chapter 3 vlsi design
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Chapter 3 VLSI Design. The Devices. March 28, 2003. Goal of this chapter. Present intuitive understanding of device operation Introduction of basic device equations Introduction of models for manual analysis Introduction of models for SPICE simulation - PowerPoint PPT PresentationTRANSCRIPT
© Digital Integrated Circuits2nd Devices
Chapter 3Chapter 3VLSI DesignVLSI Design
The DevicesThe DevicesMarch 28, 2003
© Digital Integrated Circuits2nd Devices
Goal of this chapterGoal of this chapter
Present intuitive understanding of device operation
Introduction of basic device equations Introduction of models for manual analysis Introduction of models for SPICE simulation Analysis of secondary and deep-sub-micron
effects Future trends
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The DiodeThe Diode
n
p
p
n
B A SiO2Al
A
B
Al
A
B
Cross-section of pn-junction in an IC process
One-dimensionalrepresentation diode symbol
Mostly occurring as parasitic element in Digital ICs
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Depletion RegionDepletion Regionhole diffusion
electron diffusion
p n
hole driftelectron drift
ChargeDensity
Distancex+
-
ElectricalxField
x
PotentialV
W2-W1
(a) Current flow.
(b) Charge density.
(c) Electric field.
(d) Electrostaticpotential.
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Diode CurrentDiode Current
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Models for Manual AnalysisModels for Manual Analysis
VD
ID = IS(eVD/T – 1)+
–
VD
+
–
+
–VDon
ID
(a) Ideal diode model (b) First-order diode model
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Junction CapacitanceJunction Capacitance
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Secondary EffectsSecondary Effects
–25.0 –15.0 –5.0 5.0
VD (V)
–0.1
I D (A
)
0.1
0
0
Avalanche Breakdown
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Diode ModelDiode Model
ID
RS
CD
+
-
VD
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SPICE MODELS
• SPICE: Simulation Program with Integrated Circuit Emphasis, by UCB in early 1970’s.
• Level 1: Long Channel Equations - Very Simple
• Level 2: Physical Model - Includes Velocity Saturation and Threshold Variations
• Level 3: Semi-empirical - Based on curve fitting to measured devices
• Level 4 (Berkeley Short-Channel IGFET Model, BSIM3v3): Empirical - Simple and Very Popular,.
• Full-fledged BSIM3v3 model (denoted as LEVEL 49) covers over 200 parameters.
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SPICE ParametersSPICE Parameters
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What is a Transistor?What is a Transistor?
VGS VT
RonS D
A Switch!
|VGS|
An MOS Transistor
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The MOS TransistorThe MOS Transistor
PolysiliconAluminum/Cu
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MOS Transistors -MOS Transistors -Types and SymbolsTypes and Symbols
D
S
G
D
S
G
G
S
D D
S
G
NMOS Enhancement NMOS
PMOS
Depletion
Enhancement
B
NMOS withBulk Contact
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Threshold Voltage: ConceptThreshold Voltage: Concept
n+n+
p-substrate
DSG
B
VGS
+
-
Depletion
Region
n-channel
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The Threshold VoltageThe Threshold Voltage
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The Body EffectThe Body Effect
-2.5 -2 -1.5 -1 -0.5 00.4
0.45
0.5
0.55
0.6
0.65
0.7
0.75
0.8
0.85
0.9
VBS
(V)
VT (
V)
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Current-Voltage RelationsCurrent-Voltage Relations
QuadraticRelationship
0 0.5 1 1.5 2 2.50
1
2
3
4
5
6x 10
-4
VDS (V)
I D (A
)
VGS= 2.5 V
VGS= 2.0 V
VGS= 1.5 V
VGS= 1.0 V
Resistive Saturation
VDS = VGS - VT
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Transistor in LinearTransistor in Linear
n+n+
p-substrate
D
SG
B
VGS
xL
V(x) +–
VDS
ID
MOS transistor and its bias conditions
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Transistor in SaturationTransistor in Saturation
n+n+
S
G
VGS
D
VDS > VGS - VT
VGS - VT+-
Pinch-off
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Current-Voltage RelationsCurrent-Voltage RelationsLong-Channel DeviceLong-Channel Device
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A model for manual analysisA model for manual analysis
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Current-Voltage RelationsCurrent-Voltage RelationsThe Deep-Submicron EraThe Deep-Submicron Era
LinearRelationship
-4
VDS (V)0 0.5 1 1.5 2 2.5
0
0.5
1
1.5
2
2.5x 10
I D (A
)
VGS= 2.5 V
VGS= 2.0 V
VGS= 1.5 V
VGS= 1.0 V
Early Saturation
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Velocity SaturationVelocity Saturation
(V/µm)c = 1.5
n
(m/s
)
sat = 105
Constant mobility (slope = µ)
Constant velocity
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PerspectivePerspective
IDLong-channel device
Short-channel device
VDSVDSAT VGS - VT
VGS = VDD
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IIDD versus V versus VGSGS
0 0.5 1 1.5 2 2.50
1
2
3
4
5
6x 10-4
VGS (V)
I D (A
)
0 0.5 1 1.5 2 2.50
0.5
1
1.5
2
2.5x 10-4
VGS (V)
I D (A
)
quadratic
quadratic
linear
Long Channel Short Channel
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IIDD versus V versus VDSDS
-4
VDS (V)0 0.5 1 1.5 2 2.50
0.5
1
1.5
2
2.5x 10
I D (A
)
VGS= 2.5 V
VGS= 2.0 V
VGS= 1.5 V
VGS= 1.0 V
0 0.5 1 1.5 2 2.50
1
2
3
4
5
6x 10-4
VDS (V)
I D (A
)
VGS= 2.5 V
VGS= 2.0 V
VGS= 1.5 V
VGS= 1.0 V
ResistiveSaturation
VDS = VGS - VT
Long Channel Short Channel
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A unified modelA unified modelfor manual analysisfor manual analysis
S D
G
B
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Simple Model versus SPICE Simple Model versus SPICE
0 0.5 1 1.5 2 2.50
0.5
1
1.5
2
2.5x 10
-4
VDS
(V)
I D (A
)
VelocitySaturated
Linear
Saturated
VDSAT=VGT
VDS=VDSAT
VDS=VGT
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A PMOS TransistorA PMOS Transistor
-2.5 -2 -1.5 -1 -0.5 0-1
-0.8
-0.6
-0.4
-0.2
0 x 10-4
VDS (V)
I D (A
)
Assume all variablesnegative!
