Transcript ppt
ESE370: Circuit-Level Modeling, Design, and Optimization for Digital Systems Day 12: September 24, 2014 MOS Transistor Details Capacitance 1 Penn ESE370 Fall2014 -- DeHon Last Time • Focused on I vs V relationships – Effective resistance – Steady state 2 Penn ESE370 Fall2014 -- DeHon Today • Capacitance – Model refinement • Region Example? 3 Penn ESE370 Fall2014 -- DeHon Capacitance • First order: looks like a capacitor gate drain src channel • Today: – Like resistance, it is not constant – Capacitance not just to gnd (drain) Penn ESE370 Fall2014 -- DeHon 4 Capacitance Setup 5 Penn ESE370 Fall2014 -- DeHon Capacitance • Claimed looked like a capacitor to ground • …but ground isn’t really one of our terminals – Don’t connect directly to it. – …source and body are often at ground… 6 Penn ESE370 Fall2014 -- DeHon Capacitance • Four Terminals • How many combinations? – 4 things taken 2 at a time? 7 Penn ESE370 Fall2014 -- DeHon Capacitances • GS, GB, GD, SB, DB, SD 8 Penn ESE370 Fall2014 -- DeHon Moving Plates? • What is distance from gate to conductor? – Depletion? – Strong Inversion? 9 Penn ESE370 Fall2014 -- DeHon Capacitance Decomposition 10 Penn ESE370 Fall2014 -- DeHon Overlap • What is the capacitive implication of gate/src and gate/drain overlap? 11 Penn ESE370 Fall2014 -- DeHon Overlap • Length of overlap? 12 Penn ESE370 Fall2014 -- DeHon Overlap Capacitance A C r 0 d Co ox W L drawn Leffective /2 t ox 13 Penn ESE370 Fall2014 -- 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 Fall2014 -- DeHon OX 14 Capacitance in Strong Inversion (easy case) • Looks like parallel plate Gate – Channel – What is CGC? – What is CGB? 15 Penn ESE370 Fall2014 -- DeHon Capacitance in Strong Inversion • Looks like parallel plate Gate – Channel – CGC – CGB=0 CGC CoxWLeffective 16 Penn ESE370 Fall2014 -- DeHon Capacitance in Strong Inversion • But channel isn’t a terminal – Split evenly with source and drain CGC CoxWLeffective CGCS CGCD 0.5CoxWLeffective 17 Penn ESE370 Fall2014 -- 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 Fall2014 -- DeHon 18 Channel Evolution Subthreshold 19 Penn ESE370 Fall2014 -- DeHon Capacitance Depletion • What happens to capacitance here? – Capacitor plate distance? 20 Penn ESE370 Fall2014 -- DeHon Capacitance Depletion • Capacitance becomes Gate-Body • Capacitance drops as Vgs increases toward Vth 21 Penn ESE370 Fall2014 -- DeHon Capacitance vs VGS CGC WLCOX CGCS=CGCD 0.5WLCOX CGCB 22 Penn ESE370 Fall2014 -- DeHon Saturation Capacitance? 23 Penn ESE370 Fall2014 -- DeHon Saturation Capacitance? • Source end of channel in inversion • Destination end of channel at or below threshold • Capacitance shifts to source – Total capacitance reduced 24 Penn ESE370 Fall2014 -- DeHon Saturation Capacitance CGC WLCOX (2/3)WLCOX CGCS CGCD 0.5WLCOX 0 VDS/(VGS-VT) 1 25 Penn ESE370 Fall2014 -- DeHon Contact Capacitance 26 Penn ESE370 Fall2014 -- 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 27 Penn ESE370 Fall2014 -- DeHon Contact/Diffusion Capacitance • Cj – diffusion depletion • Cjsw – sidewall capacitance • LS – length of diffusion LS Cdiff C j LSW C jsw 2LS W 28 Penn ESE370 Fall2014 -- DeHon Capacitance Roundup • • • • • CGS=CGCS+CO CGD=CGCD+CO CGB=CGCB CSB=Cdiff CDB=Cdiff 29 Penn ESE370 Fall2014 -- DeHon One Implication 30 Penn ESE370 Fall2014 -- DeHon Step Response? 31 Penn ESE370 Fall2014 -- DeHon Step Response 32 Penn ESE370 Fall2014 -- DeHon Impact of CGD • What does CGD do to the switching response here? – V2 – Vout 33 Penn ESE370 Fall2014 -- DeHon Impact of CGD 34 Penn ESE370 Fall2014 -- DeHon Idea • Capacitance – To every terminal – Voltage dependent CGC CGCS CGCB 35 Penn ESE370 Fall2014 -- DeHon Approach • Identify Region • Drives governing equations • Use to understand operation 36 Penn ESE370 Fall2014 -- DeHon Admin • • • • HW4 due Thursday Make sure read 3.3 Lecture Friday Midterm 1 on Monday – Ron review on Sunday – No lecture at noon – André office hour – Exam 7-9pm (Towne 303) 37 Penn ESE370 Fall2014 -- DeHon