Advanced MOS DevicesChapter 6: Long Channel MOSFETCullen College of EngineeringDepartment of Electrical and Computer EngineeringDr. Len TrombettaSpring 2011The MOSFETAn appropriate voltage applied to the gate (the threshold voltage) allows a current to flow from drain to source.A simpler device, the gated diode, shows some important characteristics of the MOSFET drain region.MOS capacitor (an annulus)Guard ring for isolationDiodeGated DiodeDiode is pulling charge away from inversion layer.Diode is putting additional charge into the inversion layer.We can look at the CV curve…The IV characteristics tell us something about generation/recombination processes.kTqisFSenn/)(Surface carrier concentration in inversionEFSiO2SiEiqsqFw = wTq(S - F)Uyemura, Fundamentals of MOS Digital Integrated Circuits, Addison Wesley, 1988Enhancement Mode: Application of VG> VTturns on the FET by enhancing the channel conductivity.Depletion Mode: (DMOS; VT<0) Application of VG > VT turns off the FET by depleting the channel charge.pFET (p-channel): Substrate is n-type and channel is p-type.nFET (n-channel): Substrate is p-type and channel is n-type.Uyemura, Fundamentals of MOS Digital Integrated Circuits, Addison Wesley, 1988Uyemura, Fundamentals of MOS Digital Integrated Circuits, Addison Wesley, 1988Uyemura, Fundamentals of MOS Digital Integrated Circuits, Addison Wesley, 1988p-Si substraten+y = Ly = 0xBVDB > VSBn+VSBVGBEIDGeometry for basic current – voltage analysisAssumptions:• Ignore depletion regions around source/drain• Current is entirely due to drift (no diffusion current)• Channel charge is determined by MOS capacitor formulae• Channel mobility is lower than bulk but constant across the channelIllustration of w(y) used for bulk charge calculation.yw(y)yV(y)VDSchematic illustration of variation in potential across the channelComparison of Square Law and Bulk Charge models for dox= 200 Å, NB= 2 x 1016cm-3, VT= 1 V, W/L = 5, meff= 500 cm2/V/s. Bulk charge model gives smaller IDbecause for a given VG, channel charge QIis less.0 1 2 3 4 5012345Square Law VG=5 VBulk Charge VG=5 VSquare Law VG=3 VBulk Charge VG = 3 VSquare Law and Bulk Charge ComparisonDrain Voltage VD (V)Drain Current ID (mA)0 1 2 3 4 5012345Generalized VG=5 VBulk Charge VG=5 VGeneralized VG=3 VBulk Charge VG=3 VGeneralized vs. Bulk Charge ModelsDrain Voltage VD (V)Drain Current ID (mA)Comparison of Generalized IV and Bulk Charge models for dox= 200 Å, NB= 2 x 1016cm-3, VT= 1 V, W/L = 5, meff= 500 cm2/V/s.ID: Strong Inversion Approximation])2()2[(2)(21))(2(2/32/3221SBFDBFSBDBSBDBFFBGBoxDVVVVVVVVCLWImIf we take the approximationswe arrive atSBFsoV2DBFsLV2for the drift current component of ID, which is the dominant component in inversion. This result is equivalent to the one obtained using the “Bulk Charge Model”.MOSFET Surface PotentialSurface potential vs. distance along channel L’ = x/L. MOSFET parameters are the same as for the I-V calculations.0 0.2 0.4 0.6 0.8 1012345Surface Potential vs. Channel LengthNormalized Channel LengthSurface Potential (V)Electric Field Along Channel0 0.2 0.4 0.6 0.8 1010203040Normalized Channel LengthLateral Electric Field (V/m)Electric field vs. distance along channel L’ = x/L. MOSFET parameters are the same as for the I-V calculations. The rapid rise in field near the drain is reproduced by more sophisticated calculations (2D simulation) and is a problem for device reliability.0 0.5 1 1.5 2 2.5 3110131101211011110101109110811071106110511041103ITotal VG 1 V 0 V( )ITotal VG 0.2 V 0 V( )VGSubthreshold current calculated from Generalized Model. MOSFET parameters are the same as for previous I-V calculations. Drain voltage is either 1 V (red curve) or 0.2 V (blue curve).Subthreshold Current: CalculationWe can extract threshold voltage from ID1/2vs. VDS. The FET will be in saturation if the gate and drain voltages are equal, since VDSwill always be greater than (VGS– VT).Notation: VT0is the threshold voltage with no body bias; VT1, VT2, etc. are shifted because of the application of body bias. Also, VSB= -VBS.This data was taken on a ON Semiconductor MC14007UB MOSFET chip.0.0E+002.0E-044.0E-046.0E-048.0E-041.0E-031.2E-030 0.5 1 1.5 2 2.5 3VDS (V)ID (A)VG = 1.5 VVG = 2 VVG = 3 VVG = 2.5 VMOSFET ID-VDSData1.0E-121.0E-111.0E-101.0E-091.0E-081.0E-071.0E-061.0E-051.0E-041.0E-031.0E-021.0E-010 0.5 1 1.5 2 2.5 3 3.5VGS (V)IDS (A)VD = 1.5 VVD = 0.5VSubthreshold Current: Data from MC14007UBThe subthreshold current should not be a function of drain voltage; dependence on VDis evidence of short channel effects. The subthreshold current in this device appears to be independent of VD.0.0E+005.0E-031.0E-021.5E-022.0E-022.5E-023.0E-023.5E-024.0E-020 1 2 3 4 5VGS = VDS (V)SQRT [ID (A)]VBS = 0VBS = -0.5 VVBS = -1.0 VVBS = -1.5 VSaturation Current and Body Bias: Data from MC14007UB0.0000.0010.0020.0030.0040.0050.0060.0070.0080 0.5 1 1.5 2 2.5 3 3.5Gate Voltage (V)Sqrt(ID)-14-12-10-8-6-4-200 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6Gate Voltage (V)Log(ID)Mobility tends to drop with increasing gate voltage, so ID“rolls over”, which complicates the extraction of the threshold voltage from these curves.Same data as above plotted as log IDvs. VG. Subthreshold slop is ~ 100 mV/dec.Silvaco Virtual Wafer Fab SimulationsL = 0.5 mm; tox= 100 ÅFrom TsividisParasitic source and drain resistance.Yuan Taur et al., “Shift and Ratio Method for MOSFET Channel Length Extraction”, IEEE El. Device Letters 13 (5), 267 (1992)From UyemuraSign of VTTermsVT =ms-Qot/Cox+ 2FQB/CoxnMOS- - + +pMOS- - - -If we were to use n+ poly-Si gates for nMOS and pMOS, there would be an asymmetry in the threshold voltage, as we can see by looking at the sign of the terms comprising VT. BSFBSioxFoxoxMSTVqNCCQV 2212assuming n+ poly for bothE = 0qAl 4.1 eVqSi 4.05 eVSiO2n+ polySiEiEFE = 0qAl 5.2 eVqSi 4.05 eVSiO2p+ polySiEiEFp substrate  NMOS n substrate  PMOSChoosing gate materials for symmetric threshold voltages…From Wolfe oxIBSFBSioxFFBTCqDVNqCVV 2212“impulse dose” (delta-function)2/122212SiiIBSFBSioxoxIFFBTxqDVNqCCqDVVrectangular profileFrom