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8/10/2005 PhD Defense, Anisur Rahman 1
Exploring New Channel Materials Exploring New Channel Materials for Nanoscale CMOS Devices: A for Nanoscale CMOS Devices: A
Simulation ApproachSimulation Approach
PhD Final Examination
Anisur Rahman
PhD Co-Advisors: Professor Mark Lundstrom and Professor Gerhard Klimeck
School of Electrical and Computer EngineeringPurdue University, West Lafayette, IN 47907
8/10/2005 PhD Defense, Anisur Rahman 2
Doctoral Advisory Committee:Doctoral Advisory Committee:• Professor Supriyo Datta (ECE)• Professor Ron Reifenberger (Physics)• Dr. Avik Ghosh (ECE)
Present Colleagues:Present Colleagues:• Dr. Diego Kienle• Dr. Jing Wang• Sayed Hasan• Neophytos Neophytou• Siyu Koswatta
Former Colleagues:Former Colleagues:• Dr. Zhibin Ren (IBM) • Dr. Ramesh Venugopal (TI)• Dr. Jung-Hoon Rhew (Intel)• Asst. Prof. Jing Guo (Univ. of Florida)
AcknowledgementsAcknowledgements
Funding:Funding: SRC and MARCO/FCRP-MSD
8/10/2005 PhD Defense, Anisur Rahman 3
Outline:Outline:
• Background• Objective• Generalized Effective-mass Approach
• Assessment of Ge n-MOSFETs
• A Top-of-the-barrier Ballistic Model• Semi-empirical Tight-binding Approach
• Tight-binding Application: UTB DG CMOS• Tight-binding Application: Self-consistent Electrostatics
• Summary• Future Work
Chapter 1
Chapter 2Chapter 3
Chapter 4
Chapter 5Chapter 6
Chapter 7
Chapter 8
8/10/2005 PhD Defense, Anisur Rahman 4
Background: CMOS ScalingBackground: CMOS ScalingHistorical Development:Historical Development:
• Four decades of steady scaling
• Present day MOSFETs are nanoscale (LG~30nm)
• Scaling is motivated by performance and
integration density issues
• Moore’s Law describes this steady growth
• ITRS guides the future scaling trends
Scaling Challenges:Scaling Challenges:
• Implementation of Moore’s law becoming challenging
• Off-state leakage limits scaling of planar CMOS
• Additional gates needed to curb SCE (dual/tri-gate)
• Gate oxide scaling reached the limit (direct tunneling)
• High-κ dielectric + metal gate in near future
• Interface properties of Si-SiO2 becoming less critical
Moore’s LawMoore’s Law
65nm Node Deviecs65nm Node DeviecsLLGG=35nm=35nm
8/10/2005 PhD Defense, Anisur Rahman 5
Background: New MaterialsBackground: New Materials
Motivation for Novel Materirals:Motivation for Novel Materirals:
• Ge and III-V display very high mobility,
saturation velocity, and scattering
mean-free path
• New process technology allows high-
quality
channel-insulator interface
• Ultra-high-speed, very-low-power
logic
application
• Experimental research is underwayKey Device Physics Issues:Key Device Physics Issues:
• Treatment of quantum mechanical
effects
• Trade-off between high velocity and low
DOS
• Atomic level fluctuation become
