MOSFET threshold voltage

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MOSFET threshold voltage

  1. 1. Metal Oxide SemiconductorField Effect Transistor (MOSFET)Structure:semiconductormetal oxidep+n nS G D
  2. 2. MOSFET operationIf VG=0 Assuming VD=high, VS=0GSDp+n nNo current
  3. 3. MOSFET operationIf VG=high Now if VD=high, there is a currentflow between D and SGSD++ ++
  4. 4. MOSFET structures and circuit symbolsp-type substr ateSi O2n +Depletion r egionGateSour ceDr ai n+Drain Drain DrainnGateBul kSour ce Sour ce Sour ceChannelSubstr ate(b) (c) (d)(a)(a) Schematic structure of n-channel MOSFET (NMOS) andcircuit symbols for (b) MOSFET, (c) n-channel MOSFET, and (d)n-channel MOSFET when the bulk (substrate) potential has tobe specified in a circuit.
  5. 5. Complementary MOSFET pairsGate n-channel p-channelDr ainSour ce+ Drain Drain+pnSi O2n-type wel lSi O2GateBul kp-type substrateSour ce Sour ceSubst r ateSchematic structure of Complementary MOSFET (CMOS) andcircuit symbols for p-channel MOSFET (PMOS). Minuses andpluses show the depletion regions.
  6. 6. Sub-threshold mode of MOSFET operation•VG = 0; the MOSFET conducting channelVG = 0is not formedhigher VGChannelEcSource DrainEnergyFBEFDistanceIn the subthreshold regime, the MOSFET current is a small reverse currentthrough the source • substrate and drain • substrate p-n junctions;Only a small number of electrons can pass over the potential barrierseparating the drain and the source.(B/ kT-F)n•n×eSTSource
  7. 7. Sub-threshold mode of MOSFET operation10 2010VG2-2VG110-410-6VG310SourceDrain-810VG1<VG2<VG3-1010Gate-source voltage (V)1.81.41.00.60.2-0.20.05 VV ds = 3.0 VI tIn the sub-threshold regime, the channel current is very low and increasesexponentially with the gate bias.(B/ kT-F)n•n×eSTSource
  8. 8. 0.05 VV ds = 3.0 VI t 0.05 VV ds = 3.0 VI tMOSFET threshold voltageVG1<VG2<VG3 10 20VG110SourceVG2VG3-210-410-610Drain-810-10VT10-0.2 0.2 0.6 1.0 1.4 1.8Gate-source voltage (V)At certain gate bias called the threshold voltage, the conductivity type underthe gate inverts and the barrier between the Source and the Draindisappears.Electrons can enter the region under the gate to form aconducting n-channel.At the gate voltages above the threshold, the gate and the channel form aMetal-Insulator-Semiconductor (MIS) capacitor. 8
  9. 9. MOSFET above the threshold voltageThe free electron charge in the MOSFET channel (per unit area):Q1 = CGATE×(VG –VT)(assuming that at VG = VT the free electron concentration is zero)In MOSFETs, the gate and channel form a MIS-capacitor,hence the capacitance per unit gate areac=e /d=ee / diiiir0iei = eir e0 is the total dielectric permittivity of the gate dielectric(usually, SiO2), eir is the relative dielectric permittivity of the gatedielectric.Total gate capacitance CG = ci ×A, where A is the gate areaThe sheet electron concentration above the threshold, nS is given by:qns= ci( VGS- VT)= ciVGT
  10. 10. MOSFET above the threshold voltage1.81.41.00.60.20.05 VV ds = 3.0 VI t10 2010-210-410-610-810-1010-0.2Gate-source voltage (V)qns=ci(VGS-VT)=ciVGTAbove the threshold, the sheet electron concentration and hencethe channel current increase linearly with the gate bias VG.
  11. 11. MOSFET Threshold Voltagesemiconductormetal oxidep+n nS G DDrainSource
