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Digital Integrated Circuit Design The Devices
The DevicesThe Devices
Digital Integrated Circuit Digital Integrated Circuit DesignDesignAndrea BonfantiDEIBVia Golgi 40, Milano
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Digital Integrated Circuit Design The Devices
Aims of this chapterAims of this chapter
Present intuitive understanding of device operation
Introduction of basic device equations Introduction of models for manual analysis Analysis of secondary and deep-sub-micron
effects Future trends
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Digital Integrated Circuit Design The 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
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Digital Integrated Circuit Design The 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.
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Digital Integrated Circuit Design The Devices
Diode CurrentDiode Current
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Digital Integrated Circuit Design The Devices
Forward BiasForward Bias
x
pn0
np0
-W1 W20p n
(W2)
n-regionp-region
Lp
diffusion
Typically avoided in Digital ICs6
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Digital Integrated Circuit Design The Devices
Reverse BiasReverse Bias
x
pn0
np0
-W1 W20n-regionp-region
diffusion
The Dominant Operation Mode7
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Digital Integrated Circuit Design The 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
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Digital Integrated Circuit Design The Devices
Junction CapacitanceJunction Capacitance
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Digital Integrated Circuit Design The Devices
Diffusion CapacitanceDiffusion Capacitance
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Digital Integrated Circuit Design The Devices
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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Digital Integrated Circuit Design The Devices
Diode ModelDiode Model
ID
RS
CD
+
-
VD
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Digital Integrated Circuit Design The Devices
SPICE ParametersSPICE Parameters
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Digital Integrated Circuit Design The Devices
What is a Transistor?What is a Transistor?
VGS VT
RonS D
A Switch!
|VGS|
An MOS Transistor
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Digital Integrated Circuit Design The Devices
The MOS TransistorThe MOS Transistor
Polysilicon Aluminum
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Digital Integrated Circuit Design The 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
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Digital Integrated Circuit Design The Devices
Threshold Voltage: ConceptThreshold Voltage: Concept
n+n+
p-substrate
DSG
B
VGS
+
-
Depletion
Region
n-channel
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Digital Integrated Circuit Design The Devices
The Threshold VoltageThe Threshold Voltage
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Digital Integrated Circuit Design The 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)
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Digital Integrated Circuit Design The 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
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Digital Integrated Circuit Design The Devices
Transistor in LinearTransistor in Linear
n+n+
p-substrate
D
SG
B
VGS
xL
V(x) +–
VDS
ID
MOS transistor and its bias conditions21
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Digital Integrated Circuit Design The Devices
Transistor in SaturationTransistor in Saturation
n+n+
S
G
VGS
D
VDS > VGS - VT
VGS - VT+-
Pinch-off
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Digital Integrated Circuit Design The Devices
Current-Voltage RelationsCurrent-Voltage RelationsLong-Channel DeviceLong-Channel Device
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Digital Integrated Circuit Design The Devices
A model for manual analysisA model for manual analysis
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Digital Integrated Circuit Design The 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
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Digital Integrated Circuit Design The Devices
Velocity SaturationVelocity Saturation
(V/µm)c = 1.5
n
(m/s
)
sat = 105
Constant mobility (slope = µ)
Constant velocity
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Digital Integrated Circuit Design The Devices
PerspectivePerspective
IDLong-channel device
Short-channel device
VDSVDSAT VGS - VT
VGS = VDD
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Digital Integrated Circuit Design The 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
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Digital Integrated Circuit Design The 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
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Digital Integrated Circuit Design The Devices
A unified modelA unified modelfor manual analysisfor manual analysis
S D
G
B
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Digital Integrated Circuit Design The 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
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Digital Integrated Circuit Design The 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
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Digital Integrated Circuit Design The Devices
Transistor Model Transistor Model for Manual Analysisfor Manual Analysis
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Digital Integrated Circuit Design The 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
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Digital Integrated Circuit Design The 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)
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Digital Integrated Circuit Design The Devices
The Transistor as a SwitchThe Transistor as a Switch
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Digital Integrated Circuit Design The Devices
MOS CapacitancesMOS CapacitancesDynamic BehaviorDynamic Behavior
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Digital Integrated Circuit Design The Devices
Dynamic Behavior of MOS TransistorDynamic Behavior of MOS Transistor
DS
G
B
CGDCGS
CSB CDBCGB
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Digital Integrated Circuit Design The 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
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Digital Integrated Circuit Design The 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
40
<
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Digital Integrated Circuit Design The 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
41
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Digital Integrated Circuit Design The 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
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Digital Integrated Circuit Design The Devices
Diffusion CapacitanceDiffusion Capacitance
Bottom
Side wall
Side wallChannel
SourceND
Channel-stop implant NA1
SubstrateNA
W
xj
LS
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Digital Integrated Circuit Design The Devices
Junction CapacitanceJunction Capacitance
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Digital Integrated Circuit Design The Devices
Linearizing the Junction CapacitanceLinearizing the Junction Capacitance
Replace non-linear capacitance by large-signal equivalent linear capacitance which displaces equal charge over voltage swing of interest
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Digital Integrated Circuit Design The Devices
Capacitances in 0.25 Capacitances in 0.25 m CMOS m CMOS processprocess
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Digital Integrated Circuit Design The Devices
The Sub-Micron MOS TransistorThe Sub-Micron MOS Transistor
Threshold Variations Subthreshold Conduction Parasitic Resistances
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Digital Integrated Circuit Design The Devices
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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Digital Integrated Circuit Design The 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
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Digital Integrated Circuit Design The Devices
Sub-Threshold Sub-Threshold IIDD vs vs VVGSGS
VDS from 0 to 0.5V
kT
qV
nkT
qV
D
DSGS
eeII 10
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Digital Integrated Circuit Design The Devices
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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Digital Integrated Circuit Design The 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
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Digital Integrated Circuit Design The Devices
Parasitic ResistancesParasitic Resistances
W
LD
Drain
Draincontact
Polysilicon gate
DS
G
RS RD
VGS,eff
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Digital Integrated Circuit Design The Devices
Latch-upLatch-up
54
Equivalent model
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Digital Integrated Circuit Design The Devices
Future PerspectivesFuture Perspectives
25 nm FINFET MOS transistor
55