Human Resource Management Lecture-39. Summary of Lecture-38.
Lecture #38
description
Transcript of Lecture #38
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EE130 Lecture 38, Slide 1Spring 2007
Lecture #38
OUTLINE
The MOSFET:• Bulk-charge theory• Body effect parameter• Channel length modulation parameter• PMOSFET I-V • Small-signal model
Reading: Finish Chapter 17, 18.3.4
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EE130 Lecture 38, Slide 2Spring 2007
Problem with the “Square Law Theory”
• Ignores variation in depletion width with distance y
CSTGoxeinv VVVVCQ
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EE130 Lecture 38, Slide 3Spring 2007
Modified (Bulk-Charge) Model
T
oxe
oxe
dm
W
T
C
Cm
311 where
23 since OSiSi
DSDSTGeffoxeDlin VVm
VVCL
WI )
2(
2)(2 TGeffoxeDsat VVC
mL
WI
• saturation region:m
VVVV TG
DsatD
• linear region:m
VVVV TG
DsatD
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EE130 Lecture 38, Slide 4Spring 2007
The expression that was previously derived for VT is the
gate voltage referenced to the body voltage that is required reach the threshold condition:
MOSFET Threshold Voltage, VT
ox
SBFSiAFSBFBT C
VqNVVV
)2(22
Usually, the terminal voltages for a MOSFET are all referenced to the source voltage. In this case,
and the equations for IDS areox
SBFSiAFFBT C
VqNVV
)2(22
DSDSTGSeffoxeDlin VVm
VVCL
WI )
2( 2)(
2 TGSeffoxeDsat VVCmL
WI
mVVVV TGSDSsatDS / mVVVV TGSDSsatDS /
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EE130 Lecture 38, Slide 5Spring 2007
Note that VT is a function of VSB:
The Body Effect
where is the body effect parameter
When the source-body pn junction is reverse-biased, |VT| is increased. Usually, we want to minimize so that IDsat will be the same for all transistors in a circuit
FSBFTFSBFox
SiAT
ox
SBFSiA
ox
FSiA
ox
FSiAFFB
ox
SBFSiAFFBT
VVVC
qNV
C
VqN
C
qN
C
qNV
C
VqNVV
22222
)2(2)2(2)2(22
)2(22
00
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EE130 Lecture 38, Slide 6Spring 2007
MOSFET VT Measurement
• VT can be determined by plotting IDS vs. VGS, using a low value of VDS
IDS
VGS
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EE130 Lecture 38, Slide 7Spring 2007
Channel Length Modulation Parameter, • Recall that as VDS is increased above VDsat, the width L of
the depletion region between the pinch-off point and the drain increases, i.e. the inversion layer length decreases.
L
L
LLLIDsat 1
11
DSsatDS VVL
DSsatDS VVL
L
DSsatDSTGSeffoxeDsat VVVVCmL
WI 1)(
22
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EE130 Lecture 38, Slide 8Spring 2007
P-Channel MOSFET• The PMOSFET turns on when VGS < VTp
– Holes flow from SOURCE to DRAIN DRAIN is biased at a lower potential than the SOURCE
• In CMOS technology, the threshold voltages are usually symmetric: VTp = -VTn
P+ P+
N
GATEVS VD
VG
IDS
VB
• VDS < 0
• IDS < 0
• |IDS| increases with
• |VGS - VTp|
• |VDS| (linear region)
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EE130 Lecture 38, Slide 9Spring 2007
DSDSTpGSeffpoxeDS VVm
VVCL
WI )
2(,
PMOSFET I-V
• Linear region:
• Saturation region:
2, )(
2 TpGSeffpoxeDsatDS VVCmL
WII
m
VVV
TpGS
DS
0
m
VVV
TpGS
DS
m = 1 + (3Toxe/WT) is the bulk-charge factor
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EE130 Lecture 38, Slide 10Spring 2007
Small Signal Model
• Conductance parameters:
)(
0
TGSoxeeff
constVG
Dm
Dsat
constVD
Dd
VVmL
CW
V
Ig
IV
Ig
D
G
gmddd vgvgi
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EE130 Lecture 38, Slide 11Spring 2007
Inclusion of Additional Parasitics
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EE130 Lecture 38, Slide 12Spring 2007
Cutoff Frequency
• fmax is the frequency where the MOSFET is no longer amplifying the input signal– Obtained by considering the small-signal model
with the output terminals short-circuited, and finding the frequency where |iout / iin| = 1
Increased MOSFET operating frequencies are achieved by decreasing the channel length
LVV
mL
W
C
gf TGS
eff
oxe
m 1)(
22max