Transcript ppt - SEAS
ESE370: Circuit-Level Modeling, Design, and Optimization for Digital Systems Day 10: September 29, 2010 MOS Transistors Details 1 Penn ESE370 Fall2010 -- DeHon Last Time • Focused on I vs V relationships – Effective resistance – Drive 2 Penn ESE370 Fall2010 -- DeHon Today • Capacitance – Gate – Source/Drain Contact • More threshold dependence – VDS 3 Penn ESE370 Fall2010 -- DeHon Theme • Refining model – Exploring next level of complexity 4 Penn ESE370 Fall2010 -- DeHon Capacitance • First order: looks like a capacitor gate drain src channel • Today: – Like resistance, it is not constant – Capacitance not just to src (drain) Penn ESE370 Fall2010 -- DeHon 5 Threshold • Threshold decreases with VDS VT VDS 6 Penn ESE370 Fall2010 -- DeHon Capacitance Setup 7 Penn ESE370 Fall2010 -- DeHon Capacitance • Argued looked like a capacitor to the channel • …but the channel isn’t really one of our terminals – Don’t connect directly to it. 8 Penn ESE370 Fall2010 -- DeHon Capacitance • Four Terminals • How many combinations – 4 things taken 2 at a time 9 Penn ESE370 Fall2010 -- DeHon Capacitances • GS, GB, GD, SB, DB, SD 10 Penn ESE370 Fall2010 -- DeHon Moving Plates? • What is distance from gate to conductor? – Depletion? – Strong Inversion? 11 Penn ESE370 Fall2010 -- DeHon Capacitance Decomposition 12 Penn ESE370 Fall2010 -- DeHon Overlap • What is the capacitive implication of gate/src and gate/drain overlap? 13 Penn ESE370 Fall2010 -- DeHon Overlap • Length of overlap? 14 Penn ESE370 Fall2010 -- DeHon Overlap Capacitance A C r 0 d Co ox W L drawn Leffective /2 t ox 15 Penn ESE370 Fall2010 -- DeHon Overlap Capacitance A C r 0 d W L C drawn o ox COX tOX Leffective /2 t ox Co CoxW L drawn Leffective /2 Penn ESE370 Fall2010 -- DeHon OX 16 Capacitance in Strong Inversion (easy case) • Looks like parallel plate Gate – Channel – What is CGC? – What is CGB? 17 Penn ESE370 Fall2010 -- DeHon Capacitance in Strong Inversion • Looks like parallel plate Gate – Channel – What is CGC? – CGB=0 CGC CoxWLeffective 18 Penn ESE370 Fall2010 -- DeHon Capacitance in Strong Inversion • But channel isn’t a terminal – Split evenly with source and drain CGC CoxWLeffective CGCS CGCD 0.5CoxWLeffective 19 Penn ESE370 Fall2010 -- DeHon Capacitance in Strong Inversion • Add in Overlap capacitance CGCS CGCD 0.5CoxWLeffective Co CoxW L drawn Leffective /2 CGS CGSC CO 0.5CoxWLdrawn Penn ESE370 Fall2010 -- DeHon 20 Capacitance Subthreshold • Need to refine model – What showed on Day 9 not quite right • Channel doesn’t start depleted – Starts with substrate doping 21 Penn ESE370 Fall2010 -- DeHon Channel Evolution Subthreshold 22 Penn ESE370 Fall2010 -- DeHon Capacitance Depletion • What happens to capacitance here? – Capacitor plate distance? 23 Penn ESE370 Fall2010 -- DeHon Capacitance Depletion • Capacitance becomes Gate-Body • Capacitance drops 24 Penn ESE370 Fall2010 -- DeHon Capacitance vs VGS • G CGC CGCS=CGCD CGCB 25 Penn ESE370 Fall2010 -- DeHon Saturation Capacitance? 26 Penn ESE370 Fall2010 -- DeHon Saturation Capacitance? • Source end of channel in inversion • Destination end of channel close at threshold • Capacitance shifts to source – Total capacitance reduced 27 Penn ESE370 Fall2010 -- DeHon Saturation Capacitance CGC CGCS CGCD VDS/(VGS-VT) 28 Penn ESE370 Fall2010 -- DeHon Contact Capacitance 29 Penn ESE370 Fall2010 -- DeHon Contact Capacitance • n+ contacts are formed by doping = diffusion • Depletion under contact – Contact-Body capacitance • Depletion around perimeter of contact – Also contact-Body capacitance 30 Penn ESE370 Fall2010 -- DeHon Contact/Diffusion Capacitance • Cj – diffusion depletion • Cjsw – sidewall capacitance • LS – length of diffusion LS Cdiff C j LSW C jsw 2LS W 31 Penn ESE370 Fall2010 -- DeHon Capacitance Roundup • • • • • CGS=CGCS+CO CGD=CGCD+CO CGB=CGCB CSB=Cdiff CDB=Cdiff 32 Penn ESE370 Fall2010 -- DeHon One Implication 33 Penn ESE370 Fall2010 -- DeHon Step Response? Rsmall Rlarge 34 Penn ESE370 Fall2010 -- DeHon Step Response 35 Penn ESE370 Fall2010 -- DeHon Impact of CGD • What does CGD do to the switching response here? 36 Penn ESE370 Fall2010 -- DeHon Impact of CGD 37 Penn ESE370 Fall2010 -- DeHon Threshold 38 Penn ESE370 Fall2010 -- DeHon Threshold • Describe VT as a constant • Induce enough electron collection to invert channel 39 Penn ESE370 Fall2010 -- DeHon VDS impact • In practice, VDS impacts state of channel 40 Penn ESE370 Fall2010 -- DeHon VDS impact • Increasing VDS, already depletes portions of channel 41 Penn ESE370 Fall2010 -- DeHon VDS impact • Increasing VDS, already depletes portions of channel • Need less charge, less voltage to invert 42 Penn ESE370 Fall2010 -- DeHon Drain-Induced Barrier Lowering (DIBL) VT VDS 43 Penn ESE370 Fall2010 -- DeHon DIBL Impact 44 Penn ESE370 Fall2010 -- DeHon In a Gate? • What does it impact most? – Which device, which state/operation? 45 Penn ESE370 Fall2010 -- DeHon In a Gate • VDS largest for off device – Easier to turn on IDS 2 W VDS nCOX VGS VT VDS L 2 46 Penn ESE370 Fall2010 -- DeHon In a Gate • VDS largest for off device – Easier to turn on – Leak more IDS IDS 2 W VDS nCOX VGS VT VDS L 2 W IS e L VGS nkT / q VDS kT / q 1 e 1 VDS 47 Penn ESE370 Fall2010 -- DeHon In a Gate • VDS largest for off device – Easier to turn on – Leak more IDS IDS W IS e L VGS nkT / q W IS e L VDS kT / q 1 V 1 e DS VGS VT nkT / q VDS kT / q 1 e 1 VDS 48 Penn ESE370 Fall2010 -- DeHon Admin • HW3 due Friday 49 Penn ESE370 Fall2010 -- DeHon Ideas • Capacitance – To every terminal – Voltage dependent CGC • Threshold CGCS – Voltage dependent CGCB • Generally do manual analysis without VT Penn ESE370 Fall2010 -- DeHon VDS 50