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Bermel ECE 305 F15
ECE 305: Fall 2015
MOSFET Review and Wrap-Up
Professor Peter BermelElectrical and Computer Engineering
Purdue University, West Lafayette, IN [email protected]
Pierret, Semiconductor Device Fundamentals (SDF)
Chapter 18 (pp. 645-658)
111/20/2015
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Outline
Bermel ECE 305 F152
1. Review of Key Principles
2. Major Failure Modes
3. Short Channel Effects
4. Device Variability
5. Improved Mobility
6. Conclusion
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Key Principles in MOSFETs
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• MOSFET basic geometry
• MOS band bending
• MOS junction capacitance
• MOSFET transfer and output characteristics
• MOSFET square law
• MOSFET velocity saturation
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MOSFET Geometry
4
gate oxidemetal
p-Si
n+-Si
sourcen+-Si
drain
MOS capacitor
VG
VD > 0VS = 0
ID
“channel”
L
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equilibrium e-band diagram
5
EC
EV
Ei
EF
metal
∆VS
∆Vox
Vbi = −φmsφ x( ) = 0 in the bulk
φ x = 0( ) = φS surface potential
V metal( ) = ∆Vox + φS
φS
V metal( ) = Vbi
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dE
dx=
ρ x( )ε
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band bending in p-type MOS
6Fig. 16.6, Semiconductor Device Fundamentals, R.F. Pierret
Flat band Accumulation Depletion Inversion
′VG = 0 ′VG < 0 0 < ′VG < VT ′VG > ′VT
φS = 0 φS < 0 0 < φS < 2φF φS > 2φF
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7
s.s. gate capacitance vs. d.c. gate bias
C
VG′
C =Cox
1+KOW φS( )
KS xo
depletion
VT′
flat band
Cox
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inversion
accumulation
VG
Cox
CS
φS
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Example: 32 nm N-MOS technology
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MOSFET device metrics
VGS
↑
log10 ID
mA µm( )
VDD
transfer characteristics:
ION
VDS = 0.05 V
VDS = VDD
subthreshold swing:
mV decade( )
DIBL (drain-induced barrier lowering)
mV V( )off-current
VT
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threshold voltagesupply voltage
on-current (mA/µm)
ID VGS = VDS = VDD( )
Bermel ECE 305 F15
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MOSFET device metrics
VDS
↑
ID
mA µm( )
VDD
on-current (mA/µm)
ID VGS = VDS = VDD( )
transconductance
gm ≡∆ID
∆VGS VDS
µS µm( )
on-resistance
RON Ω − µm( )output resistance:
rd Ω − µm( )
VGS
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Why does the curve roll over?
Bermel ECE 305 F1513
( )2
2
µ= −o
D G T
W CI V V
mL
ID
VDS
VGS
VDSAT = VGS − VT( )/ m
( )≈ − − −i o G TQ C V V mV
VG VD>00
Q
loss of inversion
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ID and (VGS - VT): In practice
…..
Bermel ECE 305 F1514
ID
VDS
VGS
( )( ) ~α
= −D D DD G TI V V V V
1 < α < 2
Long channelComplete
velocity
saturation
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2D energy band diagram on n-MOSFET
Bermel ECE 305 F15S.M. Sze, Physics of Semiconductor Devices, 1981 and Pao and Sah.
a) device
b) equilibrium (flat band)
c) equilibrium (φS > 0)
d) non-equilibrium with VG and VD > 0 applied
FN15
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Review Questions
Bermel ECE 305 F15 16
1) Why is the small signal conductance and diffusion capacitance
absent for MOS capacitors?
2) What is the expression for inversion capacitance? Why isn’t there
inversion capacitance in a diode?
3) What is the difference between flatband voltage vs. threshold
voltage?
4) When would you use deep depletion formula vs. small signal
formula?
5) Explain why there is a difference between low frequency response
vs. high-frequency response for a MOS-C, but there is no such
distinction for MOSFET.
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Outline
Bermel ECE 305 F15 17
1. Review of Key Principles
2. Major Failure Modes
3. Short Channel Effects
4. Device Variability
5. Improved Mobility
6. Conclusion
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Warranty, product recall and other facts of life
Bermel ECE 305 F1518
In this course, you are learning to analyze/design MOSFETs that go in an IC …
… because the ICs operate in incredibly harsh conditions, turning on and off trillions of timeduring its lifetime ….
… therefore the properties of the MOSFET keep changing. Eventually, S/D can be shorted, the gate oxide can break, etc ….
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How to Calculate Built-in or Flat-band Voltage
Bermel ECE 305 F1519
χs
Φm
Vacuum level
EV
EF
EC
( )= + − ∆ −
=
Φ
≡
bi g p
FB MS
s MqV E
qV φ
χ
qVbi
( )= −i o G TQ C V V
Therefore,
2φ
= − −
BT F
o
FB
QV
CV
( )
( ( ) )
φ
χ
= ≡ Φ − Φ
= − Φ − − −bi MS M s
M s c F bulk
qV q q
E E
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Interpretation for Bulk Charge
Bermel ECE 305 F1520
0
1,
0 0
1,
0
1
( ) ( )1
( )
ρ δ= −
= −
−∫x
T T ideal
o
T i e l
o
a
o
x
Md
V V xC x
x
x x x dx
Q xV
x C
C/Cox
VG
Ideal VT
New VT
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Acceptor and Donor Traps Combined
Bermel ECE 305 F15
C/Cox
VG
21
Donor-related stretchout
Acceptor-related stretchout
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SiO and SiH Bonds
22
Broken Si-H bonds
Negative Bias Temperature Instability (NBTI)
Hot carrier degradation (HCI)
Broken Si-O bonds
Gate dielectric Breakdown (TDDB)
Electrostatic Discharge (ESD)
Radiation induced Gate Rupture (RBD)
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Negative Bias Temperature Instability
23VD (volts)
I D(m
A)
before stress
after stress
0 1 2 3 4 0
4
3
2
1
Stress Time (sec)
% d
eg
rad
ati
on
101 103 105 107 109
5
10
15
Spec.