significant
• Full-band treatment become necessary
Qtop vinj
Current=Qtop X vinj
Qtop=CG(VG-VT) where, CG<COX
Source
Drain
Current CalculationCurrent Calculation
Si GeGaA
sInAs InSb
Electron, μn
(cm2/V-sec)600
>1000
4600
20000
30000
Vsat
(107cm/sec)
1.0 -- 1.2 3.5 5.0
MFP (nm) 28 -- 80 194 226
Electron Transport PropertyElectron Transport Property
Ashley et al., ICSICT 2004
8/10/2005 PhD Defense, Anisur Rahman 6
Objective:Objective:
For New Channel Material Nanoscale CMOS Devices:For New Channel Material Nanoscale CMOS Devices:
• Develop simulation tools and theoretical approaches
• Perform design studies, assess performance limits, and
explore
scaling characteristics
• Investigate relevance of carrier mobility
• Identify key bandstructure related issues
• Provide an improved understanding of their operation.Theoretical Approaches:Theoretical Approaches:
• Effective-mass-equation based quantum transport using NEGF
formalism
for novel-channel material n-MOSFETs
• Atomistic tight-binding approach for incorporating bandstructure
effects in
deeply scaled ballistic n- and p-MOSFETs
8/10/2005 PhD Defense, Anisur Rahman 7
Outline:Outline:
• Background• Objective• Generalized Effective-mass Approach
• Assessment of Ge n-MOSFETs
• A Top-of-the-barrier Ballistic Model• Semi-empirical Tight-binding Approach
• Tight-binding Application: UTB DG CMOS• Tight-binding Application: Self-consistent Electrostatics
• Summary and Conclusion• Future Work
Chapter 2Chapter 3
8/10/2005 PhD Defense, Anisur Rahman 8
Theory: Effective Mass Theory: Effective Mass ApproachApproach
Bottom Gate1k
2k
3k
Source DrainTop Gate
Channel
Device Device
1k
2k3k
CrystalCrystal
1 1 111 12 13
1 1 121 22 231 1 1
31 23 33
m m m
m m m
m m m
22 23 3
1 1
22 2
i ji
i i j iii ij
k kkE
m m
1
1
1
l
t
t
m
m
m
2 2 2 21 2
2 2l t
k k kE
m m
2k
1k
2k
EllipseEllipseKey Features of EMA:Key Features of EMA:
• Most widely used approach for n-
MOSFETs
• Very successful to treat quantum
transport
in (100) Si n-MOSFETs
• NEGF and mode-space decomposition
of the
transport problem is highly efficient
Outstanding Issue:Outstanding Issue:
When DCS and ECS are not
aligned, EMT is a full 3X3 matrix
and solution of EME is very difficult
in 1 2 3, ,E k k kj jk i x
Effective Mass Equation (EME)
8/10/2005 PhD Defense, Anisur Rahman 9
Theory: Generalized Effective Theory: Generalized Effective MassMass
X
Y
Z
33 331 2
31 23
m mi k k zm m
CU e
1
2122
mi k xm
TU e
Z
X
Y
Z
X
Y
Unitary transformations conserves density of states and group velocity
8/10/2005 PhD Defense, Anisur Rahman 10
Results: Ge n-MOSFETsResults: Ge n-MOSFETs
VD = 0.4 V
Si (001)/[100]
Ge (001)/[100]
Ge (111)/[110]
IDS [μA/μm]
Ballistic NEGF IBallistic NEGF IDD-V-VGG Ballistic NEGF IBallistic NEGF IDD-V-VDD
VG = 0.4 V
IDS [μA/μm]
Valley Deg.
