investigation of performance limits of germanium dg-mosfet tony low 1, y. t. hou 1, m. f. li 1,2,...
TRANSCRIPT
Investigation of Performance Limits of Germanium DG-MOSFET
Tony Low1, Y. T. Hou1, M. F. Li1,2, Chunxiang Zhu1, Albert Chin3, G. Samudra1,
L. Chan4 and D. -L. Kwong5
[1] Silicon Nano Device Lab (SNDL), National University of Singapore [2] Institute of Microelectronics, Singapore
[3] Electronics Eng., National Chiao Tung Univ., Hsinchu, Taiwan [4] Technology Development, Chartered Semiconductor, Singapore[5] Electrical and Computer Engineering, University of Texas, USA
International Electron Device Meeting 2003
Silicon Nano Device Laboratory
Motivations
Modeling Methodology
Impact of Surface Orientations
Optimizing Ballistic Drive Current
Leakage Considerations
Conclusions
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Presentation Outline
• Mobility degradations related to body confinement and high-K dielectric
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• UTB successfully demonstrated and projected to be used in 2007
S. Nakaharai et al.
• High-K dielectric on Ge-Bulk or Ge-OI processing with high mobility demonstrated
Questions: The performance limits of Ge UTB ? The possible engineering issues ?
• This propel recent research effort into Ge UTB
Prospect for future HP and LSTP applications ?
Motivations
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Quantum Simulations
Straight DG MOSFETNeudeck et al.
IEDM 2000
Body thickness <5nm explored
All possible crystal orientations explored
Abrupt heavily doped source/drain junctions
Lightly p-doped (1x1015cm-3) channel (NMOS)
Metal gate and EOT of 1nm used
Quantum transport simulated for ION and IOFF
A DG structure used, result applicable to SG at UTB
Gate work-function selected for given IOFF
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Different Surface Orientations
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Ge<100> Ge<110> Ge<111>
2D constant energy ellipses and Brillouin zone
L valley
valley
F. Stern et al. PR163, 1967
Transport mass, DOS mass, Quantization massCalculated for various surface + channel orientations
L and valleys considered due to small energy splits
L valleys electrons contribute low transport mass
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Impact of Carrier Quantization
Body quantization effect results:
L and valleys competing for
dominance
valleys sink down at TBODY < 5nm for Ge<100>
L valleys stay much below EF for Ge<111>
L valleys dominant for Ge<110> at TBODY < 5nm
Self-consistent Poisson and Schrodinger calculation
Inversion charge: 1x1013cm-2
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Impact of Carrier Quantization
Voltage Overdrive
VDD - VT
VDD and VT are defined at
inducing surface charge densities of 1x1013cm-2 & 1x1011cm-2 respectively
Ge UTB generally have better overdrive than Si
Ge<111> has poor overdrive due to low DOS mass
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Ballistic Current
ThermionicSD Tunneling
Ese
Ec
EvEsh
Non-Equilibrium Green Function for SD current
Scattering treated using simple Buttiker probes: A phenomenological treatment but efficient
Channel length 20nm used for good SS
NEGFPurdue’s Comp.
Electronics Group
S. Datta et al. IEDM 2002 R. Venugopal et al. JAP 2003
Modeling transport current from Source to Drain
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Ballistic CurrentExploring different surface and channel orientation
Ge<100> and Ge<111> relatively isotropic
Exhibits high anisotropy Optimal channel direction for electron is [110] Aligned with experimental optimal hole transport direction in Si <110> UTB
For Ge<110>:
T. Mizuno et al. VLSI 2003
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Ballistic Current
Drive current decrease for Ge<100>
Drive current increase for Ge<110>
Effect of body scaling on ballistic current
Due to increasing valleys occupation
Due to improved overdrive and high L occupation Drive current decrease for Ge<111>
Due to degradation of overdrive
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Quasi-Ballistic Current
Si 40 cm2/Vs
Ge 400 cm2/Vs
S. M. Sze
Ge<110> 60% ballistic and Si<100> 40% ballistic
TBODY=3nm LG=30nm
Higher ballistic nature of Ge UTB due to less dissipative source/drain
Ge<110> drive current at quasi-ballistic matched Si<100> ballistic current
Source/Drain mobility
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Quasi-Ballistic CurrentComparing performance metric CV/I of Si and Ge
Simulated at quasi-ballistic regime Considered only subthreshold leakage
TBODY = 3nm LG = 30nmEOT = 1nm
Appreciable improvement in intrinsic delay Need to account BTB and Gate leakages in LSTP
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BTB LeakageModeling of BTB Tunneling current
Subband-to-subband tunneling using WKB Freeman and Dahlke dispersion relation used
L. B. Freeman et al. SSE 1970
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BTB Leakage
BTB leakage depends on: Effective band gap Tunneling mass Applied supply voltage
Ge<111> exhibits very large BTB leakage
BTB leakage sets a limit on maximum supply voltage
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BTB LeakageReduction of BTB Tunneling current
BTB leakage has to be suppressed for LSTP
Ge<110> performance diminish when BTB dominates
Body thinning effectively increase allowable supply voltage due to apparent band gap widening
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Gate LeakageModeling of Gate Tunneling current
Improved WKB tunneling model used Wave reflection at abrupt interfaces accounted
Y. T. Hou et al. IEDM 2002
Only dominant CBE tunneling current considered
CBE: Conduction electrons
VBE: Valence electrons
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Gate Leakage
Gate leakage strong dependent on quantization mass
Gate leakage generally larger for Ge
Relatively insensitive to TBODY except Ge<110>
At an inversion charge of 1x1013cm-2
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Gate LeakageDielectric requirements for low voltage operation
Ge UTB requires a larger EOT (of ~1nm) for given gate voltage
Gate voltage and EOT design requirements for gate leakage of 10pA/um
(Inversion charge 1x1013cm-2)
HfO2 with k=22
TBODY=3nm and LG=30nm
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Ge<110>:
1) Largest drive current and increase with body scaling
2) High anisotropy of drive current3) Aligned optimum channel for electron and
hole transport4) Require thin body for BTB suppression 5) Demand low voltage operation for BTB
suppression6) Requires larger EOT for suppression of
gate leakages
Main FindingsGe UTB DG: Performance Limit & Design Requirement
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Ge<111>:
1) Poor voltage overdrive2) Large BTB leakage
Ge<100>:
1) Body scaling beyond 5nm not advantageous
2) At 5nm body, appreciable L valley electrons occupation obtainable
3) Relatively low BTB leakages
Main FindingsGe UTB DG: Performance Limit & Design Requirement
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We acknowledge the NEGF program NanoMOS from Purdue University Comp. Electronic Group and the help rendered by Prof Mark Lundstrom, Ramesh Venugopal and useful discussion with Rahman Anisur. This work is supported by Singapore A*STAR research grant R263000267305 and R263000266305. The author T. Low gratefully acknowledges the Scholarship from Singapore Millennium Foundation.
Acknowledgement