Progettazione di circuiti e sistemi VLSI
Anno Accademico 2010-2011
Lezione 3
16.3.2012
Dispositivi e modelli
Dispositivi e modelli
1
Goal of this chapter
• Presents 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-submicron effects
• Future trends
Dispositivi e modelli
2
The Diode
B
A
Al
SiO2
p
n
Cross-section of pn-junction in an IC process
A
Al
A
p
n
B
B
One-dimensional
representation
diode symbol
Mostly occurring as parasitic element in Digital ICs
Dispositivi e modelli
3
Depletion Region
hole diffusion
electron diffusion
(a) Current flow.
n
p
hole drift
electron drift
Charge
Density

x
Distance
+
-
Electrical
Field
(b) Charge density.

x
(c) Electric field.
V
Potential
-W 1

W2
Dispositivi e modelli
x
(d) Electrostatic
potential.
4
pn (W2)
Forward Bias
pn0
Lp
np0
p-region
-W1 0
W2
n-region
x
diffusion
Typically avoided in Digital ICs
Dispositivi e modelli
5
Reverse Bias
pn0
np0
p-region
-W1 0
W2
x
n-region
diffusion
The Dominant Operation Mode
Dispositivi e modelli
6
Diode Current
Dispositivi e modelli
7
Models for Manual Analysis
+
ID = IS(eV D/T – 1)
VD
ID
+
+
VD
–
–
VDon
–
(a) Ideal diode model
(b) First-order diode model
Dispositivi e modelli
8
Junction Capacitance
Dispositivi e modelli
9
Diode Model
RS
+
VD
ID
CD
-
Dispositivi e modelli
10
SPICE Parameters
Dispositivi e modelli
11
What is a Transistor?
A Switch!
An MOS Transistor
VGS  V T
|VGS|
Ron
S
D
Dispositivi e modelli
12
The MOS Transistor
Dispositivi e modelli
13
MOS Transistors Types and Symbols
D
D
G
G
S
S
NMOS Enhancement NMOS Depletion
D
D
G
G
B
S
S
PMOS Enhancement
NMOS with
Bulk Contact
Dispositivi e modelli
14
Threshold Voltage: Concept
+
S
VGS
-
D
G
n+
n+
Depletion
Region
n-channel
p-substrate
B
Dispositivi e modelli
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The Threshold Voltage
Dispositivi e modelli
16
The Body Effect
0.9
0.85
0.8
0.75
VT (V)
0.7
0.65
0.6
0.55
0.5
0.45
0.4
-2.5
-2
-1.5
-1
V
BS
-0.5
0
(V)
Dispositivi e modelli
17
Current-Voltage Relations
A good ol’ transistor
6
x 10
-4
VGS= 2.5 V
5
Resistive
Saturation
4
ID (A)
VGS= 2.0 V
3
VDS = VGS - VT
2
VGS= 1.5 V
1
0
VGS= 1.0 V
0
0.5
1
1.5
2
2.5
VDS (V)
Dispositivi e modelli
18
Transistor in Linear
VGS
VDS
S
G
n+
–
V(x)
ID
D
n+
+
L
x
p-substrate
B
MOS transistor and its bias conditions
Dispositivi e modelli
19
Transistor in Saturation
VGS
VDS > VGS - VT
G
D
S
n+
-
VGS - VT
+
n+
Pinch-off
Dispositivi e modelli
20
Current-Voltage Relations
Long-Channel Device
Dispositivi e modelli
21
A model for manual analysis
Dispositivi e modelli
22
Current-Voltage Relations
The Deep-Submicron Era
2.5
x 10
-4
VGS= 2.5 V
Early Saturation
2
VGS= 2.0 V
ID (A)
1.5
VGS= 1.5 V
1
0.5
0
Linear
Relationship
VGS= 1.0 V
0
0.5
1
1.5
2
2.5
VDS (V)
Dispositivi e modelli
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u n (m/s)
Velocity Saturation
usat = 105
Constant velocity
Constant mobility (slope = µ)
c = 1.5
Dispositivi e modelli
 (V/µm)
24
Perspective
ID
Long-channel device
VGS = VDD
Short-channel device
V DSAT
VGS - V T
Dispositivi e modelli
VDS
25
ID versus VGS
-4
6
x 10
-4
x 10
2.5
5
2
4
linear
quadratic
ID (A)
ID (A)
1.5
3
1
2
0.5
1
0
0
quadratic
0.5
1
1.5
2
2.5
0
0
VGS(V)
0.5
1
1.5
2
2.5
VGS(V)
Long Channel
Short Channel
Dispositivi e modelli
26
ID versus VDS
-4
6
-4
x 10
VGS= 2.5 V
x 10
2.5
VGS= 2.5 V
5
2
Resistive Saturation
ID (A)
VGS= 2.0 V
3
VDS = VGS - VT
2
1
VGS= 1.5 V
0.5
VGS= 1.0 V
VGS= 1.5 V
1
0
0
VGS= 2.0 V
1.5
ID (A)
4
VGS= 1.0 V
0.5
1
1.5
2
2.5
0
0
VDS(V)
Long Channel
0.5
1
1.5
2
2.5
VDS(V)
Short Channel
Dispositivi e modelli
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A unified model
for manual analysis
G
S
D
B
Dispositivi e modelli
28
Simple Model versus SPICE
2.5
x 10
-4
VDS=VDSAT
2
Velocity
Saturated
ID (A)
1.5
Linear
1
VDSAT=VGT
0.5
VDS=VGT
0
0
0.5
Saturated
1
1.5
2
2.5
VDS (V)
Dispositivi e modelli
29
A PMOS Transistor
-4
0
x 10
VGS = -1.0V
-0.2
VGS = -1.5V
ID (A)
-0.4
-0.6
-0.8
-1
-2.5
VGS = -2.0V
Assume all variables
negative!
VGS = -2.5V
-2
-1.5
-1
-0.5
0
VDS (V)
Dispositivi e modelli
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Transistor Model
for Manual Analysis
Dispositivi e modelli
31
The Transistor as a Switch
VGS  V T
Ron
S
ID
V GS = VD D
D
Rmid
R0
V DS
VDD/2
Dispositivi e modelli
VDD
32
The Transistor as a Switch
7
x 10
5
6
Req (Ohm)
5
4
3
2
1
0
0.5
1
1.5
V
DD
2
2.5
(V)
Dispositivi e modelli
33
The Transistor as a Switch
Dispositivi e modelli
34
MOS Capacitances
Dynamic Behavior
Dispositivi e modelli
35
Dynamic Behavior of MOS Transistor
G
CGS
CGD
D
S
CGB
CSB
CDB
B
Dispositivi e modelli
36
The Gate Capacitance
Polysilicon gate
Source
Drain
xd
n+
W
xd
n+
Gate-bulk
overlap
Ld
Top view
Gate oxide
tox
n+
L
n+
Cross section
Dispositivi e modelli
37
Gate Capacitance
G
G
CGC
CGC
D
S
Cut-off
G
CGC
D
S
Resistive
D
S
Saturation
Most important regions in digital design: saturation and cut-off
Dispositivi e modelli
38
Diffusion Capacitance
Channel-stop implant
N A1
Side wall
Source
ND
W
Bottom
xj
Side wall
LS
Channel
Substrate N A
Dispositivi e modelli
39