Penn ESE370 Fall DeHon 1 ESE370: Circuit-Level Modeling, Design, and Optimization for Digital Systems Day 12: September 25, 2013 MOS Transistors Details

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

Today Capacitance Penn ESE370 Fall DeHon 3

Theme Refining model –Exploring next level of complexity Penn ESE370 Fall DeHon 4

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 5

Capacitance Setup Penn ESE370 Fall DeHon 6

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 7

Capacitance Four Terminals How many combinations? –4 things taken 2 at a time? Penn ESE370 Fall DeHon 8

Capacitances GS, GB, GD, SB, DB, SD Penn ESE370 Fall DeHon 9

Moving Plates? What is distance from gate to conductor? –Depletion? –Strong Inversion? Penn ESE370 Fall DeHon 10

Capacitance Decomposition Penn ESE370 Fall DeHon 11

Overlap What is the capacitive implication of gate/src and gate/drain overlap? Penn ESE370 Fall DeHon 12

Overlap Length of overlap? Penn ESE370 Fall DeHon 13

Overlap Capacitance Penn ESE370 Fall DeHon 14

Overlap Capacitance Penn ESE370 Fall DeHon 15

Capacitance in Strong Inversion (easy case) Looks like parallel plate Gate – Channel –What is C GC ? –What is C GB ? Penn ESE370 Fall DeHon 16

Capacitance in Strong Inversion Looks like parallel plate Gate – Channel –C GC –C GB =0 Penn ESE370 Fall DeHon 17

Capacitance in Strong Inversion But channel isn’t a terminal –Split evenly with source and drain Penn ESE370 Fall DeHon 18

Capacitance in Strong Inversion Add in Overlap capacitance Penn ESE370 Fall DeHon 19

Channel Evolution Subthreshold Penn ESE370 Fall DeHon 20

Capacitance Depletion What happens to capacitance here? –Capacitor plate distance? Penn ESE370 Fall DeHon 21

Capacitance Depletion Capacitance becomes Gate-Body Capacitance drops as Vgs increases toward Vth Penn ESE370 Fall DeHon 22

Capacitance vs V GS Penn ESE370 Fall DeHon 23 G C GC C GCS = C GCD C GCB

Saturation Capacitance? Penn ESE370 Fall DeHon 24

Saturation Capacitance? Penn ESE370 Fall DeHon 25 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 26 C GC C GCS C GCD V DS /(V GS -V T )

Contact Capacitance Penn ESE370 Fall DeHon 27

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 28

Contact/Diffusion Capacitance C j – diffusion depletion C jsw – sidewall capacitance L S – length of diffusion Penn ESE370 Fall DeHon 29 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 30

One Implication Penn ESE370 Fall DeHon 31

Step Response? Penn ESE370 Fall DeHon 32

Step Response Penn ESE370 Fall DeHon 33

Impact of C GD What does C GD do to the switching response here? –V2 –Vout Penn ESE370 Fall DeHon 34

Impact of C GD Penn ESE370 Fall DeHon 35

Idea Capacitance –To every terminal –Voltage dependent Penn ESE370 Fall DeHon 36 C GC C GCS C GCB

Region Example Penn ESE370 Fall DeHon 37

Switching Operation Consider Inverter Start with in=0V Output voltage? NFET region of operation? How off is it? Penn ESE370 Fall DeHon 38

Switching Operation Input rises from 0V When NFET cross into new region? What region cross into? Ids Current? What happens to Ids as V continues to rise? What is happening to Vout? Penn ESE370 Fall DeHon 39

Switching Operation Input at Vdd When NFET change operating regions? Which region move into? What’s happening to Vout? What region when settles to static voltage? Penn ESE370 Fall DeHon 40

Penn ESE370 Fall DeHon 41 Retrace Transition

Approach Identify Region Drives governing equations Use to understand operation Penn ESE370 Fall DeHon 42

Admin HW4 out due Thursday Make sure read 3.3 Lecture Friday Midterm 1 on Monday –Spencer review on Sunday –No lecture at noon – Andre office hour –Exam 7-9pm (location?) Penn ESE370 Fall DeHon 43