EE 435 Lecture 37 Parasitic Capacitances in MOS Devices String DAC Parasitic Capacitances
Parasitic Capacitors in MOSFET (will initially consider two)
Parasitic Capacitors in MOSFET C GCH
Parasitic Capacitors in MOSFET Recall that pn junctions have a depletion region!
Parasitic Capacitors in pn junction capacitance MOSFET C Depletion Region For V FB <φ B /2 C JO A C φ B V FB C = CA J0 V 1- φ FB B m
Parasitic Capacitors in pn junction capacitance MOSFET The bottom and the sidewall:
Parasitic Capacitors in pn junction capacitance MOSFET C Depletion Region For a pn junction capacitor C = BOT C A BOT V 1- φ C =C A+C P J BOT SW FB B m C = SW C P SW V 1- φ FB B m
Question Are the parasitic capacitors relevant?
Observation Parasitic Capacitors are Small Consider a minimum-sized transistor 2l 3l 4l
Process Parameters from AMI 0.5u Process PROCESS PARAMETERS Sheet Resistance Contact Resistance Gate Oxide Thickness N+ACTV 81.5 64.6 140 P+ACTV 101.9 141.9 POLY 21.6 15.8 PLY2_ 1120 HR POLY2 41 26.8 MTL1 0.09 MTL2 0.09 0.8 UNITS ohms/sq ohms angstrom PROCESS PARAMETERS Sheet Resistance Contact Resistance MTL 3 0.06 0.65 N\PLY 822 N WELL 812 ohms/sq ohms COMMENTS: N\POLY is N-well under polysilicon. CAPACITANCE PARAMETERS Area (substrate) Area (N+active) Area (P+active) Area (poly) Area (poly2) Area (metal1) Area (metal2) Fringe (substrate) Fringe (poly) Fringe (metal1) Fringe (metal2) Overlap (N+active) Overlap (P+active) l=.35 microns N+ACTV 424 315 315 P+ACTV 731 247 POLY 87 2473 2382 195 239 POLY2 969 M1 32 36 56 50 72 57 M2 16 16 15 31 58 39 48 M3 10 12 10 13 39 38 28 34 55 N_WELL 39 UNITS af/um^2 af/um^2 af/um^2 af/um^2 af/um^2 af/um^2 af/um^2 af/um af/um af/um af/um af/um af/um
Size of Capacitances Gate-Channel Capacitance = 6l 2 x 2.47fF/m 2 = 1.82fF Source Diffusion-Substrate Capacitance = 12l 2 x.424ff/m 2 + 14l x.315ff/m =.624fF + 1.54fF =2.16fF Note Sidewall Capacitance larger than Bottom Capacitance Are these negligible?
Are these negligible? These small capacitors play the dominant role in the speed limitations of most digital circuits These small capacitors play a major role in the performance of many linear circuits It is essential that these capacitors (parasitic capacitors) be considered and managed when designing most integrated circuits today!
Types of Capacitors 1. Fixed Capacitors a. Fixed Geometry b. Junction 2. Operating Region Dependent a. Fixed Geometry b. Junction
Parasitic Capacitors in MOSFET Fixed Capacitors
Parasitic Capacitors in MOSFET Fixed Capacitors C GSO C GDO Overlap Capacitors: C GDO, C GSO
Parasitic Capacitance Summary D C GD G B C GS Cutoff Ohmic Saturation C GS CoxWL D CoxWL D CoxWL D C GD CoxWL D CoxWL D CoxWL D L D is a model parameter S
Parasitic Capacitors in MOSFET Fixed Capacitors C BS1 C BD1 Junction Capacitors: C BS1, C BD1
Parasitic Capacitors in MOSFET Fixed Capacitors C GSO C GDO C BS1 C BD1 Overlap Capacitors: C GDO, C GSO Junction Capacitors: C BS1, C BD1
Fixed Parasitic Capacitance Summary C GD D C BD G C GS C BS B C BOT and C SW are model parameters Cutoff Ohmic Saturation C GS CoxWL D CoxWL D CoxWL D C GD CoxWL D CoxWL D CoxWL D C BG C BS C BS1 = C BOT A S +C SW P S C BS1 = C BOT A S +C SW P S C BS1 = C BOT A S +C SW P S C BD C BD1 = C BOT A D +C SW P D C BD1 = C BOT A D +C SW P D C BD1 = C BOT A D +C SW P D S
Parasitic Capacitors in MOSFET Operation Region Dependent
Parasitic Capacitors in MOSFET Operation Region Dependent -- Cutoff C GBCO Cutoff Capacitor: C GBCO
Parasitic Capacitors in MOSFET Operation Region Dependent -- Cutoff C GBCO Note: A depletion region will form under the gate if a positive Gate voltage is applied thus decreasing the capacitance density Cutoff Capacitor: C GBCO
Parasitic Capacitors in MOSFET Operation Region Dependent and Fixed -- Cutoff C GSO C GDO C BS1 C GBCO C BD1 Overlap Capacitors: C GDO, C GSO Junction Capacitors: C BS1, C BD1 Cutoff Capacitor: C GBCO
