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Lecture 23
OUTLINE
The MOSFET (cont’d)
• Source/drain structure
• CMOS fabrication process
• The CMOS power crisis
Reading: Pierret 19.2; Hu 6.10
Optional Reading: Pierret 4; Hu 3
Source and Drain (S/D) Structure
• To minimize the short channel effect and DIBL, we want
shallow (small rj) S/D regions  but the parasitic resistance of
these regions increases when rj is reduced.
Rsource, Rdrain  r / Wrj
where r = resistivity of the S/D regions
• Shallow S/D “extensions” may be used to effectively reduce rj
with a relatively small increase in parasitic resistance
EE130/230A Fall 2013
Lecture 23, Slide 2
E-Field Distribution Along the Channel
• The lateral electric field peaks at
the drain end of the channel.
Epeak can be as high as 106 V/cm
• High E-field causes problems:
–Damage to oxide interface & bulk
(trapped oxide charge  VT shift)
–substrate current due to impact
ionization:
EE130/230A Fall 2013
Lecture 23, Slide 3
Lightly Doped Drain (LDD) Structure
• Lower pn junction doping results in lower peak E-field
 “Hot-carrier” effects are reduced
 Parasitic resistance is increased
R. F. Pierret, Semiconductor Device Fundamentals, Fig. 19.9
EE130/230A Fall 2013
Lecture 23, Slide 4
Parasitic Source-Drain Resistance
G
RS
S
RD
D
• For short-channel MOSFET, IDsat0  VGS – VT , so that
I Dsat0
I Dsat 
I Dsat0 Rs
1
(VGS  VT )
 IDsat is reduced by ~15% in a 0.1 mm MOSFET.
• VDsat is increased to VDsat0 + IDsat (RS + RD)
EE130/230A Fall 2013
Lecture 23, Slide 5
C. C. Hu, Modern Semiconductor Devices for Integrated Circuits, Figure 7-10
Summary: MOSFET OFF State vs. ON State
• OFF state (VGS < VT):
– IDS is limited by the rate at which carriers diffuse
across the source pn junction
– Minimum subthreshold swing S, and DIBL are issues
• ON state (VGS > VT):
– IDS is limited by the rate at which carriers drift across
the channel
– Punchthrough is of concern at high drain bias
• IDsat increases rapidly with VDS
– Parasitic resistances reduce drive current
• source resistance RS reduces effective VGS
• source & drain resistances RS & RD reduce effective VDS
EE130/230A Fall 2013
Lecture 23, Slide 6
CMOS Technology
Need p-type regions (for NMOS) and n-type regions (for PMOS)
on the wafer surface, e.g.:
(ND)
n-well
(NA)
Single-well technology
• n-well must be deep enough
to avoid vertical punch-through
p-substrate
(NA)
p-well
Twin-well technology
• Wells must be deep enough to
avoid vertical punch-through
(ND)
n-well
p- or n-substrate
(lightly doped)
EE130/230A Fall 2013
Lecture 23, Slide 7
Sub-Micron CMOS Fabrication Process
p-type Silicon Substrate
• A series of lithography, etch,
and fill steps are used to create
silicon mesas isolated by
silicon-dioxide
Shallow Trench Isolation (STI) - oxide
p-type Silicon Substrate
p-type Silicon Substrate
EE130/230A Fall 2013
• Lithography and implant steps
are used to form the NMOS
and PMOS wells and the
channel/body doping profiles
Lecture 23, Slide 8
• The thin gate dielectric layer is
formed
p-type Silicon Substrate
• Poly-Si is deposited and
patterned to form gate
electrodes
p-type Silicon Substrate
p-type Silicon Substrate
EE130/230A Fall 2013
• Lithography and implantation
are used to form NLDD and
PLDD regions
Lecture 23, Slide 9
• A series of steps is used to form
the deep source / drain regions
as well as body contacts
p-type Silicon Substrate
• A series of steps is used to
encapsulate the devices and
form metal interconnections
between them.
p-type Silicon Substrate
EE130/230A Fall 2013
Lecture 23, Slide 10
CMOS Technology Advancement
XTEM images with the same scale
courtesy V. Moroz (Synopsys, Inc.)
90 nm node
T. Ghani et al.,
IEDM 2003
65 nm node
(after S. Tyagi et al., IEDM 2005)
45 nm node 32 nm node
K. Mistry et al.,
IEDM 2007
P. Packan et al.,
IEDM 2009
• Gate length has not scaled proportionately with device pitch
(0.7x per generation) in recent generations.
– Transistor performance has been boosted by other means.
EE130/230A Fall 2013
Lecture 23, Slide 11
Performance Boosters
• Strained channel regions  meff
• High-k gate dielectric and metal gate electrodes  Coxe
Cross-sectional TEM views of Intel’s 32nm CMOS devices
P. Packan et al., IEDM Technical Digest, pp. 659-662, 2009
EE130/230A Fall 2013
Lecture 23, Slide 12
Historical Voltage Scaling
• Since VT cannot be scaled down aggressively, the
supply voltage (VDD) has not been scaled down in
proportion to the MOSFET gate length:
VDD
VDD – VT
Source: P. Packan (Intel),
2007 IEDM Short Course
EE130/230A Fall 2013
Lecture 23, Slide 13
Power Density Scaling – NOT!
1E+03
Power Density Prediction circa 2000
Power Density (W/cm2)
Power Density (W/cm2)
Power Density Trend
Active Power Density
1E+02
1E+01
1E+00
1E-01
1E-02
1E-03
1E-04
Passive Power Density
1E-05
0.01
0.1
1
10000
Sun’s Surface
1000
Nuclear Reactor
100
8086 Hot Plate
10 4004
P6
8008 8085
Pentium®
proc
386
286
486
8080
1
1970
1980
1990
2000
Gate Length (μm)
Year
Source: S. Borkar (Intel )
Source: B. Meyerson (IBM) Semico Conf.,
January 2004
EE130/230A Fall 2013
Rocket Nozzle
Lecture 23, Slide 14
2010
Parallelism
• Computing performance is now limited by power dissipation.
This has forced the move to parallelism as the principal means
of increasing system performance.
Energy vs. Delay per operation
100
1000
single
core
Sun’s Surface
Rocket Nozzle
Nuclear Reactor
100
Core 2
8086 Hot Plate
10 4004
P6
8008 8085
Pentium® proc
386
286
486
8080
1
1970
1980
1990
2000
2010
Year
Source: S. Borkar (Intel )
EE130/230A Fall 2013
Normalized Energy/op
Power Density (W/cm2)
10000
80
dual
core
60
Operate at a lower
energy point
(lower VDD)
40
20
Run in parallel to
recoup performance
0 0
10
Lecture 23, Slide 15
1
10
2
3
10
10
1/throughput (ps/op)
4
10
Key to VDD Reduction: Gate Control
Gate
log ID
Cox
Cdep
Source
Body
ION
Ctotal
S
Cox
Drain
VDD
VGS
• The greater the capacitive coupling between Gate and channel, the
better control the Gate has over the channel potential.
 lower VDD to achieve target ION/IOFF
 reduced short-channel effect (SCE) and
drain-induced barrier lowering (DIBL)
EE130/230A Fall 2013
Lecture 23, Slide 16
Intel Ivy Bridge Processor
EE130/230A Fall 2013
Lecture 23, Slide 17