Upload
candace-spoon
View
213
Download
0
Tags:
Embed Size (px)
Citation preview
Penn ESE370 Fall2010 -- DeHon1
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 Fall2010 -- DeHon2
Today
• Capacitance– Gate– Source/Drain Contact
• More threshold dependence– VDS
Penn ESE370 Fall2010 -- DeHon3
Theme
• Refining model– Exploring next level of complexity
Penn ESE370 Fall2010 -- DeHon4
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 Fall2010 -- DeHon5
Threshold
• Threshold decreases with VDS
Penn ESE370 Fall2010 -- DeHon6
VT
VDS
Capacitance Setup
Penn ESE370 Fall2010 -- DeHon7
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.
Penn ESE370 Fall2010 -- DeHon8
Capacitance
• Four Terminals
• How many combinations– 4 things taken 2 at a time
Penn ESE370 Fall2010 -- DeHon9
Capacitances
• GS, GB, GD, SB, DB, SD
Penn ESE370 Fall2010 -- DeHon10
Moving Plates?
• What is distance from gate to conductor?– Depletion?– Strong Inversion?
Penn ESE370 Fall2010 -- DeHon11
Capacitance Decomposition
Penn ESE370 Fall2010 -- DeHon12
Overlap
• What is the capacitive implication of gate/src and gate/drain overlap?
Penn ESE370 Fall2010 -- DeHon13
Overlap
• Length of overlap?
Penn ESE370 Fall2010 -- DeHon14
Overlap Capacitance
Penn ESE370 Fall2010 -- DeHon15
€
C = ε rε 0
A
d
€
Co = ε ox
W Ldrawn−Leffective( ) /2
tox
Overlap Capacitance
Penn ESE370 Fall2010 -- DeHon16
€
C = ε rε 0
A
d
€
COX =εOX
tOX
€
Co = ε ox
W Ldrawn−Leffective( ) /2
tox
€
Co = CoxW Ldrawn−Leffective( ) /2
Capacitance in Strong Inversion(easy case)
• Looks like parallel plate Gate – Channel– What is CGC?
– What is CGB?
Penn ESE370 Fall2010 -- DeHon17
Capacitance in Strong Inversion
• Looks like parallel plate Gate – Channel– What is CGC?
– CGB=0
Penn ESE370 Fall2010 -- DeHon18
€
CGC = CoxWLeffective
Capacitance in Strong Inversion
• But channel isn’t a terminal– Split evenly with source and drain
Penn ESE370 Fall2010 -- DeHon19
€
CGCS = CGCD = 0.5CoxWLeffective
€
CGC = CoxWLeffective
Capacitance in Strong Inversion
• Add in Overlap capacitance
Penn ESE370 Fall2010 -- DeHon20
€
CGCS = CGCD = 0.5CoxWLeffective
€
CGS = CGSC +CO = 0.5CoxWLdrawn
€
Co = CoxW Ldrawn−Leffective( ) /2
Capacitance Subthreshold
• Need to refine model– What showed on Day 9 not quite right
• Channel doesn’t start depleted– Starts with substrate doping
Penn ESE370 Fall2010 -- DeHon21
Channel Evolution Subthreshold
Penn ESE370 Fall2010 -- DeHon22
Capacitance Depletion
• What happens to capacitance here?– Capacitor plate distance?
Penn ESE370 Fall2010 -- DeHon23
Capacitance Depletion
• Capacitance becomes Gate-Body
• Capacitance drops
Penn ESE370 Fall2010 -- DeHon24
Capacitance vs VGS
Penn ESE370 Fall2010 -- DeHon25
• G CGC
CGCS=CGCD
CGCB
Saturation Capacitance?
Penn ESE370 Fall2010 -- DeHon26
Saturation Capacitance?
