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Microelectronic Circuits, Sixth Edition Sedra/Smith Copyright © 2010 by Oxford University Press, Inc.
C H A P T E R 5
MOS Field-Effect Transistors
(MOSFETs)
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Microelectronic Circuits, Sixth Edition Sedra/Smith Copyright © 2010 by Oxford University Press, Inc.
Device Structure
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Microelectronic Circuits, Sixth Edition Sedra/Smith Copyright © 2010 by Oxford University Press, Inc.
Two back-to-back diodes, High Resistance (Giga Ohms), No Current Flow
Operation with Zero Gate Voltage
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Microelectronic Circuits, Sixth Edition Sedra/Smith Copyright © 2010 by Oxford University Press, Inc.
Creating a Channel for Current Flow
-vGS (gate to source voltage)
-Vt: threshold voltage (vGS which channel
starts conducting)
-Current flowing when vDS applied
-Effective voltage (or overdrive voltage):
-Charge in the channel:
-Oxide capacitance:
-Gate to source capacitance:
effOVtGS vvVv ≡≡−
OVox vWLCQ )(=
ox
oxox
tC
ε=
WLCC ox=
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Microelectronic Circuits, Sixth Edition Sedra/Smith Copyright © 2010 by Oxford University Press, Inc.
Applying a small vDS
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Microelectronic Circuits, Sixth Edition Sedra/Smith Copyright © 2010 by Oxford University Press, Inc.
))()((
])()([(
]))([(
tGSoxnDS
DStGSoxnD
DSOVoxnD
VvL
WCg
vVvL
WCi
vvL
WCi
−=
−=
=
µ
µ
µ
Conductance gDS of the channel:
Applying a small vDS
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Microelectronic Circuits, Sixth Edition Sedra/Smith Copyright © 2010 by Oxford University Press, Inc.
Applying a small vDS
))()((
1
1
tGSoxn
DS
DS
DS
VvL
WC
r
gr
−
=
=
µ
)(L
WCk oxnn µ=
oxnn Ck µ='
Process transconductance
parameter
MOSFET transconductance
parameter
MOSFET behaves as a
linear resistance
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Microelectronic Circuits, Sixth Edition Sedra/Smith Copyright © 2010 by Oxford University Press, Inc.
Operation as vDS is increased
Channel becomes
more tapered and its
resistance increases
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Microelectronic Circuits, Sixth Edition Sedra/Smith Copyright © 2010 by Oxford University Press, Inc.
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Microelectronic Circuits, Sixth Edition Sedra/Smith Copyright © 2010 by Oxford University Press, Inc.
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Microelectronic Circuits, Sixth Edition Sedra/Smith Copyright © 2010 by Oxford University Press, Inc.
Operation for vDS>>VOV
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Microelectronic Circuits, Sixth Edition Sedra/Smith Copyright © 2010 by Oxford University Press, Inc.
P-Channel
MOSFET
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Microelectronic Circuits, Sixth Edition Sedra/Smith Copyright © 2010 by Oxford University Press, Inc.
P-Channel MOSFET
tpGS Vv ≤
tpGS Vv ≥
oxnn Ck µ='
)(L
WCk oxnn µ=
P-Channel transistor transconductance parameter
P-Channel transistor process transconductance parameter
Use absolute value
Threshold voltage
“Formulae are the same,
switch sign of voltages”
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Microelectronic Circuits, Sixth Edition Sedra/Smith Copyright © 2010 by Oxford University Press, Inc.
Figure 5.10 Cross-section of a CMOS integrated circuit. Note that the PMOS
transistor is formed in a separate n-type region, known as an n well. Another
arrangement is also possible in which an n-type body is used and the n device
is formed in a p well. Not shown are the connections made to the p-type body and to the n well; the latter functions as the body terminal for the p-channel
device.
Complementary MOS or CMOS
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Microelectronic Circuits, Sixth Edition Sedra/Smith Copyright © 2010 by Oxford University Press, Inc.
Figure 5.11 (a) Circuit symbol for the n-channel enhancement-type MOSFET.
(b) Modified circuit symbol with an arrowhead on the source terminal to
distinguish it from the drain and to indicate device polarity (i.e., n channel). (c)
Simplified circuit symbol to be used when the source is connected to the body
or when the effect of the body on device operation is unimportant.
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Microelectronic Circuits, Sixth Edition Sedra/Smith Copyright © 2010 by Oxford University Press, Inc.
Table 5.1 Regions of Operation of the
Enhancement NMOS Transistor
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Microelectronic Circuits, Sixth Edition Sedra/Smith Copyright © 2010 by Oxford University Press, Inc.
Figure 5.12 The relative levels of the terminal voltages of the enhancement
NMOS transistor for operation in the triode region and in the saturation region.
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Microelectronic Circuits, Sixth Edition Sedra/Smith Copyright © 2010 by Oxford University Press, Inc.
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Microelectronic Circuits, Sixth Edition Sedra/Smith Copyright © 2010 by Oxford University Press, Inc.
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Microelectronic Circuits, Sixth Edition Sedra/Smith Copyright © 2010 by Oxford University Press, Inc.
Figure 5.15 Large-signal equivalent-circuit model of an n-
channel MOSFET operating in the saturation
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Microelectronic Circuits, Sixth Edition Sedra/Smith Copyright © 2010 by Oxford University Press, Inc.
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Microelectronic Circuits, Sixth Edition Sedra/Smith Copyright © 2010 by Oxford University Press, Inc.
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Microelectronic Circuits, Sixth Edition Sedra/Smith Copyright © 2010 by Oxford University Press, Inc.
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Microelectronic Circuits, Sixth Edition Sedra/Smith Copyright © 2010 by Oxford University Press, Inc.
Figure 5.19 (a) Circuit symbol for the p-channel enhancement-type
MOSFET. (b) Modified symbol with an arrowhead on the source
lead. (c) Simplified circuit symbol for the case where the source is
connected to the body.
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Microelectronic Circuits, Sixth Edition Sedra/Smith Copyright © 2010 by Oxford University Press, Inc.
Table 5.2 Regions of Operation of the Enhancement PMOS Transistor
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Microelectronic Circuits, Sixth Edition Sedra/Smith Copyright © 2010 by Oxford University Press, Inc.
Figure 5.20 The relative levels of the terminal voltages of the
enhancement-type PMOS transistor for operation in the triode region
and in the saturation region.
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Microelectronic Circuits, Sixth Edition Sedra/Smith Copyright © 2010 by Oxford University Press, Inc.
Figure E5.7
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Microelectronic Circuits, Sixth Edition Sedra/Smith Copyright © 2010 by Oxford University Press, Inc.
Example 5.3.
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Microelectronic Circuits, Sixth Edition Sedra/Smith Copyright © 2010 by Oxford University Press, Inc.
Example 5.4
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Microelectronic Circuits, Sixth Edition Sedra/Smith Copyright © 2010 by Oxford University Press, Inc.
Example 5.6..
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Microelectronic Circuits, Sixth Edition Sedra/Smith Copyright © 2010 by Oxford University Press, Inc.
Example 5.7.