(4)
(2)(1)
Simulation Setup:Simulation Setup:
• End of ITRS 2001 UTB DG Ge n-MOSFETs
• NEGF ballistic and scattering simulation
• Design study performed
• nanoMOS 2.5 modified to treat Ge n-MOSFETs
• Ge devices on (100) and (111) wafers
• Process variation and mobility effects examined
S DCh
LG
Lul Lul
NSD NSD
LT=LG+2Lul
Doping Density Profile
EOT=0.6nm
Rahman et al., IEDM 2003
8/10/2005 PhD Defense, Anisur Rahman 11
IOFF [μA/μm]
Gate Length, LG [nm]
W/C device
Thinbody
Thickbody
VD = 0.4 V; VG = 0 V
Nominaltbody = 2.5 nm
tbody
Energy
YSensitivity to process variation
Results: Ge n-MOSFETsResults: Ge n-MOSFETs
VG = VD = 0.4V
IDS [μA/μm]
ch=300cm2/V-sec
S/D mobility, SD [cm2/V-sec]
ITRS 2016
Ballistic
limitch=1000cm2/V-sec
S DCh
NSD NSD
μSD μSDμCh
NEGF Scattering
8/10/2005 PhD Defense, Anisur Rahman 12
Outline:Outline:
• Background• Objective• Generalized Effective-mass Approach
• Assessment of Ge n-MOSFETs
• A Top-of-the-barrier Ballistic Model• Semi-empirical Tight-binding Approach
• Tight-binding Application: UTB DG CMOS• Tight-binding Application: Self-consistent Electrostatics
• Summary and Conclusion• Future Work
Chapter 4
8/10/2005 PhD Defense, Anisur Rahman 13
Top-of-the-Barrier Ballistic Top-of-the-Barrier Ballistic ModelModel
Source Drain
FSFD
Gate Electrostatics
S DCh
Top Gate
Bottom Gate
VDVS
VG COX
CS CD
CQ
Circuit Model for 2-D Circuit Model for 2-D ElectrostaticsElectrostatics
UTB DG Model DeviceUTB DG Model Device
Key Features:Key Features:
• Semiclassical ballistic transport
• Poisson equation solved only at top-of-barrier
• Quantum capacitance, CQ, treated
• Treats arbitrary bandstructure
• Treats floating source potential
• Two carrier fluxes, FS and FD present
• Sum of carrier density in FS and FD is total charge
• Difference of current from FS and FD is net current
Rahman et al., TED, 2003
8/10/2005 PhD Defense, Anisur Rahman 14
Outline:Outline:
• Background• Objective• Generalized Effective-mass Approach
• Assessment of Ge n-MOSFETs
• A Top-of-the-barrier Ballistic Model• Semi-empirical Tight-binding Approach
• Tight-binding Application: UTB DG CMOS• Tight-binding Application: Self-consistent Electrostatics
• Summary• Future Work
Chapter 5
8/10/2005 PhD Defense, Anisur Rahman 15
Theory: Tight-bindingTheory: Tight-binding
a
(001)
(100)
(010)
(111)
(110)
Key Features: Localized atomic orbital-like basis
set
Suitable for modeling
nanostructures
Correct full bandstructure
Strain, hetero-channel, novel
materials
Two FCC Lattices — Red and Blue
DrainChannelSource
Device — Ultra-thin Body Symmetric Dual-gate MOSFET
Challenges: Appropriate TB model
Finite dimensional
systems
Remove surface states
Treat electrostatics
Sparse matrix
techniques
Computing I-V
8/10/2005 PhD Defense, Anisur Rahman 16
NN-sp3s*
5 orbitalsNN-sp3d5s*
10 orbitals
Silicon CB ΔEc = +100meV
Vogl NEMO
“Well-behaved” TB Parameter Set
• Manageable size — two
center integrals
• Correct bandgap and
effective masses
• Scalable for strained
materialsBoykin et al.,
PRB 69(11), 2004
Approach: A Good TB ModelApproach: A Good TB Model
8/10/2005 PhD Defense, Anisur Rahman 17
Approach: Band BendingApproach: Band Bending
Transport
Potential Charge
Poisson
Charge Potential
Self-consistenceSelf-consistence
Calculate Bandstructure
Bulk/HOIMOSFETs
UTBMOSFETs
VG
EV
EC
nZ atoms
TB Domain
Poisson Domain
VG
EV
EC
nZ atomsTB Domain
VG
Band bending along thickness is less important in UTB.
Band bending along thickness is important in bulk/HOI devices.
8/10/2005 PhD Defense, Anisur Rahman 18
Outline:Outline:
• Background• Objective• Generalized Effective-mass Approach
• Assessment of Ge n-MOSFETs
• A Top-of-the-barrier Ballistic Model• Semi-empirical Tight-binding Approach
• Tight-binding Application: UTB DG CMOS• Validity of single band parabolic E-k in Ge n-MOSFETs• Compare Si, Ge, GaAs, InAs as channel materials
• Tight-binding Application: Self-consistent Electrostatics
• Summary• Future Work
Chapter 6
8/10/2005 PhD Defense, Anisur Rahman 19
ΔEc=300meV
Bulk Ge Bandstructure:Bulk Ge Bandstructure:
• The CB Δ, Λ, Г within
250meV.