  12. 12. Band Diagram at the MOS interfacesBefore ContactVacuum leveloxidemetalp+nnq .oxECqfmqqq..sssEiqfsECEFmEgEFsEVEVMETAL OXIDE SEMICONDUCTOR
  13. 13. Metal and semiconductor Fermi levels align byAfter Contactelectron transfer. Bending is the result of thepresence of transferred electronp+nnECECECEEEVVVECEiEEEFmFmFmEFsEiEVEFsEVMETAL OXIDE SEMICONDUCTOR
  14. 14. p+nnp+nnFlat band VoltageVGGate voltage making the band flatECVFB= fm-fsECEFmEiVGEFsEVEVECECEFmEi VGEFsEVVGEFmEVVG=VFBECEC
  15. 15. EiEFsEVEVVG>0 VG<VFB
  16. 16. Conductivity conversion in MOSFETp+nnLess holes at theVGVGinterface, morebendingp+nnLess p type p typeECECEiEiEFsEFsEVEVVG .VG=0 More depletion
  17. 17. p+nnp+nnp+nnVGVGLess p type p type Less p type p typeECECOnset ofChannelChannelEiEicreationcreatedEFsEFsEVEVVG ..VG ...n type Inversion n type Strong Inversion
  18. 18. Inversion condition in MOSFETECEVEFsEiqfbEquilibrium hole concentration in the bulk of semiconductorqVsqfbkTpne=iqfb is the Fermi level offset fromthe mid-gap in the bulk materialSurface potential Vsis controlled by the gate voltageAccumulation Depletion Onset of inversion InversionV<0 V<fb V=fb V>fbsss sStrong Inversion When Vs = 2fb, n-concentration at the surfaceis the same as p-concentration in the bulkVs>2fb
  19. 19. Surface potential required to reachthe MOSFET thresholdqfbqfbkTipne=ECEVEFsEiVsT=2fbfbfbkTinne=When Vs = 2fb, n-concentration at the surfaceis the same as p-concentration in the bulk
  20. 20. Surface potential and gate voltage•VG is the gate voltage, as source is grounded,ViVG=VGS•Vi is the voltage drop across the oxide/insulator•Vs is the surface potentialVGEFmV=V+V+VGSFBsiECVsECEiEFsEVEV
  21. 21. Voltage drop across the oxide layerV=V+V+VGSFBsiViVi is the voltage drop across the oxide/insulatorGate electrode and semiconductor form the VGplates of the MOS capacitor.EFmVoltage drop across the capacitor:QdV=iCiECEiEFsEVEVECwhere Qd is the capacitor charge and Ci is the capacitance.Since the charges on the metal and semiconductor plates are the same,Qd can be calculated as the charge in semiconductor.The semiconductor charge is formed by the charge of the depletion region
  22. 22. Voltage drop across the oxide layerViThe relation between the depletion region width W andthe applied voltage Vs:qNW2aVs=VG2es2eVsEFmForm this,W=qNaThe depletion region charge (per unit area):ECEiEFsEVEV2eVsQ=qNW=qN.Q=2eqNda
  23. 23. aqNdsasa
  24. 24. Voltage drop across the oxide layerQVidV=iciwhere,Q=2eqN Vd sasVGis the depletion region charge per unit area, EFmci is the MOS-capacitor capacitance per unit area:eic=ididi is the thickness of the oxide film under the gateECEiEFsEVEV
  25. 25. MOSFET threshold voltage (cont.)The MOSFET threshold voltage is defined as the Gatevoltage leading to the strong inversion, i.e. Vs = 2fbTV=V+At the onset of strong inversion:2esqNaGSFBs()()22 bbff+2eqNVsasV=V+V+ciFBciFinally, the threshold voltage,V=V+2.+.2.TFBbb
  26. 26. Nwhere the body effect constant, .=N2 saiqNc/e
  27. 27. Effect of Body BiasVGp+n nVS VDVBS .0the Threshold voltage,V = V + 2. +.T FB bN( )BSb V2 -.
  28. 28. Effect of Surface Statesp+n nVS VDVBS .0bonds are created that contributes towanted trapped charges at the interface+ + + + + + + + + +VG During the oxide growth on Si, danglingthe Threshold voltage,VT = VFB +iCssQ+ 2.b +.N (2.b - VBS )Qss : surface state charges per unit area

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