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Digital Integrated
Circuits
A Design Perspective
Jan M. Rabaey
Anantha Chandrakasan
Borivoje Nikolic
The Devices
July 30, 2002
© Digital Integrated Circuits2nd
Devices
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
 Analysis of secondary and deep-submicron effects
 Future trends

© Digital Integrated Circuits2nd
Devices
The Diode
B
A
Al
SiO2
p
n
Cross-section of pn-junction in an IC process
A
p
Al
A
n
B
One-dimensional
representation
B
diode symbol
Mostly occurring as parasitic element in Digital ICs
© Digital Integrated Circuits2nd
Devices
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
© Digital Integrated Circuits2nd

W2
x
(d) Electrostatic
potential.
Devices
Depletion Region
hole diffusion
electron diffusion
p
(a) Current flow.
n
n  N c exp( 
Ec  E f
p  N v exp( 

+
x
Distance
(b) Charge density.
qo  kT ln(
)
Eg

x
(c) Electric field.
qo  kT ln(
)
Nc Nv
)
ni2
[kT ln(
Nc
N
)  kT ln( v )]
nno
p po
nno p po
ni2
)  kT ln(
ND N A
)
2
ni
n po p po  n po p po  ni2
V
Potential
-W 1
kT
)
kT
qo  E g  (qVn  qV p )
-
Electrical
Field
kT
Ev  E f
np  ni2  N c N v exp( 
hole drift
electron drift
Charge
Density
np  ni2  (1.5 x1010 ) 2

W2
© Digital Integrated Circuits2nd
x
(d) Electrostatic
potential.
o 
p po
n
kT
kT
ln(
)
ln( no )
q
pno
q
n po
Devices
Depletion Region
W2 N D  W1 N A
hole diffusion
electron diffusion
p

(a) Current flow.
n
For 0  x  W2
hole drift
electron drift
Charge
Density
 2V  ( x)

x 2
s
For 0  x  W1

+
x
Distance
(b) Charge density.
For 0  x  W1
For 0  x  W2
| Emax ( x  0) |

x
(c) Electric field.
 2V  qN A

x 2
s
E ( x) 
E ( x) 
qN A (W1 )
s

V  qN A ( x  W1 )

x
s
V qN D ( x  W2 )

x
s
qN D (W2 )
s
Integrating twice...
x2
V ( x)  Emax ( x 
) W  W1  W2
2W
1
 0  Emax W
2
V
Potential
-W 1

Integrating...
-
Electrical
Field
 2V qN D
 2 
x
s

W2
© Digital Integrated Circuits2nd
x
(d) Electrostatic
potential.
W 
2 s
q
 N A  ND 

0
 N AND 
Devices
pn (W2)
Forward Bias
pn0
Lp
np0
Wp
p-region
-W1 0
W2
Wn
n-region
x
diffusion
Typically avoided in Digital ICs
© Digital Integrated Circuits2nd
Devices
Forward Bias
I D,P
dpn
 qAD DP
dx
with linear carrier concetration gradient
pn ( x)  
© Digital Integrated Circuits2nd
pn (W2 )  pn 0
x  const
Wn  W2
Devices
Forward Bias
 VD
p(W2 )  pn 0  e t


