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tweetbazaarElectronics - Devices

Nov 2, 2013 (3 years and 7 months ago)

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FIELD-EFFECT TRANSISTORS
￿Properties of Field-Effect Transistors
￿Types of Field-Effect Transistors
￿Characteristics and Applications
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Field-effecttransistorsaresonamedbecause
aweakelectricalsignal
cominginthroughoneelectrodecreatesanelectricalfieldthroughthe
restofthetransistor.
TheFETwasknownasa“
unipolar
”transistor.
Thetermreferstothefactthat
currentistransportedbycarriersofone
The Field Effect Transistor (FET)The Field Effect Transistor (FET)
polarity(majority),whereasintheconventionalbipolartransistor
carriersofbothpolarities(majorityandminority)areinvolved.
Thefield-effecttransistor(FET)operatesby
theeffectsofanelectricfield
ontheflowofelectronsthroughasingletype
ofsemiconductormaterial.
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TheterminalsofaJFETare
thesource,gate,anddrain
.
AJFETcanbeeitherpchannelornchannel.
Basic Operation of The Junction Field Effect Transistor (JFET)
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Theterms￿-channelandP-channelrefertothematerialwhichthe
drain
and
source
are
connected.
FET (Field Effect Transistor)
–MOSFET(metal-oxide-semiconductorfield-effecttransistors)
–JFET(JunctionField-effecttransistors)
TypesofTransistors
JFEToperationcanbecomparedtoawatertap:
Basic Operation of The Junction Field Effect Transistor (JFET)
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Thesourceofwaterpressure
–accumulatedelectronsatthenegativepoleoftheapplied
voltagefromSourcetoDrain.
Thedrainofwater
–electrondeficiency(orholes)atthepositivepoleoftheapplied
voltagefromDraintoSource.
Thecontrolofflowofwater
–Gatevoltagethatcontrolsthewidthofthen-channel,
whichinturncontrolstheflowofelectronsinthen-channelfromsourcetodrain.
Basic Operation of The Junction Field Effect Transistor (JFET)
Figure:
n
-Channel JFET.
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Drain
SYMBOLS
Drain
Drain
Basic Operation of The Junction Field Effect Transistor (JFET)
Gate
Source
n-channel JFET
Gate
Source
n-channel JFET
Offset-gate symbol
Gate
Source
p-channel JFET
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The Junction Field Effect Transistor (JFET)-Biasing
￿The“source-drain”currentcontrolledbyavoltagefieldatthe“gate”.
￿Thatfieldisdevelopedbythereversed
gate-sourcejunction(gateisconnectedtoboth
sides).￿Withmore
V
GG
(reversebias)thefieldgrowslarger.
￿Thisfieldorresistancelimitstheamountofcurrentflowthrough
R
D
.
￿Withloworno
V
GG
currentflowisatmaximum.
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The voltage on the gate produces an electric field that interferes with the flow of charge
J-FET (Junction Field Effect Transistor)
The voltage on the gate produces an electric field that interferes with the flow of charge
carriers through the channel.
Fluctuations in
gate voltage, E
G, cause changes in the current through the
channel, I
S
or
I
D. Small fluctuations in the control voltage
E
G
can cause large variations in the flow of
charge carriers through the JFET. This translates into voltage amplification in electronic
circuits.
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The more negative
E
G
becomes, the more the electric field chokes off the current though
the channel, and the smaller
I
D
becomes.
HowaJFETtransistorworks?
Whenthegateisnegative,itrepels
theelectronintheN-channel.So
thereisnowayforelectronstoflow
fromsourcetodrain.
Whenthenegativevoltageis
removedfromGate,theelectrons
canflowfreelyfromsourceto
drain.sothetransistorison.
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HowaJFETtransistorworks?
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When
E
G
becomes increasingly negative (Fig. A), a
depletion region
(solid black) begins to form in
the channel. Charge carriers cannot flow in this region; they must pass through a narrowed
channel.
The more negative
E
G
becomes, the wider the depletion region gets. If the gate becomes negative
enough,the depletion region will completely obstruct the flow of charge carriers. This is called
pinchoff
.
Transconductance
Refer to Fig. . Suppose that
E
G
is a certain value, with a corresponding
I
D
resulting. If the gate voltage changes by a small amount
dE
G
then the
drain current will also change by a certain increment
dI
D
.
The
transconductance or dynamic mutual conductance
is the ratio
dI
D
/dE
G
.
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Depletion and pinchoff
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BJT vs JFET
B
ipolar
J
unction
T
ransistor
J
unction
F
ield
E
ffect
T
ransistor
•Current-based device
•I
Base
controls I
CollectorEmitter
•Voltage-based device
•V
Gate
controls I
SourceDrain
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FET (Field Effect Transistor)
FEToperation
involvesanelectricfieldwhichcontrolstheflowofacharge
(current)throughthedevice.
The
source,drain,andgate
terminaloftheFETareanalogoustothe
In
contrast,a
bipolartransistor
employsasmallinputcurrenttocontrola
large
outputcurrent.
emitter,collector,andbase
ofabipolartransistor.
BJT FET
BaseGate
CollectorDrain
EmitterSource
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