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Bermel ECE 305 F16
ECE 305: Fall 2016
MOSFET 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)
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Outline
Bermel ECE 305 F16 2
1. Review Questions
2. Device Variability
3. Improved Mobility
4. Conclusion
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Review Questions
Bermel ECE 305 F16 3
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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Variability in Vth at Low Doping
4
2 2B Tth F F
ox ox
AQ qN WV
C C
IBM Journal of Res. And Tech. 2003.11/16/2016
Variability in Threshold Voltage
Bermel ECE 305 F16
5
2 2B Tth F F
ox ox
AQ qN WV
C C
If every transistor has different Vth and therefore different current, circuit design becomes difficult
2
,( )2
o
D G T idealI VL
VZ C
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Vth control by Metal Work-function
6
cs
Fm
Vacuum level
EV
EF
EC
qVbi
( ) i o G TQ C V V
BT s
o
FBVQ
VC
High-k/metal gate MOSFET
bi g p M
FB
qV E
qV
c F
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Tunneling Current
Bermel ECE 305 F16
7
2
( ) ( )GqViT i G th
A
nJ Q V e T E
N
EC
EV
EF
EG
T
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Ge for PMOS, Si for NMOS
Bermel ECE 305 F16 8
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
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1. Review Questions
2. Device Variability
3. Improved Mobility
4. Conclusion
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Basics of Strain ..
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Compressive
biaxial strain
Enhances mobility in the channel …
Larger lattice
Smaller lattice
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Biaxial Strain to Enhance Mobility
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11
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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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Putting it all together
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MOSFET Simulation Tool:
https://nanohub.org/tools/mosfetsat
Comparing Ge with Si
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Conclusion
Bermel ECE 305 F16 17
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 channels and variability are serious concerns for
MOSFET scaling, but strained lattices can help address
this issue, resulting in effective MOSFET channel
lengths < 15 nm
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