 I DP




pn 0  ni2 / N D
VD

pn 0  t
 qAD D p
e  1

Wn  W2 

I D  I DP  I DN
 VD 
 I S  e t  1




np0
pn 0
where Is  qAD DP
 qAD Dn
Wn  W2
W p  W1
© Digital Integrated Circuits2nd
Devices
Reverse Bias
pn0
np0
p-region
-W1 0
W2
x
n-region
diffusion
The Dominant Operation Mode
© Digital Integrated Circuits2nd
Devices
Diode Current
© Digital Integrated Circuits2nd
Devices
Models for Manual Analysis
+
ID = IS(eV D/T – 1)
VD
ID
+
+
VD
–
(a) Ideal diode model
© Digital Integrated Circuits2nd
–
VDon
–
(b) First-order diode model
Devices
Junction Capacitance
Q j  AD qW2 ( N D )  AD qW1 ( N A )
Q j  AD
Cj 
dQ j
dVd
 Cj 
2q s N A N D
(0  Vd )
N A  ND
 AD
C j0
1
© Digital Integrated Circuits2nd
2q s N A N D
(0  Vd ) 1
N A  ND
Vd
0
Devices
Diffusion Capacitance
© Digital Integrated Circuits2nd
Devices
Secondary Effects
ID (A)
0.1
0
–0.1
–25.0
–15.0
–5.0
0
5.0
VD (V)
Avalanche Breakdown
© Digital Integrated Circuits2nd
Devices
Diode Model
RS
+
VD
ID
CD
-
© Digital Integrated Circuits2nd
Devices
SPICE Parameters
© Digital Integrated Circuits2nd
Devices
What is a Transistor?
A Switch!
An MOS Transistor
VGS  V T
|VGS|
Ron
S
© Digital Integrated Circuits2nd
D
Devices
The MOS Transistor
Polysilicon
© Digital Integrated Circuits2nd
Aluminum
Devices
MOS Transistors Types and Symbols
D
D
G
G
S
S
NMOS Enhancement NMOS Depletion
D
G
G
S
PMOS Enhancement
© Digital Integrated Circuits2nd
D
B
S
NMOS with
Bulk Contact
Devices
Basic Concepts
Vg  Vox  s
E ( x)  
d ( x)
dx
s   ( x  0) surface potential
d ( x)
Es  E ( x  0)  
| x 0
dx
ps and n s are surface carrier concetration
which are of great intrest
© Digital Integrated Circuits2nd
Devices
Depletion of MOS
xd =
2 si
S
qN A
QB 0  qN A xd   2qN A siS
QS  QB 0 surface charge
VOX  
QS

where COX  OX
COX
tOX
© Digital Integrated Circuits2nd
Devices
Creating Inversion Layer
F  
kT N A
ln
Fermi potential
q
ni
S  2F
xd =
2 si
2 F
qN A
QB0   2qN A si (2 F )
QS  QB 0  QI  QB 0
© Digital Integrated Circuits2nd
Devices
Ideal Threshold Voltage
VG  Vox  s and VTO  VG
VTO 
2qN A si (2 F )
QS
 S 
 2 F
COX
COX
© Digital Integrated Circuits2nd
Devices
More Realistic Vto
VTO 
2qN A si (2 F )
COX
where VFB  GS 
 2 F  VFB
1
(QOX  QSS )
COX
flat band voltage due to impurites in oxide and substrate!
© Digital Integrated Circuits2nd
Devices
Even More Realistic Vto
 Threshold
VTO 
adjustment with ion dose Di
2qN A si (2 F )
© Digital Integrated Circuits2nd
COX
qDi
 2 F  VFB 
COX
Devices
Body Effect
QB   2qN A si (2 F  VB )
VT  VT  VTO 
2qN A si
( 2 F  VB  2 F )
COX
VT  VTO   ( 2 F  VB  2 F )

