Penn ESE370 Fall DeHon 1 ESE370: Circuit-Level Modeling, Design, and Optimization for Digital Systems Day 12: September 24, 2014 MOS Transistor Details Capacitance

Last Time Focused on I vs V relationships –Effective resistance –Steady state Penn ESE370 Fall DeHon 2

Today Capacitance –Model refinement Region Example? Penn ESE370 Fall DeHon 3

channel gate srcdrain Capacitance First order: looks like a capacitor Today: –Like resistance, it is not constant –Capacitance not just to gnd (drain) Penn ESE370 Fall DeHon 4

Capacitance Setup Penn ESE370 Fall DeHon 5

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… Penn ESE370 Fall DeHon 6

Capacitance Four Terminals How many combinations? –4 things taken 2 at a time? Penn ESE370 Fall DeHon 7

Capacitances GS, GB, GD, SB, DB, SD Penn ESE370 Fall DeHon 8

Moving Plates? What is distance from gate to conductor? –Depletion? –Strong Inversion? Penn ESE370 Fall DeHon 9

Capacitance Decomposition Penn ESE370 Fall DeHon 10

Overlap What is the capacitive implication of gate/src and gate/drain overlap? Penn ESE370 Fall DeHon 11

Overlap Length of overlap? Penn ESE370 Fall DeHon 12

Overlap Capacitance Penn ESE370 Fall DeHon 13

Overlap Capacitance Penn ESE370 Fall DeHon 14

Capacitance in Strong Inversion (easy case) Looks like parallel plate Gate – Channel –What is C GC ? –What is C GB ? Penn ESE370 Fall DeHon 15

Capacitance in Strong Inversion Looks like parallel plate Gate – Channel –C GC –C GB =0 Penn ESE370 Fall DeHon 16

Capacitance in Strong Inversion But channel isn’t a terminal –Split evenly with source and drain Penn ESE370 Fall DeHon 17

Capacitance in Strong Inversion Add in Overlap capacitance Penn ESE370 Fall DeHon 18

Channel Evolution Subthreshold Penn ESE370 Fall DeHon 19

Capacitance Depletion What happens to capacitance here? –Capacitor plate distance? Penn ESE370 Fall DeHon 20

Capacitance Depletion Capacitance becomes Gate-Body Capacitance drops as Vgs increases toward Vth Penn ESE370 Fall DeHon 21

Capacitance vs V GS Penn ESE370 Fall DeHon 22 C GC C GCS = C GCD C GCB WLC OX 0.5WLC OX

Saturation Capacitance? Penn ESE370 Fall DeHon 23

Saturation Capacitance? Penn ESE370 Fall DeHon 24 Source end of channel in inversion Destination end of channel at or below threshold Capacitance shifts to source –Total capacitance reduced

Saturation Capacitance Penn ESE370 Fall DeHon 25 C GC C GCS C GCD V DS /(V GS -V T ) (2/3)WLC OX WLC OX 0.5WLC OX 01

Contact Capacitance Penn ESE370 Fall DeHon 26

Contact Capacitance n + contacts are formed by doping = diffusion Depletion under contact –Contact-Body capacitance Depletion around perimeter of contact –Also contact-Body capacitance Penn ESE370 Fall DeHon 27

Contact/Diffusion Capacitance C j – diffusion depletion C jsw – sidewall capacitance L S – length of diffusion Penn ESE370 Fall DeHon 28 LSLS

Capacitance Roundup C GS =C GCS +C O C GD =C GCD +C O C GB =C GCB C SB =C diff C DB =C diff Penn ESE370 Fall DeHon 29

One Implication Penn ESE370 Fall DeHon 30

Step Response? Penn ESE370 Fall DeHon 31

Step Response Penn ESE370 Fall DeHon 32

Impact of C GD What does C GD do to the switching response here? –V2 –Vout Penn ESE370 Fall DeHon 33

Impact of C GD Penn ESE370 Fall DeHon 34

Idea Capacitance –To every terminal –Voltage dependent Penn ESE370 Fall DeHon 35 C GC C GCS C GCB

Approach Identify Region Drives governing equations Use to understand operation Penn ESE370 Fall DeHon 36

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) Penn ESE370 Fall DeHon 37