainelectric fieldduringhighdrainvoltageapplicationwhenthepotentialbarrierheightforchannelcarriersattheedgeofthesourcedecreases[17].Thereisnocurrentflowthathasbeendetectedwhennegativeinterfacechargeandnointerfacechargeisappliedtothesurfaceoftheunderlapregion.

Fig.3showsthattheelectronconcentrationalongthechannelwhentheunderlapstructureareahasbeensubjectedfordifferentcharges(positive,negativeandnocharge).Initially,thechannelhas doped with n‐type impurities. At the area underneath the channel region, the electronconcentration changes from low to high concentration when the underlap is charging withpositive and negative charges, respectively. Hence higher drain current can obtained thepositive charge due to an increase of electron concentration at the channel. Therefore, theconductinglayerofmobileelectronsformedattheSisurfacebeneaththeoxidehastheabilitytocarrycurrentbetweenthesourceanddrain,throughthechannel.

Figure2.Theeffectofdifferentgatelengthtowardpositive,negativeandwithoutinterfacechargesatsinglegatebiasing.

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Figure3.(a)TonyplotoftheFETstructure(b)Electronconcentrationalongthechannel.

Fig. 4 shows the current – voltage (I‐V) characteristics for different gate lengths graphwhensubjected to synchronous doublegate biasing. In synchronous doublegate, the front gate hasconnectedwiththeback‐gatebyutilizingonlyonevoltagesupply.Wecanseethat,for500nmgate lengthwithasynchronousdoublegateandapositiveinterfacechargeatthesensingareahassignificantlyincreasedtheIDcomparedtosinglegateoperationasdiscussedpreviously(Fig.2).Insteadforthe100nmgatelength,itshowedthatthereisaleakagecurrentforallthethreesituations.Thereductionhascausedanincrementofinjectedcarriersintothechannelfromthesource that leads to the off current enlargement. Thus, it is not suitable for it to be used ofbiosensingapplicationandnotinterestingtofocusinthediscussion.

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Figure4.Theeffectofdifferentgatelengthwithpositive,negativeandwithoutinterfacechargeat

synchronousdoublegatebiasing.Fig. 5 shows a graph of asynchronous double‐gate biasing. At this condition the back‐gate issuppliedwith+3Vvoltage.Clearlyattheexpensesofhighoff‐statecurrent,theincreaseofdraincurrent is higher when single is being compared to synchronous doublegates.. This outcomeoccursatallinterfacechargescondition;i.e.positive,negativeandnointerfacecharge.

Figure5.Theeffectofdifferentgatelengthwithpositive,negativeandwithoutinterfacechargesof

asynchronousdoublegate,Vbg=+3V.

Fig. 6 summarized the performance of 500 nm gate length for different voltage biasingcondition; i.e. singlegate, synchronous and asynchronous doublegates with positive interfacecharge condition. The drain current is increasing significantly when it used asynchronousdouble‐gatebiasing.The increased isbetween100%and200% if compared tosynchronousand singlegate biasing respectively. Apart from this, it is found that the off‐state current isincreasingforthesynchronousback‐gatebiasing.

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Figure6.Draincurrentcharacteristicfor500nmgatelengthwithsinglegate,synchronousdoublegate

andasynchronous3Vback‐gatebiasing.

Fig. 7 shows the electron concentration for the result obtained in Fig. 6. The electronconcentrationcuthadbeendoneatthemiddleofthechannel(Fig.7a)andatthesubstrate/BOxinterface (Fig. 7b). At the channel area, the electron concentration is higher under theasynchronous+3Vback‐gatesupplyandasynchronousdoublegateifcomparedtosinglegate.Higherelectronconcentrationatthechannelindicatesthatthedraincurrentflowbetweentheterminals of source and drain alsowill be high.While at the substrate/BOx interface, higherconcentrationisobservedwithsinglegate.Asseenfromthechannelarea,thereisnosignificantdifference canbe seenwith theelectron concentrationbetween the synchronousdouble‐gateandasynchronousdouble‐gatebiases(Fig.7c).

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Figure7.Electronconcentrationfor500nmgatelengthindifferentbiasingalongtheterminals.

4.CONCLUSIONThe simulated results on n‐p‐n junction, between source, channel and respectively, show thatpositiveinterfacecharge(whichistranslatedintothepositivelychargedbiomolecules)hasthehighestdraincurrentcomparedto theneutralandnegativelychargedmoleculesof thedevice.Amongthethreetypesofbiasing,theasynchronousdoublegatewith+3Vback‐gatesupplyhasgenerated the highest ID. From the simulated data, it shows that the presence of underlapstructureinFETwitheithersynchronousorasynchronousdouble‐gatewith+3Vbiasinghas itpotentialtoenhancetheI‐Vresponses(i.e.exhibitshighsensitivityforlowtohighconcentrationofanalyteontheunderlap'sensing'surface)ofthedevice.Howeverthetrade‐offperformanceofsynchronousdoublegatedevicebetweenanon‐statetooff‐statecurrenthasbettersensitivityifcompared to theasynchronous+3Vback‐gatebiasing.Therefore, the findings agreed that theunderlap FET architecture holds an unprecedented potential to be used as a highly sensitivebiosensors.

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