2qN A si
COX
© Digital Integrated Circuits2nd
Devices
MOSFET Operation
© Digital Integrated Circuits2nd
Devices
MOSFET GCA Analysis
V ( y) : 0  Vds as y : 0  L
xdm ( y) 
2 si
[ 2F  V ( y)
qN A
depletion depth increases from y : 0  L !!!
QI ( y )  COX [Vgs  VT  V ( y )]
charge density decreases from y : 0  L !!!
© Digital Integrated Circuits2nd
Devices
MOSFET GCA Analysis
dy
dR  
 nWQI ( y )
© Digital Integrated Circuits2nd
Devices
MOSFET GCA Analysis
QI ( y )  COX [Vgs  VT  V ( y )]
dy
dR  
 nWQI ( y )
I D dy
dV  I D dR  
 nWQI ( y )
L
I D  dy    nW
0
Vds
 Q ( y)dV
I
0
Vds
W
I D   nCOX ( )  (Vgs  VT  V )dV
L 0
© Digital Integrated Circuits2nd
Devices
Non saturation mode
2
Vds
W
I D   nCOX ( )[(Vgs  VT )Vds 
]
L
2
© Digital Integrated Circuits2nd
Devices
Saturation Mode
 Find
the maximum in Id equation
dI D
W
  nCOX ( )[Vgs  VT  Vds ]  0
dVds
L
 Vds , sat  Vgs  VT
I D   nCOX
© Digital Integrated Circuits2nd
W 1
(Vgs  VT ) 2
L 2
Devices
Saturation Mode
QI ( L)  COX [Vgs  VT  Vds ]  0
© Digital Integrated Circuits2nd
Devices
Channel Length Modulation
V ( L ')  Vds , sat
QI ( L ')  0
© Digital Integrated Circuits2nd
L 
2 si
(Vds  Vdssat )
qN A
Devices
Saturation Mode (Channel Length
Modulation)
1
W
I D   nCOX
(Vgs  VT ) 2
L
2
L(1 
)
L
1
L
 1
 1  Vds
L
L
(1 
)
L
I D   nCOX
© Digital Integrated Circuits2nd
W 1
(Vgs  VT ) 2 (1  Vds )
L 2
Devices
The Threshold Voltage
© Digital Integrated Circuits2nd
Devices
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
© Digital Integrated Circuits2nd
-0.5
0
(V)
Devices
Body Bias
© Digital Integrated Circuits2nd
Devices
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
Quadratic
Relationship
VGS= 1.0 V
0
0.5
1
1.5
2
2.5
VDS (V)
© Digital Integrated Circuits2nd
Devices
Current-Voltage Relations
Long-Channel Device
© Digital Integrated Circuits2nd
Devices
A model for manual analysis
© Digital Integrated Circuits2nd
Devices
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)
© Digital Integrated Circuits2nd
Devices
Velocity Saturation
ideally u n   n E
u n (m/s)
o
but  n 
(1  Vgs )
usat = 105
Constant velocity
Constant mobility (slope = µ)
c = 1.5
© Digital Integrated Circuits2nd
 (V/µm)
Devices
Perspective
ID
Long-channel device
VGS = VDD
Short-channel device
V DSAT
© Digital Integrated Circuits2nd
VGS - V T
VDS
Devices
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
VGS(V)
Long Channel
© Digital Integrated Circuits2nd
2
2.5
0
0
0.5
1
1.5
2
2.5
VGS(V)
Short Channel
Devices
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
VDS(V)
Long Channel
© Digital Integrated Circuits2nd
2
2.5
0
0
0.5
1
1.5
2
VDS(V)
Short Channel
Devices
2.5
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)
© Digital Integrated Circuits2nd
Devices
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 relative to
Vdd!
VGS = -2.5V
-2
-1.5
-1
-0.5
0
VDS (V)
© Digital Integrated Circuits2nd
Devices
Transistor Model
for Manual Analysis
© Digital Integrated Circuits2nd
Devices
MOS Capacitances
Dynamic Behavior
© Digital Integrated Circuits2nd
Devices
Dynamic Behavior of MOS Transistor
G
CGS
CGD
D
S
CGB
CSB
CDB
B
© Digital Integrated Circuits2nd
Devices
The Gate Capacitance
Polysilicon gate
SPICE Parameter:
CGSO, CGDO = Xd*Cox
Source
Drain
xd
n+
xd
Ld
W
n+
Gate-bulk
overlap
Top view
Gate oxide
tox
n+
L
n+
Cross section
© Digital Integrated Circuits2nd
Devices
Gate Capacitance
G
G
CGC
CGC
D
S
G
Cut-off
CGC
D
S
Resistive
D
S
Saturation
Most important regions in digital design: saturation and cut-off
For speed considerations, assume worst-case scenario = W*L*Cox
© Digital Integrated Circuits2nd
Devices
Gate Capacitance
CG C
WLC ox
WLC ox
CGC B
C G CS = CG CD
2
VG S
Capacitance as a function of VGS
(with VDS = 0)
© Digital Integrated Circuits2nd
WLC ox
CG C
2WLC ox
CG CS
WLC ox
2
3
CGCD
0
VDS /(VG S-VT)
1
Capacitance as a function of the
degree of saturation
Devices
Measuring the Gate Cap
x 10-16
10
9
V GS
8
I
7
6
5
Cgs 
I
dVgs / dt
© Digital Integrated Circuits2nd
4
Gate
3 Capacitance (F)
2
- 2 - 1.5 - 1 - 0.5 0 0.5
V GS (V)
1
1.5
2
Devices
Diffusion Capacitance
Channel-stop implant
N A+
Side wall
Source
ND
W
Bottom
xj
Side wall
LS
© Digital Integrated Circuits2nd
Channel
Substrate N A
Devices
Junction Capacitance
© Digital Integrated Circuits2nd
Devices
Capacitances in 0.25 m CMOS
process
© Digital Integrated Circuits2nd
Devices
Capacitances in 0.5 m CMOS
process
NMOSFET
PMOSFET
K
19.6 uA/V2
5.4 uA/V2
VTO
0.74 V
-0.74 V

