ece 2c, notes set 2: fet models · mosfet physical structure n + s o u rce n + d ra in s o u rce co...
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ECE 2C, notes set 2: Active Devices
Mark Rodwell
University of California, Santa Barbara
[email protected] 805-893-3244, 805-893-3262 fax
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Goals of this note set:
model limited- velocityfashioned-old-lessslighly
model. limited-mobility fashioned-old
MOSFETs of models almathematicRough
stics.characteri voltage-current FET
operation FET of sense physicalRough
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N-Channel MOSFET
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Field-Effect Transistor Operation
source drain
gate
Positive Gate Voltage
→ reduced energy barrier
→ increased drain current
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Field-Effect Transistor Operation
source drain
gate
Positive Gate Voltage
→ reduced energy barrier
→ increased drain current
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N-Channel MOSFET
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Physical Sketch
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MOSFET Physical Structure
N+ source N+ drain
source contact (silicide) drain contact (silicide)
N+ polygate
gate metal(silicide) dielectric
sidewall
gate oxide
P substrate
S D S D S D S
G
G
Wg
Cross-Section Layout
P substrate
gatedielectric
N+polysilicongate
inversionlayer
(6 FETs, each of gate width Wg , connected in parallel)
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MOSFET I-V characteristics
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MOSFET I-V characteristics
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Id
VgsVth
V
MOSFETs: Three Regions of Gate Voltage
2
, )(
limitedmobility iscurrent , thresholdabove littlea is When
thgsD
gs
VVI
V
)(
limited velocityiscurrent , thresholdabovefar is When
, VVVI
V
thgsvD
gs
mobility-limited
velocity-limited
ld"subthresho" :off (almost) isr transisto, threshold thanless is When gsV
almost off
/gsatLvV
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MOSFET DC Characteristics: Mobility-Limited Case
:current limitedmobility
Id
VgsVth
mobility-limited
velocity-limited
voltage,knee thenlarger tha voltagesdrainfor Applies
ID
VDS
increasingVGS
)1())(2/( 2
, DSthgsggoxD VVVLWcI
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MOSFET DC Characteristics: Velocity-Limited Case
))(1(, VVVVvWcI thgsDSsatgoxvD
Id
VgsVth
mobility-limited
velocity-limited
ID
VDS
increasingVGS
voltage,knee thenlarger tha voltagesdrainfor Applies
current limitedvelocity
/gsatLvV
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DC Characteristics---Far Above Threshold
/ where
1/)(for )(
gsat
thgsthgssatgoxD
LvV
VVVVVVvWcI
Id
VgsVth
V
classes. advanced moreyour in it
use pleasebut ece2C,in term theignore willWe V
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Paramers and Typical #s
V 20-3 typically1/ :sticscharacterioutput of slope gives
FETs-N modernfor V 0.4-0.2usually --- voltaged threshol
m10 nm 1about - thicknessoxide equivalent
)SiOfor 3.8 ( /area unit per ecapacitanc gate
FETs-N of that halfabout are and both FETs,channel-For P
MOSFET-Nfor s)-/(Vcm 400300~surfaceat mobility carrier
MOSFETs-Nfor cm/s 10~city drift velo saturation
9-
20
2
7
th
ox
roxrox
sat
sat
V
T
Tc
v
v
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Knee Voltage: Mobility-Limited Case
VGD=V
th
regionscurrent -constant and Ohmic the
betweenboundary thedefines voltageknee TheID
VDS
increasingVGS
Oh
mic
constant-current
thgsdsdg VVVV
whenoccurs curve in knee the
regime, limited-mobility theIn
IDRD
VGD=V
th
IDRS
s.resistance drain & source parasitic theacross drops
by voltage increasedfurther is Voltage KneeThe
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Knee Voltage: Velocity-Limited Case
/ whenoccurs
curve in knee theregime, limited- velocity theIn
gsatds LvV
s.resistance drain & source
parasitic theacross drops by voltage
increasedfurther is Voltage Knee theAgain,
VDS=v
satLg/
IDRD
VDS=v
satLg/
IDRS
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Which Model to use When ?
Id
VgsVth
/gsatLvV
mobility-limited
velocity-limited
modelvelocity -constant theuse , If
modelmobility -constant theuse ,/ where If
VVV
LvVVVV
thgs
gsatthgs
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Linear vs. Square-Law Characteristics: 90 nm
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Which Model to use When ?
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90 nm MOSFET DC Characteristics
3V~1/ V 6.0||
mm/S 7.0μm/mS 7.0/
channel-P
3V~1/ V 6.0
mm/S 4.1μm/mS 4.1/
channel-N
th
satoxgm
th
satoxgm
V
vcWg
V
vcWg
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180 nm MOSFET DC Characteristics
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P-Channel MOSFET
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P-Channel MOSFET
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P-Channel MOSFET
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