• Parabolic E-k valid at low
energy
• Quantum confinement
can
alter the order of the
bands.
NN-sp3d5s*-SO
ΔEc=170meV
Red-Eff. mass Black-TB
Study: Validity of Parabolic E-k in Study: Validity of Parabolic E-k in GeGe
Simulation SetupSimulation Setup:
• Validity of parabolic E-k in UTB
unstrained Ge n-MOSFET examined
• 20 band sp3d5s*-SO TB used
• Three thicknesses: 16nm, 4nm, 2nm
• Band bending not treated
• I-V calculated from top-of-barrier model
Rahman et al., IEDM 2004
8/10/2005 PhD Defense, Anisur Rahman 20
Results: Thick Body Limit Results: Thick Body Limit (16nm)(16nm)
1 2 3
2D E-k~ 16nm (113 atomic layers) Ge bodySize of H ~ 2200 X 2200Electrostatic potential not treated(100) wafer quantization along [100]Conduction band subbandsNon-parabolicity important at high energyI-V shown for (001)/[100] device
12
3
2D DOS Ballistic I-V
VT not adjusted
VDS = 0.4V
[110] [100]
L
XГ
8/10/2005 PhD Defense, Anisur Rahman 21
Results: Going Thinner (4nm)Results: Going Thinner (4nm)
VT not adjusted
25% lower ION for
eff. mass
ΔVT=55 meV
~ 4nm (30 atomic layers) Ge
body
Size of H ~ 600 X 600
Conduction band Subbands
L and Г- Valleys came closer
Non-parabolicity affects ground
state
Ge (001)/[100] device
VT shifted by 55 meV (not
adjusted)
1 2 3[110] [100]
2D E-k
2 31
ΔVT
2D DOS
VDS = 0.4V
Ballistic I-V
L
XГ
8/10/2005 PhD Defense, Anisur Rahman 22
123
1
23 VT adjusted
2D E-k
2D DOS15%
higher ION for
eff. mass
ΔVT=570 meVΔVT
~ 2nm (12 atomic layers) Ge body
Size of H ~ 240 X 240
Conduction band Subbands
At Г, X Valleys form ground state
Strong non-parabolicity affects L-
valleys
VT shifted by 570 meV (adjusted)
[110] [100]
Ballistic I-V
Results: Extreme Scaling Results: Extreme Scaling (2nm)(2nm)
L
XГ
8/10/2005 PhD Defense, Anisur Rahman 23
Outline:Outline:
• Background• Objective• Generalized Effective-mass Approach
• Assessment of Ge n-MOSFETs
• A Top-of-the-barrier Ballistic Model• Semi-empirical Tight-binding Approach
• Tight-binding Application: UTB DG CMOS• Validity of single band parabolic E-k in Ge n-MOSFETs• Compare Si, Ge, GaAs, InAs as channel materials
• Tight-binding Application: Self-consistent Electrostatics
• Summary• Future Work
Chapter 6
8/10/2005 PhD Defense, Anisur Rahman 24
Study: New Channel MaterialsStudy: New Channel Materials
Energy, [eV]
Si
Ge
Eeff = 1.3eV
Eeff=1.04eV
GaAs
InAs
Eeff=1.72eV
Eeff=0.78eV
Energy, [eV]
Group IVGroup IV
Group III-VGroup III-V
Objective:Objective:To explore and compare scaling characteristics of CMOS with Si, Ge, GaAs, InAs as channel materials.
Simulation Setup:Simulation Setup:
• 2016 device specification from ITRS 2004
(22nm HP)
• UTB DG with, body: 19 AL (~2.5nm),
EOT=0.5nm
• Unstrained material
• Semiclassical, top-of-barrier, ballistic model
Bandstructure:Bandstructure:
• CB and VB split into subbands
• Quantum confinement increase effective band gap
• Very high vinj expected for III-V
• Lowest CB are X2(Si), L4(Ge), Г1(GaAs, InAs)
• Si and Ge display higher CB DOS compared to III-VRahman et al., to appear in IEDM 2005.
