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

Impact of Surface Orientations

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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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Optimizing Ballistic Drive Current

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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

Leakage Considerations for

LSTP Applications

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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