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Metal-Oxide-Semiconductor
Field Effect Transistors
(MOSFETs)
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Types of Field Effect Transistors(The Classification)
JFET
MOSFET(IGFET)
n-Channel JFET
p-Channel JFET
n-ChannelEMOSFET
p-ChannelEMOSFET
EnhancementMOSFET
DepletionMOSFET
n-ChannelDMOSFET
p-ChannelDMOSFET
FET
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Circuit Symbol (NMOS)enhancement-type: no channel at zero gate
voltage
G
D
S
B (IB=0, should be reverse biased)
ID= IS
IS
IG= 0
G-GateD-Drain
S-Source
B-Substrate or Body
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Figure: Circuit symbol for an enhancement-mode n-channel
MOSFET.
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Types of Field Effect Transistors(The Classification)
JFET
MOSFET(IGFET)
n-Channel JFET
p-Channel JFET
n-ChannelEMOSFET
p-ChannelEMOSFET
EnhancementMOSFET
DepletionMOSFET
n-ChannelDMOSFET
p-ChannelDMOSFET
FET
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Figure: ForvGS< Vto thepn junction between drain and body is
reverse biased and iD=0.
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Structure: n-channelMOSFET(NMOS)
pn+n+
metal
LW
source
S
gate: metal orheavily doped poly-Si
Gdrain
D
body
B
oxide
IG=0
ID=ISIS
x
y
(bulk or
substrate)
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For this choice of materials, VGS
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Flatbands < VGS < VT (Includes VGS=0 here).n+-depletion-n+
structure I
D~ 0
pn+n+
n++
L
W
source
S
gate
Gdrain
D
body
B
oxide
+-
+++
VD=Vs
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VGS > VTn+-n-n+ structure inversion
pn+n+
n++
L
W
source
S
gate
Gdrain
D
body
B
oxide
+-
+++
++++++
- - - - -
VD=Vs
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Triode RegionA voltage-controlled resistor@small VDS
G
pn+n+
metal
S DB
oxide
+-
+++
+++
- - - -
VGS1>Vt
pn+n+
metal
S DB
oxide
+-
+++
++++++
- - - - - -
VGS2>VGS1
pn+n+
metal
S DB
oxide
+-
+++
++++++
- - - - - - - - -
VGS3>VGS2+++
ID
VDS0.1 v
increasing
VGS
Increasing VGS puts more
charge in the channel, allowing
more drain current to flow
cut-off
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Figure: As vDSincreases, the channel pinches down at the drain
end and iD increases more slowly.
Finally forvDS> vGS-Vto, iD becomes constant.
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Current-Voltage Relationship of
n-EMOSFET
Locus of points where
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The MOSFET Device and S Model
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The S circuit model of the n-channel MOSFET inverter
The transfer
characteristic of the inverter
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STATIC ANALYSIS USING THE S MODEL
A mapping between logic values and
voltage levels corresponding to a static
discipline appropriate for the inverter.
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THE SR MODEL OF THE MOSFET
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STATIC ANALYSIS USING THE SR MODEL
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ACTUAL MOSFET CHARACTERISTICS
The MOSFET operates in its triode region
VDS< VGS VTand VGS VT
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If
then
Characteristics of the MOS device in the saturation region
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SIGNAL AMPLIFICATION
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The MOSFET is in saturation when
The drain-to-source current of theMOSFET is given by
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the node equation
VO versus VINcurve for the amplifier
