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Berkeley ELENG 130 - MOSFET in the On-state

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Semiconductor Devices for Integrated Circuits (C. Hu) Slide 6-1Chapter 6 MOSFET in the On-stateThe MOSFET (MOS Field-Effect Transistor) is the building block of Gb memory chips, GHz microprocessors, analog, and RF circuits.Match the following MOSFET characteristics with their applications:•small size• high speed• low power• high gainSemiconductor Devices for Integrated Circuits (C. Hu) Slide 6-26.1 Introduction to the MOSFETBasic MOSFET structure and IV characteristicsSemiconductor Devices for Integrated Circuits (C. Hu) Slide 6-36.1 Introduction to the MOSFETTwo ways of representing a MOSFET:Semiconductor Devices for Integrated Circuits (C. Hu) Slide 6-4Invention of the Field-Effect TransistorSemiconductor Devices for Integrated Circuits (C. Hu) Slide 6-5Invention of the Field-Effect TransistorIn 1935, a British patent was issued to Oskar Heil. A working MOSFET was not demonstrated until 1955.Using today’s terminology, what are 1, 2, and 6?Semiconductor Devices for Integrated Circuits (C. Hu) Slide 6-6Today’s MOSFET TechnologyGate oxides as thin as 1.2 nm can be manufactured reproducibly.Large tunneling current through the oxide limits oxide-thicknessreduction.Semiconductor Devices for Integrated Circuits (C. Hu) Slide 6-76.2 Complementary MOSFETsWhen Vg = Vdd, the NFET is on and the PFET is off. When Vg= 0, the PFET is on and the NFET is off.NFET PFETSemiconductor Devices for Integrated Circuits (C. Hu) Slide 6-8CMOS (Complementary MOS) InverterA CMOS inverter is made of a PFET pull-up device and a NFET pull-down device. Vout= ? if Vin= 0 V.C: Vin Vdd PFETNFET0V 0V S D D SVout etc.) (of interconnect, capacitanceSemiconductor Devices for Integrated Circuits (C. Hu) Slide 6-9CMOS (Complementary MOS) Inverter• NFET and PFET can be fabricated on the same chip. VinVoutVdd0V N-wellP+N+PFETNFETConta ctVddVout 0V Vin N-well P-substrate P+N+N+N+ P+ P+• basic layout of a CMOS inverterSemiconductor Devices for Integrated Circuits (C. Hu) Slide 6-106.3 Surface Mobilities of Electrons and HolesLVVVWCLVWQWQvQWIdsnstgoxedsnsinvnsinvinvds/)(/µµµ−===⋅⋅=How to measure the surface mobility:Vg= Vdd, Vgs= VddIdsVds> 0Semiconductor Devices for Integrated Circuits (C. Hu) Slide 6-11Surface mobility is a function of the average of the fields at the bottom and the top of the inversion charge layer, band t .From Gauss’s Law,gateP-body-- - - - - - -NNVgToxeWdmaxtbb= – Qdep/εsoxedepstfbtCQVV /−+=φ)(stfbtsoxebVVCφε+−=Therefore,)()(//)(stfbgssoxetgssoxebsinvbsinvdeptVVCVVCQQQφεεεε+−=−+=−=+−=oxetgstgssoxestfbtgssoxetbTVVVVCVVVC6V2.0)V2.0(2)22(2)(21++=++≈−−+=+εφε∴Semiconductor Devices for Integrated Circuits (C. Hu) Slide 6-12Mobility is a function of Vgs, Vt, and Toxe.What suppresses the surface mobility:• phonon scattering• coulombic scattering• surface roughness scattering (Vgs + Vt + 0.2)/6Toxe(M V/cm)–(Vgs + 1.5Vt – 0.25)/6Tox e(MV/cm) (NFET) (PFET)Universal Surface MobilitiesSemiconductor Devices for Integrated Circuits (C. Hu) Slide 6-13EXAMPLE: What is the surface mobility at Vgs=1 V in an N-channel MOSFET with Vt=0.3 V