8/10/2005 PhD Defense, Anisur Rahman 25
Results: New Channel Results: New Channel MaterialsMaterials
GeGe
SiGaAsInAs
ID-VDDevice: Deeply scaled body (19AL)
and oxide (0.5nm)
Ballistic IBallistic IDD-V-VDD::• Ge performs best (n or p-FET)• GaAs or InAs cannot compete with Si or Ge• InAs n-FETs performs worst
GaAs
InAs
GeSi
vinj-VG
Injection velocity, vInjection velocity, vinjinj vs. V vs. VGG::
• Very high vinj for III-V materials
• Electron vinj in InAs is highest, as expected
• Beyond 0.4V, GaAs vinj drops (Г→L transfer)
GeSi
GaAs
InAs
Qtop-VG
Carrier density, QCarrier density, Qtoptop vs. V vs. VGG::
• Ge: High CB DOS is key to its success
• InAs: Electron Qtop and CG is strongly degraded
• Beyond 0.4V, CG in GaAs improves (Г→L transfer)
Low CB DOS strongly degrades deeply scaled III-V device performances
8/10/2005 PhD Defense, Anisur Rahman 26
tox=0.5nm
Ge
InAs
Si
GaAs
tox=0.5nm Ge
InAs
Si
GaAs
Ge
InAs
SiGaAs
tox=0.5nm
Thin OxideThin Oxide
Thin EOT + Thick Body (Small Thin EOT + Thick Body (Small CCQQ/C/COXOX):):
• Ge n-MOSFETs perform best
• Qtops in III-V suffer, CG strongly degraded
tox=1.0nm
Results: New Channel Results: New Channel MaterialsMaterials
Si
GeInAs & GaAs
Ge
InAs
Si
GaAs
tox=1.0nm
Ge
InAs
Si
GaAs
IIDD-V-VDD
QQtoptop-V-VGG
tox=1.0nm
vvinjinj-V-VGG
Thick OxideThick OxideEffects of ratioEffects of ratio CCQQ/C/COXOX explored:explored:
• 100 AL (~15nm) body thickness
• Only n-MOSFETs treated
• Ballistic transport using top-of-barrier model
• Less QC effect, subbands closely separated
• Thick body increase 2D DOS, consequently, CQ
• EOT =1.0nm, and 0.5nm (thin) considered.
Thick EOT + Thick Body (Large Thick EOT + Thick Body (Large CCQQ/C/COXOX):):
• III-V n-MOSFETs perform best
• Silicon performs worst (slow X4 valleys)
8/10/2005 PhD Defense, Anisur Rahman 27
Outline:Outline:
• Background• Objective• Generalized Effective-mass Approach
• Assessment of Ge n-MOSFETs
• A Top-of-the-barrier Ballistic Model• Semi-empirical Tight-binding Approach
• Tight-binding Application: UTB DG CMOS• Tight-binding Application: Self-consistent Electrostatics
• Mobility behavior in strained bulk p- and n- MOSFETs• Hole density profile in HOI structure
• Summary• Future Work
Chapter 7
8/10/2005 PhD Defense, Anisur Rahman 28
Study: Strained Bulk p-Study: Strained Bulk p-MOSFETsMOSFETs
SiSi
Source
Drain
Si
UnstrainedUnstrained
SiGe
Si SiSi
Source
Drain
Substrate InducedSubstrate InducedBiaxial TensionBiaxial Tension
SiGe
Drain
SiGe
Si
Source
Process InducedProcess InducedUniaxial CompressionUniaxial Compression
Rim et al. (Biaxial) 2002
Rim et al. (Biaxial)
1995
Universal Hole Mobility(Unstrained)
Mob
ilit
y [
cm
2/V
-sec]
Eeff [MV/cm]
Intel 90 nm(Uniaxial)
Experimental Hole MobilityExperimental Hole MobilityOverview:Overview:• Devices on (100) wafers• Strain can be substrate-induced (biaxial)
or
process-induced (uniaxial)• Strain deforms crystal by changing bond
