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Special Topics in Nanodevices

Special Topics in Nanodevices. 3 rd Lecture: Nanowire MOSFETs Byung-Gook Park. Nanowire MOSFETs. MOSFET Scaling and Issues Evolution of MOSFET Device Structure Double Gate Structures Multiple Gate Structures Ballistic Electron Transport in Nanowires

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Special Topics in Nanodevices

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  1. Special Topics in Nanodevices 3rd Lecture: Nanowire MOSFETs Byung-Gook Park Special Issues on Nanodevices

  2. Nanowire MOSFETs • MOSFET Scaling and Issues • Evolution of MOSFET Device Structure • Double Gate Structures • Multiple Gate Structures • Ballistic Electron Transport in Nanowires • Effectof Scattering – Landauer’s Formula Ref : H.S. Min, Y.J. Park, B.G. Park, H.C. Shin, Semiconductor Devices with NANOCAD, Ch. 8 S. Datta, Electronic Transport in Mesoscopic Systems, Ch. 2 Special Issues on Nanodevices

  3. MOSFET Scaling - The Grand View D.R. = 20e - 0.116(Y-1960) q ~ 3 yrs : 2-1/2 reduction q~ 20 yrs : 10-1 reduction ITRS Roadmap (’05) 2016 : 9 nm (MPU Lphy) 2019 : 6 nm (MPU Lphy) Special Issues on Nanodevices

  4. Short Channel Effects (1) q Phenomenon : roll-off of VT as a function of gate length L q Cause : charge sharing Special Issues on Nanodevices

  5. Short Channel Effects (2) Special Issues on Nanodevices

  6. Dopant Number Fluctuation and VT q Number of dopants in the depletion region : - The number of dopant atoms in the depletion region decreases as the device dimension decreases. - As the number of dopants decreases, the statistical fluctuation of the number of dopants becomes more important. qExample : L = W = 50 nm, Na = 1018 cm-3 Wdm = 35 nm  N = Na LWWdm = 87.5  sN = N1/2 = 9.35 (~ 10.7%) qThreshold voltage variation due to dopant number fluctuation : Special Issues on Nanodevices

  7. Evolution of Device Structure (1) q Tightness of gate control over the channel Double gate SOI Bulk SiO2 Special Issues on Nanodevices

  8. Evolution of Device Structure (2) q Tightness of gate control over the channel Special Issues on Nanodevices

  9. Short Channel Effects <Single gate> <Double gate> • qDesign guideline • SG: tsi Lchannel/3 • DG: tsi 2Lchannel/3 • NW: tsi Lchannel Special Issues on Nanodevices

  10. Various Double Gate Structures Type I Type II Type III L: horizontal W: horizontal L: vertical W: horizontal L: horizontal W: vertical Special Issues on Nanodevices

  11. FinFETs q FinFET Schematic q FinFET Issue G L L S D G G G G W W G fin fin t S S D D SOI SOI G Hfin=tSOI S D Wfin<LG G Special Issues on Nanodevices

  12. Electric Field and Charge Distribution VG<VT VG>VT q Electric Field q Charge Special Issues on Nanodevices

  13. Basic Equations for DGMOSFETs q Due to symmetry q Voltage, electric field, and channel charge Special Issues on Nanodevices

  14. Threshold Voltage and Drain Current q Threshold voltage * Usually, Qb 0 to suppress the dopant # fluctuation effect  negative threshold voltage for n-channel  work function engineering required q Drain current * Two devices in one! Special Issues on Nanodevices

  15. Inversion Charge in the Channel • Charge distribution : - assumption: Charge distribution is dominated by the ground state. <Thick channel> <Thin channel> Bulk inversion – two channels are merged Surface inversion – two channels are separated Special Issues on Nanodevices

  16. VT vs. Channel Thickness • Threshold voltage for thicker channel : VT • Threshold voltage for thin channel : - dominated by the energy level quantization - higher for thinner body Tch Special Issues on Nanodevices

  17. MG MOSFETs and Corner Effects • qQuadruple gate MOSFET • The gate surrounds the • channel. • qCorner effect : • Field concentration at • corners. Gate Gate Oxide Oxide Channel Channel Special Issues on Nanodevices

  18. Coaxial Gate MOSFETs Gate Oxide Channel • qIdeal shape for NW MOSFET • No corner effect • 2D analysis with cylindrical coordinates Special Issues on Nanodevices

  19. Carbon Nanotube FETs q CNT FETs • Schottky contact at S/D junction • High k dielectric for gate insulator Special Issues on Nanodevices

  20. Ballistic Transport in Nanowire (1) Contact 1 Contact 2 L Ballistic Conductor y W x q Large conductor (L >> mean free path): G = sW/L (Ohmic scaling) qG  for L  0? q Ballistic conductor (L << mean free path): G Gc for L  0 Gc-1 : “contact” resistance Special Issues on Nanodevices

  21. Ballistic Transport in Nanowire (2) q Assumption : ‘reflectionless contacts’ Electrons can enter a wide contact from a narrow conductor without suffering reflections. => +k states : occupied by electrons originating in the left contact -k states : occupied by electrons originating in the right contact q Quasi-Fermi levels : q Dispersion relation : N : transverse mode number eN : cut-off energy for mode N E N = 3 2 1 -eV1 -eV2 e3 e2 e1 k Special Issues on Nanodevices

  22. Ballistic Transport in Nanowire (3) q Number of transverse modes : q Current by a single transverse mode : +k states are occupied according to the function f(E-Ef+) Special Issues on Nanodevices

  23. Ballistic Transport in Nanowire (4) q Current by multiple transverse modes : q Total current : q At low temperature : Special Issues on Nanodevices

  24. Landauer’s Formula (1) Contact 1 Contact 2 Conductor y Lead 1 Lead 2 T x Special Issues on Nanodevices

  25. Landauer’s Formula (2) Contact 1 Contact 2 Conductor y Lead 1 Lead 2 T x Special Issues on Nanodevices

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