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© Digital Integrated Circuits2nd Devices
Digital Integrated Digital Integrated CircuitsCircuitsA Design PerspectiveA Design Perspective
The DevicesThe Devices
Jan M. RabaeyAnantha ChandrakasanBorivoje Nikolic
July 30, 2002
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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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Forward BiasForward Bias
x
pn0
np0
-W1 W20p n
(W2)
n-regionp-region
Lp
diffusion
Typically avoided in Digital ICs
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Reverse BiasReverse Bias
x
pn0
np0
-W1 W20n-regionp-region
diffusion
The Dominant Operation Mode
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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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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
Polysilicon Aluminum
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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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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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A model for manual analysisA model for manual analysis
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Current-Voltage RelationsCurrent-Voltage RelationsA good ol’ transistorA good ol’ transistor
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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Process DataProcess Data
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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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Velocity Saturation (Computation)Velocity Saturation (Computation)
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PerspectivePerspective
IDLong-channel device
Short-channel device
VDSVDSAT VGS - VT
VGS = VDD
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IIDD versus V versus VDSDS
-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
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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Transistor Model Transistor Model for Manual Analysisfor Manual Analysis
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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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The MOS TransistorThe MOS Transistor
Polysilicon Aluminum
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MOSFET Capacitance ModelMOSFET Capacitance Model
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MOSFET Gate CapacitanceMOSFET Gate Capacitance
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MOSFET DB and SB CapacitanceMOSFET DB and SB Capacitance
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Junction CapacitanceJunction Capacitance
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More Process DataMore Process Data
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Problems (2)Problems (2)
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Problems (3)Problems (3)
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Problems (8)Problems (8)
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Problems (17)Problems (17)
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Latch-upLatch-up