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Penn ESE370 Fall2014 -- DeHon 1 ESE370: Circuit-Level Modeling, Design, and Optimization for Digital Systems Day 12: September 24, 2014 MOS Transistor.

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Presentation on theme: "Penn ESE370 Fall2014 -- DeHon 1 ESE370: Circuit-Level Modeling, Design, and Optimization for Digital Systems Day 12: September 24, 2014 MOS Transistor."— Presentation transcript:

1 Penn ESE370 Fall2014 -- DeHon 1 ESE370: Circuit-Level Modeling, Design, and Optimization for Digital Systems Day 12: September 24, 2014 MOS Transistor Details Capacitance

2 Last Time Focused on I vs V relationships –Effective resistance –Steady state Penn ESE370 Fall2014 -- DeHon 2

3 Today Capacitance –Model refinement Region Example? Penn ESE370 Fall2014 -- DeHon 3

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 Fall2014 -- DeHon 4

5 Capacitance Setup Penn ESE370 Fall2014 -- DeHon 5

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 Fall2014 -- DeHon 6

7 Capacitance Four Terminals How many combinations? –4 things taken 2 at a time? Penn ESE370 Fall2014 -- DeHon 7

8 Capacitances GS, GB, GD, SB, DB, SD Penn ESE370 Fall2014 -- DeHon 8

9 Moving Plates? What is distance from gate to conductor? –Depletion? –Strong Inversion? Penn ESE370 Fall2014 -- DeHon 9

10 Capacitance Decomposition Penn ESE370 Fall2014 -- DeHon 10

11 Overlap What is the capacitive implication of gate/src and gate/drain overlap? Penn ESE370 Fall2014 -- DeHon 11

12 Overlap Length of overlap? Penn ESE370 Fall2014 -- DeHon 12

13 Overlap Capacitance Penn ESE370 Fall2014 -- DeHon 13

14 Overlap Capacitance Penn ESE370 Fall2014 -- DeHon 14

15 Capacitance in Strong Inversion (easy case) Looks like parallel plate Gate – Channel –What is C GC ? –What is C GB ? Penn ESE370 Fall2014 -- DeHon 15

16 Capacitance in Strong Inversion Looks like parallel plate Gate – Channel –C GC –C GB =0 Penn ESE370 Fall2014 -- DeHon 16

17 Capacitance in Strong Inversion But channel isn’t a terminal –Split evenly with source and drain Penn ESE370 Fall2014 -- DeHon 17

18 Capacitance in Strong Inversion Add in Overlap capacitance Penn ESE370 Fall2014 -- DeHon 18

19 Channel Evolution Subthreshold Penn ESE370 Fall2014 -- DeHon 19

20 Capacitance Depletion What happens to capacitance here? –Capacitor plate distance? Penn ESE370 Fall2014 -- DeHon 20

21 Capacitance Depletion Capacitance becomes Gate-Body Capacitance drops as Vgs increases toward Vth Penn ESE370 Fall2014 -- DeHon 21

22 Capacitance vs V GS Penn ESE370 Fall2014 -- DeHon 22 C GC C GCS = C GCD C GCB WLC OX 0.5WLC OX

23 Saturation Capacitance? Penn ESE370 Fall2014 -- DeHon 23

24 Saturation Capacitance? Penn ESE370 Fall2014 -- DeHon 24 Source end of channel in inversion Destination end of channel at or below threshold Capacitance shifts to source –Total capacitance reduced

25 Saturation Capacitance Penn ESE370 Fall2014 -- DeHon 25 C GC C GCS C GCD V DS /(V GS -V T ) (2/3)WLC OX WLC OX 0.5WLC OX 01

26 Contact Capacitance Penn ESE370 Fall2014 -- DeHon 26

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 Fall2014 -- DeHon 27

28 Contact/Diffusion Capacitance C j – diffusion depletion C jsw – sidewall capacitance L S – length of diffusion Penn ESE370 Fall2014 -- DeHon 28 LSLS

29 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 Fall2014 -- DeHon 29

30 One Implication Penn ESE370 Fall2014 -- DeHon 30

31 Step Response? Penn ESE370 Fall2014 -- DeHon 31

32 Step Response Penn ESE370 Fall2014 -- DeHon 32

33 Impact of C GD What does C GD do to the switching response here? –V2 –Vout Penn ESE370 Fall2014 -- DeHon 33

34 Impact of C GD Penn ESE370 Fall2014 -- DeHon 34

35 Idea Capacitance –To every terminal –Voltage dependent Penn ESE370 Fall2014 -- DeHon 35 C GC C GCS C GCB

36 Approach Identify Region Drives governing equations Use to understand operation Penn ESE370 Fall2014 -- DeHon 36

37 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 Fall2014 -- DeHon 37

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