Transcript ppt - SEAS

ESE370:
Circuit-Level
Modeling, Design, and Optimization
for Digital Systems
Day 10: September 29, 2010
MOS Transistors Details
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Penn ESE370 Fall2010 -- DeHon
Last Time
• Focused on I vs V relationships
– Effective resistance
– Drive
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Today
• Capacitance
– Gate
– Source/Drain Contact
• More threshold dependence
– VDS
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Penn ESE370 Fall2010 -- DeHon
Theme
• Refining model
– Exploring next level of complexity
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Penn ESE370 Fall2010 -- DeHon
Capacitance
• First order: looks like a capacitor
gate
drain
src
channel
• Today:
– Like resistance, it is not constant
– Capacitance not just to src (drain)
Penn ESE370 Fall2010 -- DeHon
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Threshold
• Threshold decreases with VDS
VT
VDS
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Penn ESE370 Fall2010 -- DeHon
Capacitance Setup
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Penn ESE370 Fall2010 -- DeHon
Capacitance
• Argued looked like a capacitor to the
channel
• …but the channel isn’t really one of our
terminals
– Don’t connect directly to it.
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Penn ESE370 Fall2010 -- DeHon
Capacitance
• Four Terminals
• How many combinations
– 4 things taken 2 at a time
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Penn ESE370 Fall2010 -- DeHon
Capacitances
• GS, GB, GD, SB, DB, SD
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Penn ESE370 Fall2010 -- DeHon
Moving Plates?
• What is distance from gate to conductor?
– Depletion?
– Strong Inversion?
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Capacitance Decomposition
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Overlap
• What is the capacitive implication of
gate/src and gate/drain overlap?
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Overlap
• Length of overlap?
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Penn ESE370 Fall2010 -- DeHon
Overlap Capacitance
A
C   r 0
d
Co   ox


W L drawn Leffective /2
t ox
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Penn ESE370 Fall2010 -- DeHon
Overlap Capacitance
A
C   r 0
d
W L
C 
drawn
o
ox


COX 
tOX

Leffective /2
t ox

Co  CoxW L drawn Leffective /2
Penn ESE370 Fall2010 -- DeHon
OX
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Capacitance in Strong Inversion
(easy case)
• Looks like parallel plate Gate – Channel
– What is CGC?
– What is CGB?
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Penn ESE370 Fall2010 -- DeHon
Capacitance in Strong Inversion
• Looks like parallel plate Gate – Channel
– What is CGC?
– CGB=0
CGC  CoxWLeffective

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Penn ESE370 Fall2010 -- DeHon
Capacitance in Strong Inversion
• But channel isn’t a terminal
– Split evenly with source and drain
CGC  CoxWLeffective
CGCS  CGCD  0.5CoxWLeffective

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Penn ESE370 Fall2010 -- DeHon
Capacitance in Strong Inversion
• Add in Overlap capacitance
CGCS  CGCD  0.5CoxWLeffective


Co  CoxW L drawn Leffective /2
CGS  CGSC  CO  0.5CoxWLdrawn

Penn ESE370 Fall2010 -- DeHon
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Capacitance Subthreshold
• Need to refine model
– What showed on Day 9 not quite right
• Channel doesn’t start depleted
– Starts with substrate doping
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Channel Evolution
Subthreshold
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Capacitance Depletion
• What happens to capacitance here?
– Capacitor plate distance?
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Capacitance Depletion
• Capacitance becomes Gate-Body
• Capacitance drops
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Capacitance vs VGS
• G
CGC
CGCS=CGCD
CGCB
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Saturation Capacitance?
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Saturation Capacitance?
• Source end of channel in inversion
• Destination end of channel close at
threshold
• Capacitance shifts to source
– Total capacitance reduced
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Penn ESE370 Fall2010 -- DeHon
Saturation Capacitance
CGC
CGCS
CGCD
VDS/(VGS-VT)
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Contact Capacitance
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Penn ESE370 Fall2010 -- DeHon
Contact Capacitance
• n+ contacts are formed by doping = diffusion
• Depletion under contact
– Contact-Body capacitance
• Depletion around perimeter of contact
– Also contact-Body capacitance
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Penn ESE370 Fall2010 -- DeHon
Contact/Diffusion Capacitance
• Cj – diffusion depletion
• Cjsw – sidewall capacitance
• LS – length of diffusion
LS
Cdiff  C j LSW  C jsw 2LS  W 
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Penn ESE370 Fall2010 -- DeHon
Capacitance Roundup
•
•
•
•
•
CGS=CGCS+CO
CGD=CGCD+CO
CGB=CGCB
CSB=Cdiff
CDB=Cdiff
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One Implication
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Step Response?
Rsmall
Rlarge
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Step Response
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Impact of CGD
• What does CGD do to the switching
response here?
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Impact of CGD
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Threshold
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Threshold
• Describe VT as a constant
• Induce enough electron collection to
invert channel
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Penn ESE370 Fall2010 -- DeHon
VDS impact
• In practice, VDS impacts state of channel
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VDS impact
• Increasing VDS, already depletes
portions of channel
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Penn ESE370 Fall2010 -- DeHon
VDS impact
• Increasing VDS, already depletes
portions of channel
• Need less charge, less voltage to invert
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Penn ESE370 Fall2010 -- DeHon
Drain-Induced Barrier
Lowering (DIBL)
VT
VDS
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DIBL Impact
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In a Gate?
• What does it impact most?
– Which device, which state/operation?
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Penn ESE370 Fall2010 -- DeHon
In a Gate
• VDS largest for off device
– Easier to turn on
IDS
2 
W 
VDS
 nCOX  VGS VT VDS 

 L 
2 
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Penn ESE370 Fall2010 -- DeHon
In a Gate
• VDS largest for off device
– Easier to turn on
– Leak more
IDS
IDS
2 
W 
VDS
 nCOX  VGS VT VDS 

 L 
2 
W 
 IS  e
 L 
 VGS 


nkT / q 
 VDS 

kT / q 
1  e  1 VDS 


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Penn ESE370 Fall2010 -- DeHon
In a Gate
• VDS largest for off device
– Easier to turn on
– Leak more
IDS
IDS
W 
 IS  e
 L 
 VGS 


nkT / q 
W 

 IS  e
 L 
 VDS 

kT / q 

1 V 
1

e

DS


VGS VT 


 nkT / q 
 VDS 

kT / q 
1  e  1 VDS 


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Penn ESE370 Fall2010 -- DeHon
Admin
• HW3 due Friday
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Penn ESE370 Fall2010 -- DeHon
Ideas
• Capacitance
– To every terminal
– Voltage dependent
CGC
• Threshold
CGCS
– Voltage dependent
CGCB
• Generally do manual
analysis without
VT
Penn ESE370 Fall2010 -- DeHon
VDS
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