and Toxe=2nm?Solution: 1 MV is a megavolt (106V). From the mobility figure, µns=190 cm2/Vs, which is several times smaller than the bulk mobility.MV/cm 25.1cm1012/V 5.16/)2.0(7=×=++−oxetgsTVVSemiconductor Devices for Integrated Circuits (C. Hu) Slide 6-14How to Measure the Vtof a MOSFET6.4 MOSFET Vtand the Body EffectVds = 50mV IdsVgsVtVtis measured by extrapolating the Idsversus Vgscurve to Ids= 0.tgsdsnstgsoxedsatVVVVVCLWI −∝−=µ)(Semiconductor Devices for Integrated Circuits (C. Hu) Slide 6-156.4 MOSFET Vtand the Body EffectmaxdsdepWCε=sbdeptgsoxeinvVCVVCQ+−−=)())((sboxedeptgsoxeVCCVVC +−−=• Two capacitors => two chargecomponentssbtsboxedeptsbtVVVCCVVVα+=+=00)(•RedefineVtasSemiconductor Devices for Integrated Circuits (C. Hu) Slide 6-16• body effect:Vtis a function of Vsb• body effect coefficient:α = Cdep/Coxe= 3Toxe/ WdmaxIs the body effect a good thing? How can it be reduced?6.4 MOSFET Vtand the Body Effect××××××××××××××××××× data model××××××××××××××××-2 -1 0 1 2 Vsb (V)NFETPFETVt 0Vt00.6 -0.2 -0.60.4 -0.4Vt (V)0.2 When the source-body junctionis reverse-biased, the NFET Vtincreases and the PFET Vtbecomes more negative.Semiconductor Devices for Integrated Circuits (C. Hu) Slide 6-17Retrograde Body Doping Profiles• Wdepdoes not vary with Vsb.• Retrograde doping is popular because it reduces off-stateleakage. 0.0 0.1 0.2 0.3 0.4 Depth (µm) 1016 1017 1018Body Doping (cm-3)××××××××××××××××××× data model××××××××××××××××-2 -1 0 1 2 Vsb (V)NFETPFETVt 0Vt00.6 -0.2 -0.60.4 -0.4Vt (V)0.2Semiconductor Devices for Integrated Circuits (C. Hu) Slide 6-18Uniform Body DopingWhen the source/body junction is reverse-biased, there are two quasi-Fermi levels (Efnand Efp) which are separated by qVsb. An NMOSFET reaches threshold of inversion when Ecis close to Efn, not Efp. This requires the band-bending to be 2φB+ Vsb, not 2φB.)22()22(200BsbBtBsbBoxesattVVVCqNVVφφγφφε−++≡−++=γis the body-effect parameter.Semiconductor Devices for Integrated Circuits (C. Hu) Slide 6-196.5 Qinvin MOSFET• Channel voltageVc=Vsat x = 0 andVc=Vdat x = L. • Qinv= – Coxe(Vgs– Vcs– Vt0 –α (Vsb+Vcs)= – Coxe(Vgs– Vcs – (Vt0 +α Vsb) –α Vcs)= – Coxe(Vgs– mVcs– Vt)• m ≡ 1 +α =1+ 3Toxe/Wdmax m is called the body-effect factor or bulk-charge factorSemiconductor Devices for Integrated Circuits (C. Hu) Slide 6-206.6 Basic MOSFET IV ModelIds= WQinvv= WQinvµns= WCoxe(Vgs– mVcs– Vt)µnsdVcs/dxcsLVtcsgsnsoxedsdVVmVVWCdxIds)(00∫∫−−=µIdsL = WCoxeµns(Vgs– Vt– mVds/2)VdsdsdstgssoxedsVVmVVCLWI )2( −−=µSemiconductor Devices for Integrated Circuits (C. Hu) Slide 6-21Vdsat: Drain Saturation Voltage)(0dstgsnsoxedsdsmVVVCLWdVdI−−==µmVVVtgsdsat−=Semiconductor Devices for Integrated Circuits (C. Hu) Slide 6-22I = µnQin vdVcs/dxIdsat0 L xI = µnQin vdVcs/dxIdsat0 L x 0 L 0 L x x 0 L 0 L x xQinv = Cox(Vg − mVcs − Vt)Qinv(b) (f)(c) (g)Ecsour cedrain Ecsource drain--- - -- (d)(h)(a) (e)Vds = Vds at VdsVdsatVds = VdsatVds > Vdsat Vcs VcsVds − VdsatSemiconductor Devices for Integrated Circuits (C. Hu) Slide 6-23Saturation Current and Transconductance• transconductance: gm=


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Berkeley ELENG 130 - MOSFET in the On-state

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