Transcript ppt - SEAS
ESE370: Circuit-Level Modeling, Design, and Optimization for Digital Systems Day 11: September 22, 2014 MOS Transistor Operating Regions Part 2 1 Penn ESE370 Fall2014 -- DeHon Today • Operating Regions (continued) – Resistive – Saturation – Velocity Saturation – Subthreshold • Drain Induced Barrier Lowering 2 Penn ESE370 Fall2014 -- DeHon Last Time… 3 Penn ESE370 Fall2014 -- DeHon Channel Evolution Increasing Vgs 4 Penn ESE370 Fall2014 -- DeHon Threshold • Voltage where strong inversion occurs N A threshold voltage F T ln – Around 2ϕF ni – Engineer by controlling doping (NA) 5 Penn ESE370 Fall2014 -- DeHon Resistive Region COX • VGS>VT, VDS small OX tOX IDS 2 W VDS nCOX VGS VT VDS L 2 6 Penn ESE370 Fall2014 -- DeHon Channel Field • When voltage gap VG-Vxdrops below VT, drops out of inversion – Occurs when: VGS-VDS< VT – Channel is “pinched off” – Current will flow, but cannot increase any further 7 Penn ESE370 Fall2014 -- DeHon Saturation • VDS> VGS-VT IDS 2 VGS VT W 2 n COX VGS VT L 2 IDS n COX W 2 VGS VT L 2 8 Penn ESE370 Fall2014 -- DeHon Saturation Region Blue curve marks transition from Linear to Saturation 9 Penn ESE370 Fall2014 -- DeHon New Stuff 10 Penn ESE370 Fall2014 -- DeHon Preclass 1 • What is electrical field in channel? – Leff=25nm, VDS=1V – Field = VDS/Leff • Velocity: v=F*μ – Electron mobility: μn = 500 cm2/(Vs) • What is electron velocity? 11 Penn ESE370 Fall2014 -- DeHon Same Phenomena • I = (1/R) × V • I increases linearly in V • What’s I? DQ/Dt – Speed at which charge moves • F=V/L • v=×F=(/L) ×V • Velocity increases linearly in V • What’s a moving electron? 12 Penn ESE370 Fall2014 -- DeHon Short Channel S • Model assumes carrier velocity increases with field – Increases with voltage • Are there limits to how fast things (including electrons) can move? 13 Penn ESE370 Fall2014 -- DeHon D Short Channel S • Model assumes carrier velocity increases with field – Increases with voltage • There is a limit to how fast carriers can move – Limited by scattering to 105m/s < speed-of-light • How relate to preclass 1b velocity? • Encounter when channel short – Modern processes, L is short enough Penn ESE370 Fall2014 -- DeHon 14 D Velocity Saturation • At what voltage do we hit the speed limit? (preclass 1c) • This is Vdsat – The voltage at which velocity (current) saturates 15 Penn ESE370 Fall2014 -- DeHon Velocity Saturation • Once velocity saturates: IDS VDSAT satCOX W VGS VT 2 VDSAT L sat n 16 Penn ESE370 Fall2014 -- DeHon S Velocity Saturation • Once velocity saturates: IDS VDSAT L sat n VDSAT satCOX W VGS VT 2 • Can still increase current with parallelism – W – make it wider – Vgs-Vt – make it deeper 17 Penn ESE370 Fall2014 -- DeHon D Velocity Saturation • How does this Vdsat compare to the threshold we have been using? 