lengths
and bond orientations• Biaxial tensile strain: Hole mobility
improves
at low gate bias but disappears at high bias• Uniaxial compressive strain: Hole
mobility
improvement at low VG retained at high VG
Thompson et al., TED 04
8/10/2005 PhD Defense, Anisur Rahman 29
Top VB HH
Second VB LH
Unstrained
Strained
Consequence of Strain: Consequence of Strain: spsp33dd55ss**-SO-SO
Unstrained Bulk Si VBUnstrained Bulk Si VB
Strained Bulk Si VBStrained Bulk Si VB
8/10/2005 PhD Defense, Anisur Rahman 30
Results: Strained Bulk p-Results: Strained Bulk p-MOSFETsMOSFETs
and High VG
Ballistic IBallistic IONON Ratio vs. E Ratio vs. Eeffeff
Rim et al. (Biaxial) 2002
Rim et al. (Biaxial)
1995
Universal Hole Mobility(Unstrained)
Mob
ilit
y [
cm
2/V
-sec]
Eeff [MV/cm]
Intel 90 nm(Uniaxial)
Experimental Hole MobilityExperimental Hole Mobility
• Bulk p-FETs: Self-consistent sp3d5s*-SO TB
approach
• Low VG: Top VB LH for both uniaxial and
biaxial
• High VG: Top VB HH for biaxial, LH for
uniaxial
• QC nullify strain splitting of LH,HH in biaxial
case
• Ballistic simulation explains mobility behavior
in
strained p-MOSFETs
2D E-k, Low VG
Eeff=q(Ndep+p/3)/εSi
8/10/2005 PhD Defense, Anisur Rahman 31
Study: Strained Bulk n-MOSFETsStudy: Strained Bulk n-MOSFETs
Ballistic n-MOSFETsBallistic n-MOSFETs
Eeff=q(Ndep+p/2)/εSi
SiGe
Drain
SiGe
Si
Source
Process InducedProcess InducedUniaxial TensionUniaxial TensionMobility Behavior in n-MOSFETs:Mobility Behavior in n-MOSFETs:
• Experimentally, both substrate-induced biaxial-
tension and process-induced uniaxial-tension
improves electron mobility
• Electronic mobility enhancement is observed over
the
entire range of gate bias
• Such mobility enhancement is often explained in
terms
of degeneracy removal of X2 and X4 valleys, which
is
recently questioned (Fischetti et al., JAP, 2002.)
Simulation Setup and Observation:Simulation Setup and Observation:
• Self-consistent sp3d5s*-SO TB model and the top-of-
barrier ballistic model does not show any
enhancement
of strained device performance
• Bandstructure alone cannot explain the electronic
mobility enhancement in strained planar n-MOSFETs
8/10/2005 PhD Defense, Anisur Rahman 32
Outline:Outline:
• Background• Objective• Generalized Effective-mass Approach
• Assessment of Ge n-MOSFETs
• A Top-of-the-barrier Ballistic Model• Semi-empirical Tight-binding Approach
• Tight-binding Application: UTB DG CMOS• Tight-binding Application: Self-consistent Electrostatics
• Mobility behavior in strained bulk p- and n- MOSFETs• Hole density profile in HOI structure
• Summary• Future Work
Chapter 7
8/10/2005 PhD Defense, Anisur Rahman 33
ε-Si (y=0.24)
ε-Si (y=0.24)
ε-Si0.5Ge0.5
TOX
BOX
Biaxial tensile
Biaxial tensile
Biaxial compressiv
e
30 AL~ 4 nm
30 AL~ 4 nm
21 AL~ 3 nm
E
EV EC
Simulation Setup:Simulation Setup:
• The 20 band sp3d5s*-SO TB model with
self-
consistent electrostatics.