Warr
anty
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Dielectric Breakdown
Bermel ECE 305 F1524
ln (time)
Gate
Curr
ent
Breakdown
ln(-
ln(1
-F))
VG1>VG2>VG3VDD6420-2-4-6-8
-10
-2 0 2 4 6 8 10log(TBD)
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Time-dependent Bulk Trap
Bermel ECE 305 F1525
C/Cox
VG
Ideal VT
Actual VT* 1 1
0
( , )oxth th
ox
x Q xV V
t
x C= −
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Radiation Induced Charge Buildup
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C/Cox
VG
Ideal VT
Actual VT* 1 1
0
( , )oxth th
ox
x Q xV V
t
x C= −
50 Mrad(Si) 8MeV elec
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Outline
Bermel ECE 305 F1527
1. Review of Key Principles
2. Major Failure Modes
3. Short Channel Effects
4. Device Variability
5. Improved Mobility
6. Conclusion
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Short Channel Effect: Vth Roll-off
Bermel ECE 305 F1528
Vth
Lch
2 2B A Tth F F
ox ox
Q qN WV
C Cφ φ= − = +
Lch∆Vth ?!
Lmin
0α
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How to reduce Vth roll-off …
Bermel ECE 305 F15 29
min
0
21 1
α
= + −
A T
o J
TJ WqNL
r
rW
C
Reduced substrate doping NA
consider WT and junction breakdown
Shallow junction/geometry of transistorslaser annealing of junctions, FINFETs
Thinner gate oxides Consider tunneling currentHigher gate dielectricConsider bulk trapsHigh-k/metal gate MOSFET
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Advantages of High-k Dielectric
…
30
0
0
21 1
ox
A T J Tc
J
qN W r WL
r
x
ε ακ
= + −
High-k/metal gate MOSFET
2
,( )2
µ= −D G T idea
olVI V
L
CZ
Thicker oxide (x0) for same capacitance …
… ensures the drive-current is not reduced
, but tunneling current is suppressed.
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Solution: Ultra-thin Body SOI
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EF
VG
VG
tox
tSi
N A
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Example: FINFET, OmegaFET, X-FET
Bermel ECE 305 F1532
Gate
Source Drain
Cross-section
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Outline
Bermel ECE 305 F1533
1. Review of Key Principles
2. Major Failure Modes
3. Short Channel Effects
4. Device Variability
5. Improved Mobility
6. Conclusion
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Variability in Vth at Low Doping
34
2 2B Tth F F
ox ox
AQ qN W
VC C
φ φ= − = +
IBM Journal of Res. And Tech. 2003.
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Variability in Threshold Voltage
Bermel ECE 305 F1535
2 2B Tth F F
ox ox
AQ qN W
VC C
φ φ= − = +
If every transistor has different Vth and therefore different current, circuit design becomes difficult
2
,( )2
µ= −o
D G T idealI V
LV
Z C
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Vth control by Metal Work-function
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χs
Φm
Vacuum level
EV
EF
EC
qVbi
( )= −i o G T
Q C V V
φ
= − + −
BT s
o
FBV
QV
C
High-k/metal gate MOSFET
( )bi g p M
FB
qV E
qV
χ= + − ∆ − Φ
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Tunneling Current
Bermel ECE 305 F1537
2
( ) ( )GqViT i G th
A
nJ Q V e T E
N
β υ− = −
EC
EV
EF
EG
T
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Ge for PMOS, Si for NMOS
Bermel ECE 305 F15 38
Gatep-MOS
Active Area
n-MOS
[1’10]
[001]
90°
(110)-Plane
Gate
135°A
B
p-MOS
[1’10]
[100]
n-MOS
(001)-Plane
B
’
Strained-Si
Pillar
Takagi, TED 52, p.367, 2005
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Outline
Bermel ECE 305 F15 39
1. Review of Key Principles
2. Major Failure Modes
3. Short Channel Effects
4. Device Variability
5. Improved Mobility
6. Conclusion
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Basics of Strain ..
Bermel ECE 305 F15 40
Compressive
biaxial strain
Enhances mobility in the channel …
Larger lattice
Smaller lattice
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Uniaxial Compressive Strain to Enhance Mobility
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Biaxial Strain to Enhance Mobility
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Adapted from Chang et. al, IEDM 2005.
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New Channel Materials for improved mobility
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Conclusion
Bermel ECE 305 F15 45
1) The basic behavior of MOSFETs can be captured by
band diagrams, transfer & output characteristics
2) There are a variety of failure modes that can degrade
performance over time
3) Short channel effects + variability are a serious concern
for MOSFET scaling, but many novel approaches have
been proposed to solve them, resulting in effective
MOSFET channel lengths < 15 nm
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