18 Penn ESE370 Fall2014 -- DeHon Velocity Saturation 19 Penn ESE370 Fall2014 -- DeHon Subthreshold 20 Penn ESE370 Fall2014 -- DeHon Below Threshold • Transition from insulating to conducting is non-linear, but not abrupt • Current does flow – But exponentially dependent on VGS 21 Penn ESE370 Fall2014 -- DeHon Subthreshold IDS W IS e L VGS nkT / q VDS kT / q 1 e 1 VDS 22 Penn ESE370 Fall2014 -- DeHon Subthreshold S • W/L dependence follow from resistor behavior (parallel, series) – Not shown explicitly in text • λ is a channel width modulation effect IDS W IS e L VGS nkT / q VDS kT / q 1 e 1 VDS 23 Penn ESE370 Fall2014 -- DeHon D Steady State • What current flows in steady state? • What causes (and determines) the magnitude of current flow? • Which device? 24 Penn ESE370 Fall2014 -- DeHon Leakage • Call this steady-state current flow leakage • Idsleak 25 Penn ESE370 Fall2014 -- DeHon Subthreshold Slope • Exponent in VGS determines how steep the turnoff is kT – Every S Volts S n ln 10 – Divide IDS by 10 q IDS W IS e L VGS nkT / q VDS kT / q 1 V 1 e DS 26 Penn ESE370 Fall2014 -- DeHon Subthreshold Slope • Exponent in VGS determines how steep the turnoff is – Every S Volts (S not related to source) kT – Divide IDS by 10 S n ln 10 q • n – depends on electrostatics – n=1 S=60mV at Room Temp. (ideal) – n=1.5 S=90mV – Single gate structure showing S=90-110mV 27 Penn ESE370 Fall2014 -- DeHon IDS vs. VGS 28 Penn ESE370 Fall2014 -- DeHon Subthreshold Slope • If S=100mV and Vth=300mV, what is Ids(Vgs=300mV)/Ids(Vgs=0V) ? • What if S=60mV? IDS W IS e L kT S n ln 10 q VGS nkT / q VDS kT / q 1 V 1 e DS 29 Penn ESE370 Fall2014 -- DeHon Threshold 30 Penn ESE370 Fall2014 -- DeHon Threshold • Describe VT as a constant • Induce enough electron collection to invert channel 31 Penn ESE370 Fall2014 -- DeHon VDS impact • In practice, VDS impacts state of channel 32 Penn ESE370 Fall2014 -- DeHon VDS impact • Increasing VDS, already depletes portions of channel 33 Penn ESE370 Fall2014 -- DeHon VDS impact • Increasing VDS, already depletes portions of channel • Need less charge, less voltage to invert 34 Penn ESE370 Fall2014 -- DeHon Drain-Induced Barrier Lowering (DIBL) VT VDS 35 Penn ESE370 Fall2014 -- DeHon DIBL Impact 36 Penn ESE370 Fall2014 -- DeHon In a Gate? • What does it impact most? – Which device, has large Vds? • Vin=Vdd ? • Vin=Gnd ? – How effect operation? • Speed of switching? • Leakage? 37 Penn ESE370 Fall2014 -- DeHon In a Gate • VDS largest for off device – Easier to turn on IDS VDSAT satCOX W VGS VT 2 38 Penn ESE370 Fall2014 -- DeHon In a Gate • VDS largest for off device – Easier to turn on – Leak more IDS W IS e L VGS nkT / q VDS kT / q 1 e 1 VDS 39 Penn ESE370 Fall2014 -- DeHon PMOS • Similar phenomena to NMOS • Signs different – Negative Vth – turn on when less than this • Reason based on carriers 40 Penn ESE370 Fall2014 -- DeHon Approach • Identify Region • Drives governing equations • Use to understand operation 41 Penn ESE370 Fall2014 -- DeHon Big Idea • 3 Regions of operation for MOSFET – Subthreshold – Resistive – Saturation • Pinch Off • Velocity Saturation – Short channel 42 Penn ESE370 Fall2014 -- DeHon Admin • Text 3.3.2 – highly recommend read – Finish it up on Wednesday • Office Hours: M, T, W • HW4 due Thursday • Midterm 1 next Monday – Midterms from 2010, 2011, 2012, 2013 • All online … both with and without answers • Suggest start without answers 43 Penn ESE370 Fall2014 -- DeHon