Carbon Nanotube FieldEffect Transistors Critique of HighFrequency Performance
Carbon Nanotube Field-Effect Transistors: Critique of High-Frequency Performance D. L. Pulfrey Department of Electrical and Computer Engineering University of British Columbia Vancouver, B. C. V 6 T 1 Z 4, Canada pulfrey@ece. ubc. ca http: //nano. ece. ubc. ca L. C. Castro D. L. John Li Chen
Carbon Nanotubes n Hybridized carbon atom graphene monolayer carbon nanotube 1 s orbital, 2 e- sp 2 hybridized orbital, 3 e( -bonds) 2 p orbital, 1 e( -bonds) n High mobility – quasi-1 D, low m*, no surface states n Small SCE - coaxial geometry L. C. Castro
Employment of metallic CNTs T. Iwai et al. , (Fujitsu), 257, IEDM, 2005
Fabricated Carbon Nanotube FETs 300 nm SB-CNFET A. Le Louarn et al. , APL, 90, 233108, 2007 Single-tube drawbacks: Imax ~ A Zout ~ k 80 nm C-CNFET A. Javey et al. , Nano Lett. , 5, 345, 2005
High-frequency Carbon Nanotube FET A. Le Louarn et al. , APL, 233108, 2007
Experimental results for f. T "Ultimate"
Carbon nanotube FETs: model structures SB-CNFET K. Alam et al. , APL, 87, 073104, 2005 C-CNFET D. L. Pulfrey et al. , IEEE TNT, 2007
Ballistic transport
vsig and SD
SB-CNFET: summary of predictions "Ultimate"
C-CNFET: summary of predictions (July 2007)
C-CNFET: summary of predictions (latest)
Regional delay times Energy where most ∂Q occurs 7. 6 THz D. L. John et al. , WOCSDICE, 2007 D. L. Pulfrey et al. , IEEE TNT, 2007
Image charges in transistors BJT FET _ + _ + QB QC + _ + QB+qb qe BJT: qb < |qe| _ _ QS+qs + + _ + QC+qc QG+qg + + + FET: qg |qe| _ + qe QD+qd
Q(E, z) in CNFETs SB-CNFET -5. 5 e. V C-CNFET Insignificant resonance in channel
Comparison of vband: Si NW, Si planar and CNT Si NW and planar Si J. Wang et al. , (11, 0) CNT Tight-binding APL, 86, 093113, 2005 vb, max (CNT) higher by factor of ~ 5
Si MOSFET and CNFET: comparison CN oxide S. Lee et al. , IEDM, 241, 2005 W (um) Lg (nm) Tox (nm) gm (m. S) Cgg (a. F) Ft (THz) FET Status Si MOS Exptl. (IBM) 80 27 1. 05 108 52 0. 33 C-CN coax Theor. (UBC) 80 7 2 448 37 1. 93 Gate
Conclusions • Multi-channel CNFETs needed for high current and for impedance matching. • HF performance appears to be ultimately limited by vb, max. • CNs have a vb, max advantage over Si of ~ 5 times. • This could lead to a gm advantage (in C-CNFETs). • Translating this advantage into superior f. T and fmax will necessitate keeping CGG low, which may be a technological issue. • Seek applications not suited to Si.
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