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2007. 1.Special Issues on Nanodevices1 Special Topics in Nanodevices 3 rd Lecture: Nanowire MOSFETs Byung-Gook Park
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2007. 1.Special Issues on Nanodevices2 Nanowire MOSFETs MOSFET Scaling and Issues Evolution of MOSFET Device Structure Double Gate Structures Multiple Gate Structures Ballistic Electron Transport in Nanowires Effect of 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
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2007. 1.Special Issues on Nanodevices3 MOSFET Scaling - The Grand View D.R. = 20e - 0.116(Y-1960) ~ 3 yrs : 2 -1/2 reduction ~ 20 yrs : 10 -1 reduction ITRS Roadmap (’05) 2016 : 9 nm (MPU L phy ) 2019 : 6 nm (MPU L phy )
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2007. 1.Special Issues on Nanodevices4 Short Channel Effects (1) Phenomenon : roll-off of V T as a function of gate length L Cause : charge sharing
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2007. 1.Special Issues on Nanodevices5 Short Channel Effects (2)
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2007. 1.Special Issues on Nanodevices6 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. Example : L = W = 50 nm, N a = 10 18 cm -3 W dm = 35 nm N = N a LWW dm = 87.5 s N = N 1/2 = 9.35 (~ 10.7%) Threshold voltage variation due to dopant number fluctuation : Dopant Number Fluctuation and V T
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2007. 1.Special Issues on Nanodevices7 Evolution of Device Structure (1) Tightness of gate control over the channel SiO 2 Double gate SOI Bulk
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2007. 1.Special Issues on Nanodevices8 Evolution of Device Structure (2) Tightness of gate control over the channel
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2007. 1.Special Issues on Nanodevices9 Short Channel Effects Design guideline SG: t si L channel /3 DG: t si 2L channel /3 NW: t si L channel
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2007. 1.Special Issues on Nanodevices10 Various Double Gate Structures L: horizontal W: horizontal L: vertical W: horizontal L: horizontal W: vertical Type IType IIType III
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2007. 1.Special Issues on Nanodevices11 FinFETs G S D L G t SOI W fin G S D L G SOI W fin SD G G G G SD FinFET Schematic FinFET Issue H fin =t SOI W fin <L G
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2007. 1.Special Issues on Nanodevices12 Electric Field and Charge Distribution Electric Field Charge V G >V T V G <V T
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2007. 1.Special Issues on Nanodevices13 Basic Equations for DGMOSFETs Due to symmetry Voltage, electric field, and channel charge
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2007. 1.Special Issues on Nanodevices14 Threshold Voltage and Drain Current Threshold voltage Drain current * Usually, Q b 0 to suppress the dopant # fluctuation effect negative threshold voltage for n-channel work function engineering required * Two devices in one!
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2007. 1.Special Issues on Nanodevices15 Inversion Charge in the Channel Charge distribution : - assumption: Charge distribution is dominated by the ground state. Surface inversion – two channels are separated Bulk inversion – two channels are merged
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2007. 1.Special Issues on Nanodevices16 V T vs. Channel Thickness VTVT T ch Threshold voltage for thicker channel : Threshold voltage for thin channel : - dominated by the energy level quantization - higher for thinner body
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2007. 1.Special Issues on Nanodevices17 MG MOSFETs and Corner Effects Gate Oxide Channel Gate Oxide Channel Quadruple gate MOSFET - The gate surrounds the channel. Corner effect : - Field concentration at corners.
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2007. 1.Special Issues on Nanodevices18 Coaxial Gate MOSFETs Gate Oxide Channel Ideal shape for NW MOSFET - No corner effect - 2D analysis with cylindrical coordinates
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2007. 1.Special Issues on Nanodevices19 Carbon Nanotube FETs CNT FETs -Schottky contact at S/D junction -High dielectric for gate insulator
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2007. 1.Special Issues on Nanodevices20 Ballistic Transport in Nanowire (1) Contact 1Contact 2 W L Ballistic Conductor x y Large conductor (L >> mean free path): G = W/L (Ohmic scaling) G for L 0? Ballistic conductor (L << mean free path): G G c for L 0 G c “contact” resistance
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2007. 1.Special Issues on Nanodevices21 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 Quasi-Fermi levels : Dispersion relation : N : transverse mode number N : cut-off energy for mode N k E N = 3 2 1 11 22 33 eV 1 eV 2 Ballistic Transport in Nanowire (2)
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2007. 1.Special Issues on Nanodevices22 Number of transverse modes : Current by a single transverse mode : +k states are occupied according to the function f(E E f + ) Ballistic Transport in Nanowire (3)
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2007. 1.Special Issues on Nanodevices23 Current by multiple transverse modes : Total current : At low temperature : Ballistic Transport in Nanowire (4)
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2007. 1.Special Issues on Nanodevices24 Landauer ’ s Formula (1) Contact 1Contact 2 Conductor x y T Lead 1 Lead 2
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2007. 1.Special Issues on Nanodevices25 Landauer ’ s Formula (2) Contact 1Contact 2 Conductor x y T Lead 1 Lead 2
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