Penn ESE370 Fall2010 -- DeHon27
• Source end of channel in inversion
• Destination end of channel close at threshold
• Capacitance shifts to source– Total capacitance reduced
Saturation Capacitance
Penn ESE370 Fall2010 -- DeHon28
CGC
CGCS
CGCD
VDS/(VGS-VT)
Contact Capacitance
Penn ESE370 Fall2010 -- DeHon29
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 Fall2010 -- DeHon30
Contact/Diffusion Capacitance
• Cj – diffusion depletion
• Cjsw – sidewall capacitance
• LS – length of diffusion
Penn ESE370 Fall2010 -- DeHon31
€
Cdiff = C jLSW +C jsw 2LS +W( )
LS
Capacitance Roundup
• CGS=CGCS+CO
• CGD=CGCD+CO
• CGB=CGCB
• CSB=Cdiff
• CDB=Cdiff
Penn ESE370 Fall2010 -- DeHon32
One Implication
Penn ESE370 Fall2010 -- DeHon33
Step Response?
Penn ESE370 Fall2010 -- DeHon34
Rsmall
Rlarge
Step Response
Penn ESE370 Fall2010 -- DeHon35
Impact of CGD
• What does CGD do to the switching response here?
Penn ESE370 Fall2010 -- DeHon36
Impact of CGD
Penn ESE370 Fall2010 -- DeHon37
Threshold
Penn ESE370 Fall2010 -- DeHon38
Threshold
• Describe VT as a constant
• Induce enough electron collection to invert channel
Penn ESE370 Fall2010 -- DeHon39
VDS impact
• In practice, VDS impacts state of channel
Penn ESE370 Fall2010 -- DeHon40
VDS impact
• Increasing VDS, already depletes portions of channel
Penn ESE370 Fall2010 -- DeHon41
VDS impact
• Increasing VDS, already depletes portions of channel
• Need less charge, less voltage to invert
Penn ESE370 Fall2010 -- DeHon42
Drain-Induced Barrier Lowering (DIBL)
Penn ESE370 Fall2010 -- DeHon43
VT
VDS
DIBL Impact
Penn ESE370 Fall2010 -- DeHon44
In a Gate?
• What does it impact most?– Which device, which state/operation?
Penn ESE370 Fall2010 -- DeHon45
In a Gate
• VDS largest for off device
– Easier to turn on
Penn ESE370 Fall2010 -- DeHon46
€
IDS = μnCOX
W
L
⎛
⎝ ⎜
⎞
⎠ ⎟ VGS −VT( )VDS −
VDS2
2
⎡
⎣ ⎢
⎤
⎦ ⎥
In a Gate
• VDS largest for off device
– Easier to turn on– Leak more
Penn ESE370 Fall2010 -- DeHon47
€
IDS = μnCOX
W
L
⎛
⎝ ⎜
⎞
⎠ ⎟ VGS −VT( )VDS −
VDS2
2
⎡
⎣ ⎢
⎤
⎦ ⎥
€
IDS = ISW
L
⎛
⎝ ⎜
⎞
⎠ ⎟e
VGS
nkT / q
⎛
⎝ ⎜
⎞
⎠ ⎟1− e−
VDS
kT / q
⎛
⎝ ⎜
⎞
⎠ ⎟
⎛
⎝ ⎜
⎞
⎠ ⎟1+ λVDS( )
In a Gate
• VDS largest for off device
– Easier to turn on– Leak more
Penn ESE370 Fall2010 -- DeHon48
€
IDS = ISW
L
⎛
⎝ ⎜
⎞
⎠ ⎟e
VGS
nkT / q
⎛
⎝ ⎜
⎞
⎠ ⎟1− e−
VDS
kT / q
⎛
⎝ ⎜
⎞
⎠ ⎟
⎛
⎝ ⎜
⎞
⎠ ⎟1+ λVDS( )
€
IDS = IS′W
L
⎛
⎝ ⎜
⎞
⎠ ⎟e
VGS −VT
nkT / q
⎛
⎝ ⎜
⎞
⎠ ⎟1 − e−
VDS
kT / q
⎛
⎝ ⎜
⎞
⎠ ⎟
⎛
⎝ ⎜
⎞
⎠ ⎟1+ λVDS( )
Admin
• HW3 due Friday
Penn ESE370 Fall2010 -- DeHon49
Ideas
• Capacitance– To every terminal– Voltage dependent
• Threshold– Voltage dependent
• Generally do manual analysis without
Penn ESE370 Fall2010 -- DeHon50
VT
VDS
CGC
CGCS
CGCB