Parasitic Capacitance Summary C GD D C BD G B C GS C BS S C BG Cutoff Ohmic Saturation C GS CoxWL D CoxWL D CoxWL D C GD CoxWL D CoxWL D CoxWL D C BG CoxWL (or less) C BS C BOT A S +C SW P S C BS1 = C BOT A S +C SW P S C BS1 = C BOT A S +C SW P S C BD C BOT A D +C SW P D C BD1 = C BOT A D +C SW P D C BD1 = C BOT A D +C SW P D
Parasitic Capacitors in MOSFET Operation Region Dependent -- Ohmic C GCH C BCH Note: The Channel is not a node in the lumped device model so can not directly include this distributed capacitance in existing models Note: The distributed channel capacitance is usually lumped and split evenly between the source and drain nodes Ohmic Capacitor: C GCH, C BCH
Parasitic Capacitors in MOSFET Operation Region Dependent and Fixed -- Ohmic C GSO C GCH C GDO C BCH C BS1 C BD1 Overlap Capacitors: C GDO, C GSO Junction Capacitors: C BS1, C BD1 Ohmic Capacitor: C GCH, C BCH
Parasitic Capacitance Summary C GD D C BD G B C GS C BS S C BG Cutoff Ohmic Saturation C GS CoxWL D CoxWL D CoxWL D C GD CoxWL D CoxWL D CoxWL D C BG CoxWL (or less) C BS C BOT A S +C SW P S C BS1 = C BOT A S +C SW P S C BS1 = C BOT A S +C SW P S C BD C BOT A D +C SW P D C BD1 = C BOT A D +C SW P D C BD1 = C BOT A D +C SW P D
Parasitic Capacitors in MOSFET Operation Region Dependent -- Saturation C GCH C BCH Note: Since the channel is an extension of the source when in saturation, the distributed capacitors to the channel are generally lumped to the source node Saturation Capacitors: C GCH, C BCH
Parasitic Capacitors in MOSFET Operation Region Dependent and Fixed --Saturation C GSO C BS1 C GCH C BCH C GDO C BD1 Overlap Capacitors: C GDO, C GSO Junction Capacitors: C BS1, C BD1 Saturation Capacitors: C GCH, C BCH
Parasitic Capacitance C GD Summary D C BD G B C GS C BS S C BG Cutoff Ohmic Saturation C GS CoxWL D CoxWL D + 0.5C OX WL CoxWL D +(2/3)C OX WL C GD CoxWL D CoxWL D + 0.5C OX WL CoxWL D C BG CoxWL (or less) 0 0 C BS C BOT A S +C SW P S C BOT A S +C SW P S +0.5WLC BOTCH C BOT A S +C SW P S +(2/3)WLC BOTCH C BD C BOT A D +C SW P D C BOT A D +C SW P D +0.5WLC BOTCH C BOT A D +C SW P D
Parasitic Capacitance Summary C GD D C BD G B C GS C BS S C BG Cutoff Ohmic Saturation C GS CoxWL D CoxWL D + 0.5C OX WL CoxWL D +(2/3)C OX WL C GD CoxWL D CoxWL D + 0.5C OX WL CoxWL D C BG CoxWL (or less) 0 0 C BS C BOT A S +C SW P S C BOT A S +C SW P S +0.5WLC BOTCH C BOT A S +C SW P S +(2/3)WLC BOTCH C BD C BOT A D +C SW P D C BOT A D +C SW P D +0.5WLC BOTCH C BOT A D +C SW P D
R-String DAC V REF X IN n Decoder b 3 b 3 b 2 b 2 b 1 b 1 R-String V OUT Tree Decoder Parasitic Capacitances in Tree Decoder
R-String DAC V REF X IN n Decoder Example: < 0 1 0 > b 3 b 3 b 2 b 2 b 1 b 1 R-String V OUT V 3 Tree Decoder Previous-Code Dependent Settling Assume all C s initially with 0V Red denotes V, black denotes 0V, Purple some other voltage
Previous-Code Dependent Settling Assume all C s initially with 0V Red denotes V, green denotes V, black denotes 0V, Purple some other vo V REF R-String DAC Transition from <010> to <101> X IN n Example: < 1 0 1 > Decoder b 3 b 3 b 2 b 2 b 1 b 1 V 6 R-String V OUT V 3 Tree Decoder
Transition from <010> to <101> V REF R-String DAC X IN n Decoder b 3 b 3 b 2 b 2 b 1 b 1 White boxes show capacitors dependen upon previous code <010> Example: < 1 0 1 > V 6 R-String V OUT V 3 Tree Decoder Previous-Code Dependent Settling Assume all C s initially with 0V Red denotes V 3, green denotes V 6, black denotes 0V, Purple some other voltage
R-String DAC V OUT V DD Decoder b 3 b 3 b 2 b 2 b 1 b 1 Tree Decoder Tree-Decoder in Digital Domain Single transistor used at each marked intersection to for PTL AND gates Do the resistors that form part of PTL dissipate any substantial power? No because only one will be conducting for any DAC output
R-String DAC V REF X IN n Decoder b 1 b 1 b 2 b 2 b 3 b 3 R-String V OUT Tree Decoder
End of Lecture 37