Penn ESE370 Fall DeHon 1 ESE370: Circuit-Level Modeling, Design, and Optimization for Digital Systems Day 10: September 29, 2010 MOS Transistors Details

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

Today Capacitance –Gate –Source/Drain Contact More threshold dependence –V DS 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 src (drain) Penn ESE370 Fall DeHon 5

Threshold Threshold decreases with V DS Penn ESE370 Fall DeHon 6 VTVT V DS

Capacitance Setup Penn ESE370 Fall DeHon 7

Capacitance Argued looked like a capacitor to the channel …but the channel isnt really one of our terminals –Dont connect directly to it. Penn ESE370 Fall DeHon 8

Capacitance Four Terminals How many combinations –4 things taken 2 at a time Penn ESE370 Fall DeHon 9

Capacitances GS, GB, GD, SB, DB, SD Penn ESE370 Fall DeHon 10

Moving Plates? What is distance from gate to conductor? –Depletion? –Strong Inversion? Penn ESE370 Fall DeHon 11

Capacitance Decomposition Penn ESE370 Fall DeHon 12

Overlap What is the capacitive implication of gate/src and gate/drain overlap? Penn ESE370 Fall DeHon 13

Overlap Length of overlap? Penn ESE370 Fall DeHon 14

Overlap Capacitance Penn ESE370 Fall DeHon 15

Overlap Capacitance Penn ESE370 Fall DeHon 16

Capacitance in Strong Inversion (easy case) Looks like parallel plate Gate – Channel –What is C GC ? –What is C GB ? Penn ESE370 Fall DeHon 17

Capacitance in Strong Inversion Looks like parallel plate Gate – Channel –What is C GC ? –C GB =0 Penn ESE370 Fall DeHon 18

Capacitance in Strong Inversion But channel isnt a terminal –Split evenly with source and drain Penn ESE370 Fall DeHon 19

Capacitance in Strong Inversion Add in Overlap capacitance Penn ESE370 Fall DeHon 20

Capacitance Subthreshold Need to refine model –What showed on Day 9 not quite right Channel doesnt start depleted –Starts with substrate doping Penn ESE370 Fall DeHon 21

Channel Evolution Subthreshold Penn ESE370 Fall DeHon 22

Capacitance Depletion What happens to capacitance here? –Capacitor plate distance? Penn ESE370 Fall DeHon 23

Capacitance Depletion Capacitance becomes Gate-Body Capacitance drops Penn ESE370 Fall DeHon 24

Capacitance vs V GS Penn ESE370 Fall DeHon 25 G C GC C GCS = C GCD C GCB

Saturation Capacitance? Penn ESE370 Fall DeHon 26

Saturation Capacitance? Penn ESE370 Fall DeHon 27 Source end of channel in inversion Destination end of channel close at threshold Capacitance shifts to source –Total capacitance reduced

Saturation Capacitance Penn ESE370 Fall DeHon 28 C GC C GCS C GCD V DS /(V GS -V T )

Contact Capacitance Penn ESE370 Fall DeHon 29

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 30

Contact/Diffusion Capacitance C j – diffusion depletion C jsw – sidewall capacitance L S – length of diffusion Penn ESE370 Fall DeHon 31 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 32

One Implication Penn ESE370 Fall DeHon 33

Step Response? Penn ESE370 Fall DeHon 34 R small R large

Step Response Penn ESE370 Fall DeHon 35

Impact of C GD What does CGD do to the switching response here? Penn ESE370 Fall DeHon 36

Impact of C GD Penn ESE370 Fall DeHon 37

Threshold Penn ESE370 Fall DeHon 38

Threshold Describe V T as a constant Induce enough electron collection to invert channel Penn ESE370 Fall DeHon 39

V DS impact In practice, V DS impacts state of channel Penn ESE370 Fall DeHon 40

V DS impact Increasing V DS, already depletes portions of channel Penn ESE370 Fall DeHon 41

V DS impact Increasing V DS, already depletes portions of channel Need less charge, less voltage to invert Penn ESE370 Fall DeHon 42

Drain-Induced Barrier Lowering (DIBL) Penn ESE370 Fall DeHon 43 VTVT V DS

DIBL Impact Penn ESE370 Fall DeHon 44

In a Gate? What does it impact most? –Which device, which state/operation? Penn ESE370 Fall DeHon 45

In a Gate V DS largest for off device –Easier to turn on Penn ESE370 Fall DeHon 46

In a Gate V DS largest for off device –Easier to turn on –Leak more Penn ESE370 Fall DeHon 47

In a Gate V DS largest for off device –Easier to turn on –Leak more Penn ESE370 Fall DeHon 48

Admin HW3 due Friday Penn ESE370 Fall DeHon 49

Ideas Capacitance –To every terminal –Voltage dependent Threshold –Voltage dependent Generally do manual analysis without Penn ESE370 Fall DeHon 50 VTVT V DS C GC C GCS C GCB