0.6
0.6

0.06 V-1
0.19 V-1
Xd (Under Diffusion)
6 nm
1 nm
NSUB
1.3 x 10^(16) cm-3
4.8 x 10^(15) cm-3
COX
1.1 x 10^(-3) F/m
1.1 x 10^(-3) F/m
CGDO = CGSO
9.6 x 10^(-12) F/m
1.7 x 10^(-12) F/m
CJ
2.8 x 10^(-4) F/m2
3.0 x 10^(-4) F/m2
CJSW
1.7 x 10^(-10) F/m
2.6 x 10^(-10) F/m
© Digital Integrated Circuits2nd
Devices
The Sub-Micron MOS Transistor
 Threshold
Variations
 Subthreshold Conduction
 Parasitic Resistances
© Digital Integrated Circuits2nd
Devices
Threshold Variations
VT
VT
Long-channel threshold
L
Threshold as a function of
the length (for low VDS )
© Digital Integrated Circuits2nd
Low VDS threshold
VDS
Drain-induced barrier lowering
(for low L)
Devices
Sub-Threshold Conduction
The Slope Factor
qVgs
-2
10
I D  I 0e nkT (1  e
Linear
kT
for Vds 
q
-4
10
qVgs
-6
ID (A)
 0
and
CD
COX
S is VGS for ID2/ID1 =10
-8
10
-10
Exponential
-12
VT
10
10
)(1  Vds )
I D  I 0e nkT , n  1 
Quadratic
10
qVds
kT
0
0.5
1
1.5
VGS (V)
© Digital Integrated Circuits2nd
2
2.5
Typical values for S:
60 .. 100 mV/decade
Devices
Sub-Threshold ID vs VGS
I D  I 0e
qVGS
nkT
qV
 DS

1  e kT






VDS from 0 to 0.5V
© Digital Integrated Circuits2nd
Devices
Sub-Threshold ID vs VDS
I D  I 0e
qVGS
nkT
qV
 DS

1  e kT



1    VDS 


VGS from 0 to 0.3V
© Digital Integrated Circuits2nd
Devices
Summary of MOSFET Operating
Regions
 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 Resistances
Polysilicon gate
LD
G
Drain
contact
D
S
RS
W
VGS,eff
RD
Drain
© Digital Integrated Circuits2nd
Devices
Latch-up
VD D
VDD
p
+
n
+
+
n
+
p
+
+
p
n-well
Rnwell
n
Rnwell
Rpsubs
n-source
p-substrate
(a) Origin of latchup
© Digital Integrated Circuits2nd
p-source
Rpsubs
(b) Equivalent circuit
Devices
Future Perspectives
25 nm FINFET MOS transistor
© Digital Integrated Circuits2nd
Devices