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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
© 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
© Digital Integrated Circuits2nd Devices
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
© Digital Integrated Circuits2nd Devices
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.
© Digital Integrated Circuits2nd Devices
Diode CurrentDiode Current
© Digital Integrated Circuits2nd Devices
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
© Digital Integrated Circuits2nd Devices
Diode ModelDiode Model
ID
RS
CD
+
-
VD
© Digital Integrated Circuits2nd Devices
SPICE ParametersSPICE Parameters
© Digital Integrated Circuits2nd Devices
What is a Transistor?What is a Transistor?
VGS VT
RonS D
A Switch!
|VGS|
An MOS Transistor
© Digital Integrated Circuits2nd Devices
The MOS TransistorThe MOS Transistor
Polysilicon Aluminum
© Digital Integrated Circuits2nd Devices
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
© Digital Integrated Circuits2nd Devices
Threshold Voltage: ConceptThreshold Voltage: Concept
n+n+
p-substrate
DSG
B
VGS
+
-
Depletion
Region
n-channel
© Digital Integrated Circuits2nd Devices
The Threshold VoltageThe Threshold Voltage
© Digital Integrated Circuits2nd Devices
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)
© Digital Integrated Circuits2nd Devices
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
© Digital Integrated Circuits2nd Devices
Transistor in LinearTransistor in Linear
n+n+
p-substrate
D
SG
B
VGS
xL
V(x) +–
VDS
ID
MOS transistor and its bias conditions
© Digital Integrated Circuits2nd Devices
Transistor in SaturationTransistor in Saturation
n+n+
S
G
VGS
D
VDS > VGS - VT
VGS - VT+-
Pinch-off
© Digital Integrated Circuits2nd Devices
Current-Voltage RelationsCurrent-Voltage RelationsLong-Channel DeviceLong-Channel Device
© Digital Integrated Circuits2nd Devices
A model for manual analysisA model for manual analysis
© Digital Integrated Circuits2nd Devices
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
© Digital Integrated Circuits2nd Devices
Velocity SaturationVelocity Saturation
(V/µm)c = 1.5
n
(m/s
)
sat = 105
Constant mobility (slope = µ)
Constant velocity
© Digital Integrated Circuits2nd Devices
PerspectivePerspective
IDLong-channel device
Short-channel device
VDSVDSAT VGS - VT
VGS = VDD
© Digital Integrated Circuits2nd Devices
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
© Digital Integrated Circuits2nd Devices
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
© Digital Integrated Circuits2nd Devices
A unified modelA unified modelfor manual analysisfor manual analysis
S D
G
B
NB For PMOS, use Vmax=max(…) instead of Vmin
© Digital Integrated Circuits2nd Devices
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
© Digital Integrated Circuits2nd Devices
A PMOS TransistorA PMOS Transistor
-2.5 -2 -1.5 -1 -0.5 0-1
-0.8
-0.6
-0.4
-0.2
0x 10
-4
VDS (V)
I D (
A)
Assume all variablesnegative!
VGS = -1.0V
VGS = -1.5V
VGS = -2.0V
VGS = -2.5V
© Digital Integrated Circuits2nd Devices
Transistor Model Transistor Model for Manual Analysisfor Manual Analysis
© Digital Integrated Circuits2nd Devices
The Transistor as a SwitchThe Transistor as a Switch
VGS VT
RonS D
ID
VDS
VGS = VD D
VDD/2 VDD
R0
Rmid
ID
VDS
VGS = VD D
VDD/2 VDD
R0
Rmid
© Digital Integrated Circuits2nd Devices
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)
© Digital Integrated Circuits2nd Devices
The Transistor as a SwitchThe Transistor as a Switch
© Digital Integrated Circuits2nd Devices
Unified Model ExamplesUnified Model Examples
© Digital Integrated Circuits2nd Devices
MOS CapacitancesMOS CapacitancesDynamic BehaviorDynamic Behavior
© Digital Integrated Circuits2nd Devices
Dynamic Behavior of MOS TransistorDynamic Behavior of MOS Transistor
DS
G
B
CGDCGS
CSB CDBCGB
© Digital Integrated Circuits2nd Devices
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
© Digital Integrated Circuits2nd Devices
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
© Digital Integrated Circuits2nd Devices
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
© Digital Integrated Circuits2nd Devices
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
© Digital Integrated Circuits2nd Devices
Diffusion CapacitanceDiffusion Capacitance
Bottom
Side wall
Side wallChannel
SourceND
Channel-stop implant NA1
SubstrateNA
W
xj
LS
© Digital Integrated Circuits2nd Devices
Junction CapacitanceJunction Capacitance
© Digital Integrated Circuits2nd Devices
Linearizing the Junction CapacitanceLinearizing the Junction Capacitance
Replace non-linear capacitance bylarge-signal equivalent linear capacitance
which displaces equal charge over voltage swing of interest
© Digital Integrated Circuits2nd Devices
Capacitances in 0.25 Capacitances in 0.25 m CMOS m CMOS processprocess
© Digital Integrated Circuits2nd Devices
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
© Digital Integrated Circuits2nd Devices
Sub-Threshold Sub-Threshold IIDD vs vs VVGSGS
VDS from 0 to 0.5V
kT
qV
nkT
qV
D
DSGS
eeII 10
© Digital Integrated Circuits2nd Devices
Sub-Threshold Sub-Threshold IIDD vs vs VVDSDS
DSkT
qV
nkT
qV
D VeeIIDSGS
110
VGS from 0 to 0.3V
© Digital Integrated Circuits2nd Devices
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
© Digital Integrated Circuits2nd Devices
Parasitic ResistancesParasitic Resistances
W
LD
Drain
Draincontact
Polysilicon gate
DS
G
RS RD
VGS,eff