ecen325: electronics summer 2018 - electrical & computer ...spalermo/ecen325/mosfet.pdf ·...
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Sam PalermoAnalog & Mixed-Signal Center
Texas A&M University
ECEN325: ElectronicsSummer 2018
Metal-Oxide-Semiconductor Field Effect Transistor (MOSFET)
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Announcements & Reading
• HW5 due today• Exam 2 on July 20
• MOSFET Reading• Razavi Ch6 – MOSFET Models• Razavi Ch7 – MOSFET Amplifiers
2
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MOSFET Circuit Symbols
3
NMOS PMOS
• MOSFETs are 4-terminal devices• Drain, Gate, Source, & Body
• Body terminal generally has small impact in normal operation modes, thus device is generally considered a 3-terminal device• Drain, Gate, and Source are respectively similar to the Collector, Base,
and Emitter of the BJT
• 2 complementary MOSFETS: NMOS, PMOS
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NMOS Physical Structure
4[Karsilayan]
n+
n+
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CMOS Physical Structure
5[Karsilayan]
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VTH Definition
• The threshold voltage, VTH, is the voltage at which an “inversion layer” is formed• For an NMOS this is when the concentration of
electrons equals the concentration of holes in the p- substrate
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[Silva]
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CH 6 Physics of MOS Transistors 7
Drain Current Derivation: Channel Charge Density
The incremental channel charge density is equal to the gate capacitance times the gate-channel voltage in excess of the threshold voltage.
ox
oxox
THGCox
tC
VVWCQ
:area gateunit per Capacitane where
)(
[Razavi]
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CH 6 Physics of MOS Transistors 8
Drain Current Derivation: Charge Density at a Point
Let x be a point along the channel from source to drain, and V(x) its potential; the expression above gives the charge density (per unit length).
THGSox VxVVWCxQ )()(
[Razavi]
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CH 6 Physics of MOS Transistors 9
Drain Current Derivation: Charge Density and Current
The current that flows from source to drain (electrons) is related to the charge density in the channel by the charge velocity.
vQI
[Razavi]
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CH 6 Physics of MOS Transistors 10
Drain Current Derivation: Triode Region (Small VDS) Current Equation
DSDSTHGSoxnD
THGSLx
xVxV
xV oxnD
nTHGSoxD
n
VVVVLWCI
xdVVxVVWCdxI
dxxdVVxVVWCvxQI
dxdVv
DS
21
)()(
)()(
:VelocityElectron
0 0
[Razavi]
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Triode or Linear Region
• Channel depth and transistor current is a function of the overdrive voltage, VGS-VT, and VDS
• Because VDS is small, VGC is roughly constant across channel length and channel depth is roughly uniform
11
x=0 x=L
VDS
DSDSTnGSOXnDS VVVVCLWI 5.0
00 V DSVLV LxVxV DS
LxVVxVVxV DSGSGSGC
TnGSox
DS
VVCLWR
1
For small VDS
[Silva]
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L
W
tox
N+ N+
VDSGNDVGS Drain current: Expression used in SPICE level 1
W
tox
N+ N+
VDSGNDVGS
Non-linear channel
Linear approximation
VDS
ID
VDSsat
IDsat
VGS > VT
MOS Equations in Triode Region (Large VDS)
This doesn’t really happen
DSDSTnGSOXnDS VVVVCLWI 5.0
TnGSDSsat VVV
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Triode Region Channel Profile
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[Sedra/Smith] LxVVxVVxV DSGSGSGC
• If VGC is always above VT throughout the channel length, the transistor current obeys the triode region current equation
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Saturation Region Channel Profile
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[Sedra/Smith]
LxVVxVVxV DSGSGSGC
• When VDS VGS-VTH=VOV, VGC no longer exceeds VTH, resulting in the channel “pinching off” and the current saturating to a value that is no longer a function of VDS (ideally)
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Saturation Region
• Channel “pinches-off” when VDS=VGS-VTH and the current saturates• After channel charge goes to 0, the high lateral field “sweeps” the
carriers to the drain and drops the extra VDS voltage
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x=0 x=L
VDSsat=VGS-VT
00 V DSVLV LxVxV DS
LxVVxVVxV DSGSGSGC VDS-VDSsat
TnGSDSsat VVV
[Silva]
22 TnGS
OXnDS VV
LWCI
TnGSDS VVV
DSDS
TnGSOXnDS VVVVLWCI
2
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NMOS ID – VDS Characteristics
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TNGSOV VVV [Sedra/Smith]
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MOS “Large-Signal” Output Characteristic
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[Sedra/Smith]
Note: Vov=VGS-VT
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What about the PMOS device?
• The current equations for the PMOS device are the same as the NMOS EXCEPT you swap the current direction and all the voltage polarities
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DSDSTnGSOXnDS VVVVCLWI 5.0 SDSDTpSGOXpSD VVVVC
LWI 5.0
22 TnGSOXnDS VVCL
WI 22 TpSGOXpSD VVCL
WI
Linear:
Saturation:
NMOS PMOS
NMOS PMOS
[Silva]
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PMOS ID – VSD Characteristics
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[Karsilayan]
(Saturation)
TPSGOV VVV
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NMOS DC Operation (w/ infinite rout)
20
22
,(Active) Saturation
2 ,(Linear) Triode
0Cutoff
Condition BiasRegion
TNGSoxn
DSTNGSDSTNGS
DSDS
TNGSoxnDSTNGSDSTNGS
DSTNGS
DS
VVLWCIVVVVV
VVVVLWCIVVVVV
IVV
I
• In transistor model, often combine nCox term as a parameter KPN with units A/V2
• In lab, we combine nCox(W/L) term as a parameter N with units A/V2
[Karsilayan]
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PMOS DC Operation (w/ infinite rout)
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22
,(Active) Saturation
2 ,(Linear) Triode
0Cutoff
Condition BiasRegion
TPSGoxp
SDTPSGSDTPSG
SDSD
TPSGoxpSDTPSGSDTPSG
SDTPSG
SD
VVLWC
IVVVVV
VVVVLWCIVVVVV
IVV
I
• In transistor model, often combine pCox term as a parameter KPP with units A/V2
• In lab, we combine pCox(W/L) term as a parameter P with units A/V2
[Karsilayan]