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Penn ESE370 Fall2013 -- DeHon 1 ESE370: Circuit-Level Modeling, Design, and Optimization for Digital Systems Day 12: September 25, 2013 MOS Transistors.

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Presentation on theme: "Penn ESE370 Fall2013 -- DeHon 1 ESE370: Circuit-Level Modeling, Design, and Optimization for Digital Systems Day 12: September 25, 2013 MOS Transistors."— Presentation transcript:

1 Penn ESE370 Fall2013 -- DeHon 1 ESE370: Circuit-Level Modeling, Design, and Optimization for Digital Systems Day 12: September 25, 2013 MOS Transistors Details

2 Last Time Focused on I vs V relationships –Effective resistance –Steady state Penn ESE370 Fall2013 -- DeHon 2

3 Today Capacitance Penn ESE370 Fall2013 -- DeHon 3

4 Theme Refining model –Exploring next level of complexity Penn ESE370 Fall2013 -- DeHon 4

5 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 Fall2013 -- DeHon 5

6 Capacitance Setup Penn ESE370 Fall2013 -- DeHon 6

7 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 Fall2013 -- DeHon 7

8 Capacitance Four Terminals How many combinations? –4 things taken 2 at a time? Penn ESE370 Fall2013 -- DeHon 8

9 Capacitances GS, GB, GD, SB, DB, SD Penn ESE370 Fall2013 -- DeHon 9

10 Moving Plates? What is distance from gate to conductor? –Depletion? –Strong Inversion? Penn ESE370 Fall2013 -- DeHon 10

11 Capacitance Decomposition Penn ESE370 Fall2013 -- DeHon 11

12 Overlap What is the capacitive implication of gate/src and gate/drain overlap? Penn ESE370 Fall2013 -- DeHon 12

13 Overlap Length of overlap? Penn ESE370 Fall2013 -- DeHon 13

14 Overlap Capacitance Penn ESE370 Fall2013 -- DeHon 14

15 Overlap Capacitance Penn ESE370 Fall2013 -- DeHon 15

16 Capacitance in Strong Inversion (easy case) Looks like parallel plate Gate – Channel –What is C GC ? –What is C GB ? Penn ESE370 Fall2013 -- DeHon 16

17 Capacitance in Strong Inversion Looks like parallel plate Gate – Channel –C GC –C GB =0 Penn ESE370 Fall2013 -- DeHon 17

18 Capacitance in Strong Inversion But channel isn’t a terminal –Split evenly with source and drain Penn ESE370 Fall2013 -- DeHon 18

19 Capacitance in Strong Inversion Add in Overlap capacitance Penn ESE370 Fall2013 -- DeHon 19

20 Channel Evolution Subthreshold Penn ESE370 Fall2013 -- DeHon 20

21 Capacitance Depletion What happens to capacitance here? –Capacitor plate distance? Penn ESE370 Fall2013 -- DeHon 21

22 Capacitance Depletion Capacitance becomes Gate-Body Capacitance drops as Vgs increases toward Vth Penn ESE370 Fall2013 -- DeHon 22

23 Capacitance vs V GS Penn ESE370 Fall2013 -- DeHon 23 G C GC C GCS = C GCD C GCB

24 Saturation Capacitance? Penn ESE370 Fall2013 -- DeHon 24

25 Saturation Capacitance? Penn ESE370 Fall2013 -- DeHon 25 Source end of channel in inversion Destination end of channel at or below threshold Capacitance shifts to source –Total capacitance reduced

26 Saturation Capacitance Penn ESE370 Fall2013 -- DeHon 26 C GC C GCS C GCD V DS /(V GS -V T )

27 Contact Capacitance Penn ESE370 Fall2013 -- DeHon 27

28 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 Fall2013 -- DeHon 28

29 Contact/Diffusion Capacitance C j – diffusion depletion C jsw – sidewall capacitance L S – length of diffusion Penn ESE370 Fall2013 -- DeHon 29 LSLS

30 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 Fall2013 -- DeHon 30

31 One Implication Penn ESE370 Fall2013 -- DeHon 31

32 Step Response? Penn ESE370 Fall2013 -- DeHon 32

33 Step Response Penn ESE370 Fall2013 -- DeHon 33

34 Impact of C GD What does C GD do to the switching response here? –V2 –Vout Penn ESE370 Fall2013 -- DeHon 34

35 Impact of C GD Penn ESE370 Fall2013 -- DeHon 35

36 Idea Capacitance –To every terminal –Voltage dependent Penn ESE370 Fall2013 -- DeHon 36 C GC C GCS C GCB

37 Region Example Penn ESE370 Fall2013 -- DeHon 37

38 Switching Operation Consider Inverter Start with in=0V Output voltage? NFET region of operation? How off is it? Penn ESE370 Fall2013 -- DeHon 38

39 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 Fall2013 -- DeHon 39

40 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 Fall2013 -- DeHon 40

41 Penn ESE370 Fall2013 -- DeHon 41 Retrace Transition

42 Approach Identify Region Drives governing equations Use to understand operation Penn ESE370 Fall2013 -- DeHon 42

43 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 Fall2013 -- DeHon 43

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