VGS = -1.0V
VGS = -1.5V
VGS = -2.0V
VGS = -2.5V
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Transistor Model Transistor Model for Manual Analysisfor Manual Analysis
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The Transistor as a SwitchThe Transistor as a SwitchVGS VT
RonS D
ID
VDS
VGS = VD D
VDD/2 VDD
R0
Rmid
ID
VDS
VGS = VD D
VDD/2 VDD
R0
Rmid
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The Transistor as a SwitchThe Transistor as a Switch
0.5 1 1.5 2 2.50
1
2
3
4
5
6
7x 10
5
VDD
(V)
Req
(O
hm)
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The Transistor as a SwitchThe Transistor as a Switch
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MOS CapacitancesMOS CapacitancesDynamic BehaviorDynamic Behavior
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Dynamic Behavior of MOS TransistorDynamic Behavior of MOS Transistor
DS
G
B
CGDCGS
CSB CDBCGB
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The Gate CapacitanceThe Gate Capacitance
tox
n+ n+
Cross section
L
Gate oxide
xd xd
L d
Polysilicon gate
Top view
Gate-bulkoverlap
Source
n+
Drain
n+W
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Gate CapacitanceGate Capacitance
S D
G
CGC
S D
G
CGC
S D
G
CGC
Cut-off Resistive Saturation
Most important regions in digital design: saturation and cut-off
Cutoff SaturationTriode
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Gate CapacitanceGate Capacitance
WLCox
WLCox
2
2WLCox
3
CGC
CGCS
VDS /(VGS-VT)
CGCD
0 1
CGC
CGCS = CGCDCGC B
WLCox
WLCox
2
VGS
Capacitance as a function of VGS(with VDS = 0)
Capacitance as a function of the degree of saturation
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Measuring the Gate CapMeasuring the Gate Cap
2 1.52 12 0.5 0
3
4
5
6
7
8
9
103 102 16
2
VGS (V)
VGS
Gate
Ca
paci
tan
ce (
F)
0.5 1 1.5 22 2
I
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Diffusion CapacitanceDiffusion Capacitance
Bottom
Side wall
Side wallChannel
SourceND
Channel-stop implant NA1
SubstrateNA
W
xj
LS
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Capacitances in 0.25 Capacitances in 0.25 m CMOS m CMOS processprocess
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The Sub-Micron MOS TransistorThe Sub-Micron MOS Transistor
Threshold Variations Subthreshold Conduction Parasitic Resistances
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Threshold VariationsThreshold Variations
VT
L
Long-channel threshold Low VDS threshold
Threshold as a function of the length (for low VDS)
Drain-induced barrier lowering (for low L)
VDS
VT
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Sub-Threshold ConductionSub-Threshold Conduction
0 0.5 1 1.5 2 2.510-12
10-10
10-8
10-6
10-4
10-2
VGS (V)
I D (A
)
VT
Linear
Exponential
Quadratic
Typical values for S:60 .. 100 mV/decade
The Slope Factor
ox
DnkT
qV
D C
CneII
GS
1 ,~ 0
S is VGS for ID2/ID1 =10
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Sub-Threshold Sub-Threshold IIDD vs vs VVGSGS
VDS from 0 to 1.0V
kT
qV
nkT
qV
D
DSGS
eeII 10
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Sub-Threshold Sub-Threshold IIDD vs vs VVDSDS
DSkT
qV
nkT
qV
D VeeIIDSGS
110
VGS from 0 to 0.3V
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Summary of MOSFET Operating Summary of MOSFET Operating RegionsRegions
Strong Inversion VGS > VT
Linear (Resistive)
VDS < VDSAT
Saturated (Constant Current)
VDS VDSAT
Weak Inversion (Sub-Threshold) VGS VT
Exponential in VGS with linear VDS dependence
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Parasitic ResistancesParasitic Resistances
W
LD
Drain
Draincontact
Polysilicon gate
DS
G
RS RD
VGS,eff
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Latch-up (Effect of Parasitic Resistance)Latch-up (Effect of Parasitic Resistance)
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SPICE Transistors Parameters
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MAIN MOS SPICE PARAMETERS
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SPICE Parameters for Parasitics
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SPICE Model for NMOS and PMOSSPICE Model for NMOS and PMOS
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SPICE Deck for a CMOS InverterSPICE Deck for a CMOS Inverter
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SPICE Simulation ResultSPICE Simulation Result
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Future PerspectivesFuture Perspectives
25 nm FINFET MOS transistor
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SummarySummary
Transistor Modeling is important for CMOS circuit design
(updated by IC manufacturing companies based on
ongoing technologies).
SPICE parameters provide a good link between
manufacturer and designers.
SPICE model/parameters need to be updated to reflect
the behavior of the MOS in deep sub-micron technology.
Recently, capacitor/inductor modeling become important
for Radio-frequency (RF) designs.