Investigation of Performance Limits of Germanium DG-MOSFET Tony Low 1, Y. T. Hou 1, M. F. Li 1,2, Chunxiang Zhu 1, Albert Chin 3, G. Samudra 1, L. Chan 4 and D. -L. Kwong 5 [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 2 Presentation Outline

Mobility degradations related to body confinement and high-K dielectric 3 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

4 Quantum Simulations Straight DG MOSFET Neudeck et al. IEDM 2000 Body thickness <5nm explored All possible crystal orientations explored Abrupt heavily doped source/drain junctions Lightly p-doped (1x10 15 cm -3 ) channel (NMOS) Metal gate and EOT of 1nm used Quantum transport simulated for I ON and I OFF A DG structure used, result applicable to SG at UTB Gate work-function selected for given I OFF

Impact of Surface Orientations

5 Different Surface Orientations Ge 2D constant energy ellipses and Brillouin zone L valley  valley F. Stern et al. PR163, 1967 Transport mass, DOS mass, Quantization mass Calculated for various surface + channel orientations L and  valleys considered due to small energy splits L valleys electrons contribute low transport mass

6 Impact of Carrier Quantization Body quantization effect results: L and  valleys competing for dominance  valleys sink down at T BODY L valleys stay much below E F for Ge L valleys dominant for Ge at T BODY < 5nm Self-consistent Poisson and Schrodinger calculation Inversion charge: 1x10 13 cm -2

7 Impact of Carrier Quantization Voltage Overdrive V DD - V T V DD and V T are defined at inducing surface charge densities of 1x10 13 cm -2 & 1x10 11 cm -2 respectively Ge UTB generally have better overdrive than Si Ge has poor overdrive due to low DOS mass

8 Optimizing Ballistic Drive Current

8 Ballistic Current 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 NEGF Purdue’s Comp. Electronics Group S. Datta et al. IEDM 2002 R. Venugopal et al. JAP 2003 Modeling transport current from Source to Drain

9 Ballistic Current Exploring different surface and channel orientation Ge and Ge relatively isotropic Exhibits high anisotropy Optimal channel direction for electron is [110] Aligned with experimental optimal hole transport direction in Si UTB For Ge : T. Mizuno et al. VLSI 2003

10 Ballistic Current Drive current decrease for Ge Drive current increase for Ge 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 Due to degradation of overdrive

11 Quasi-Ballistic Current Si40 cm 2 /Vs Ge400 cm 2 /Vs S. M. Sze Ge 60% ballistic and Si 40% ballistic T BODY =3nm L G =30nm Higher ballistic nature of Ge UTB due to less dissipative source/drain Ge drive current at quasi-ballistic matched Si ballistic current Source/Drain mobility

12 Quasi-Ballistic Current Comparing performance metric CV/I of Si and Ge Simulated at quasi-ballistic regime Considered only subthreshold leakage T BODY = 3nm L G = 30nm EOT = 1nm Appreciable improvement in intrinsic delay Need to account BTB and Gate leakages in LSTP

Leakage Considerations for LSTP Applications

13 BTB Leakage Modeling of BTB Tunneling current Subband-to-subband tunneling using WKB Freeman and Dahlke dispersion relation used L. B. Freeman et al. SSE 1970

14 BTB Leakage BTB leakage depends on: Effective band gap Tunneling mass Applied supply voltage Ge exhibits very large BTB leakage BTB leakage sets a limit on maximum supply voltage

15 BTB Leakage Reduction of BTB Tunneling current  BTB leakage has to be suppressed for LSTP Ge performance diminish when BTB dominates Body thinning effectively increase allowable supply voltage due to apparent band gap widening

16 Gate Leakage Modeling 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

17 Gate Leakage Gate leakage strong dependent on quantization mass  Gate leakage generally larger for Ge Relatively insensitive to T BODY except Ge At an inversion charge of 1x10 13 cm -2

18 Gate Leakage Dielectric 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 1x10 13 cm -2 ) HfO 2 with k=22 T BODY =3nm and L G =30nm

19 Ge : 1)Largest drive current and increase with body scaling 2) High anisotropy of drive current 3)Aligned optimum channel for electron and hole transport 4)Require thin body for BTB suppression 5)Demand low voltage operation for BTB suppression 6)Requires larger EOT for suppression of gate leakages Main Findings Ge UTB DG: Performance Limit & Design Requirement

20 Ge : 1)Poor voltage overdrive 2) Large BTB leakage Ge : 1)Body scaling beyond 5nm not advantageous 2) At 5nm body, appreciable L valley electrons occupation obtainable 3) Relatively low BTB leakages Main Findings Ge UTB DG: Performance Limit & Design Requirement

21 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 R and R The author T. Low gratefully acknowledges the Scholarship from Singapore Millennium Foundation. Acknowledgement