• Compressively strained Si0.5Ge0.5
sandwiched
between two Si layers under tensile
strain.
• Top and bottom oxides are 2 nm and 10
nm
thick, respectively.
Heterostructure on Insulator Heterostructure on Insulator ((HOIHOI))
• Utilizes the high mobility central SiGe
channel to improve hole mobility
• Band discontinuity moves holes to
center
• Band bending due to VG rearranges
hole
profile
Buried Oxide
Poly-Si
ε-Siε-SiGe
Hoyt Group (MIT)
Schematic Representation
Study: Hole Profile in HOIStudy: Hole Profile in HOI
8/10/2005 PhD Defense, Anisur Rahman 34
Results: HOIResults: HOI
BOXSi/SiGe/Si
Hole ProfileHole ProfileVB ProfileVB Profile
BOX
Si/SiGe/Si
BOXSi/SiGe/Si
Observations:Observations:
• Tri-layer (Si-SiGe-Si) structure
• tOX= 2 nm(top), tOX=10 nm(bot)
• Low VG: band discontinuity
moves holes to central channel.
• High VG: hole profile moves
near the surface.
Q-VQ-VGG
EF = 0.45 eV
EF = 0.45 eV
EF = 0.45 eV
8/10/2005 PhD Defense, Anisur Rahman 35
Outline:Outline:
• Background• Objective• Generalized Effective-mass Approach
• Assessment of Ge n-MOSFETs
• A Top-of-the-barrier Ballistic Model• Semi-empirical Tight-binding Approach
• Tight-binding Application: UTB DG CMOS• Tight-binding Application: Self-consistent Electrostatics
• Summary• Future Work
Chapter 8
8/10/2005 PhD Defense, Anisur Rahman 36
Summary:Summary:
• Effective mass approach extended to treat n-MOSFETs on arbitrary
wafer
orientations.
• An NEGF study of LG=10nm end of ITRS Ge n-MOSFET reveals that:
• Ge (001)/[100] device performs best and can meet target
ION
• Gate under-lap improves short-channel-effects
• High mobility in the S/D region is crucial to limit RS
degradation
• A strict process tolerance in body thickness necessary to
limit VT fluctuation across the chip.
• A physics based top-of-the-barrier semiclassical ballistic transport
model
developed and its application demonstrated.
Continued to next slide
8/10/2005 PhD Defense, Anisur Rahman 37
Summary (cont.):Summary (cont.):• A semi-empirical 20 band sp3d5s*-SO TB model used to assess strained
and
unstrained UTB novel channel material CMOS devices, and was reveled
that:
• Ge n-MOSFETs: Below 4nm thickness, use of single band
parabolic E-k is limited by non-parabolicity. Below 2nm thickness,
its use is further limited by band reordering and multi-valley
conduction.
• A trade-off exists between high vinj and low DOS. III-V devices
outperform Si or Ge n-MOSFETs only for thick body and thick EOT.
For deeply scaled devices Ge displays the best ballistic
performances.
• Self-consistent gate electrostatics was treated in sp3d5s*-SO TB model
and was revealed that:
• Experimental hole mobility behavior in strained planar p-
MOSFETs can be explained by bandstructure modulation. Similar
behavior for electron mobility in n-MOSFETs cannot be explained by
bandstructure alone.
• HOI simulation shows that at high gate field, the hole-profile
moves near the surface and not at the central high-mobility SiGe
layer.
8/10/2005 PhD Defense, Anisur Rahman 38
Future WorkFuture Work
1. Separate the effects of bandstructure and scattering in published
experimental mobility data
2. Discretize the TB Hamiltonian for arbitrarily oriented wafers—Only
(100) wafers treated here
3. Mode-space representation of TB Hamiltonian for UTB MOSFETs—A
full 2D representation is not feasible, computationally
4. Employ zone-unfolding technique to treat SRS and random alloy
effects
5. Employ self-consistent TB approaches for III-V HEMT and QWFET
devices— Ballistic simulation is more relevant here due to their
very high mobility