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8/4/2019 junction field effect transistors
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CONTENTCONTENT
Introduction to Junction Field EffectIntroduction to Junction Field Effect
Transistor (JFET).Transistor (JFET).
Structure and function of a JFET.Structure and function of a JFET.Schematic symbols of a JFET.Schematic symbols of a JFET.
Formation of JFET & its Response toFormation of JFET & its Response to
biasing.biasing.Characteristic curves of JFET.Characteristic curves of JFET.
Applications.Applications.
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INTRODUCTION TO JFETINTRODUCTION TO JFET
The Junction gate field-effect transistor(JFET ) is the simplest type of Field
Effect Transistor.
The common Junction transistor,has the disadvantage of a low input
impedance because the base of the transistor is the signal input and the base-
emitter diode is forward biased. Another device achieved transistor action with
the input diode junction reversed biased, and this device is called a|JUNCTION FIELD EFFECT TRANSISTOR|.
For an n-channel JFET, the device is constructed from a bar ofn-type
material, with the shaded areas composed of a p-type material as a Gate.
Between the Source and the Drain, the n-type material acts as a resistor. Thecurrent flow consists of the majority carriers (electrons for n-type material).
Since the Gate junction is reverse biased and there is no minority carrier
contribution to the flow through the device, the input impedance is extremely
high.
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The control element for the JFET
comes from depletion of chargecarriers from the n-channel.
When the Gate is made more
negative,it depletes the majority
carriers from a larger depletion
zone around the gate. Thisreduces the current flow for a
given value of Source-to-Drain
voltage. Modulating the Gate
voltage modulates the currentflow through the device.
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STRUCTUREOF A JFETSTRUCTUREOF A JFETCircuit diagram for an n-Channel,p-gateJFET is shown.
The JFET is a long channel of
semiconductormaterial, doped to
contain an abundance of positive
charge carriers (p-type), or of negativecarriers (n-type).
Contacts at each end form the
source(S) and drain(D). The gate(G)
(control)terminal has doping opposite
to that of the channel, which itsurrounds, so that there is a P-N
junction at the interface. Terminals to
connect with the outside are usually
made ohmic.
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A pair of metallic contacts are placed at
each end of the channel. When we apply a
voltage between these, a current can flow
along the channel from one contact to the
other. The contact which launches charges
along the channel is called the source, the
one that 'eats' them at the other end is
called the drain.
In an n-channel device, the channel is
made of n-type semiconductor, so the
charges free to move along the channel are
negatively (hence n) charged - they are
electrons.
In a p-channel device the free charges
whichmove from end-to-end are positively(hence p) charged - they are
holes.Rememberthata hole is the absence
of anelectron.
In each case the source puts new charges
into the channel while the drain removes
them at the other end.
A TYPICAL DIAGRAM OF JFET
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Function of a JFETFunction of a JFET
JFET functions like that of aJFET functions like that of a garden hosegarden hose. The flow of. The flow ofwater through a hose can be controlled by squeezing itwater through a hose can be controlled by squeezing itto reduce theto reduce the cross sectioncross sectionsimilar is the function of JFET.similar is the function of JFET.
Electric chargeElectric charge flows through a semiconducting channelflows through a semiconducting channelbetween "source" and "drain" terminals. By applying abetween "source" and "drain" terminals. By applying abiasbias voltagevoltage to a "gate" terminal, the channel isto a "gate" terminal, the channel is"pinched", so that the"pinched", so that the electric currentelectric current is impeded oris impeded orswitched off completely.switched off completely.
The flow ofThe flow ofelectricchargeelectriccharge
through a JFET is controlled bythrough a JFET is controlled byconstricting the currentconstricting the current--carrying channel. The currentcarrying channel. The currentdepends also on the electric field between source anddepends also on the electric field between source anddrain (analogous to the difference indrain (analogous to the difference in pressurepressureon eitheron eitherend of the hose).end of the hose).
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SYMBOLSOF A JFETSYMBOLSOF A JFET
Figure (a) shows the side cross-sectional view of an N-channel.
Figure (b) shows schematic
symbols of N & P channel
devices.
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Formationofa JFETFormationofa JFET
GENERAL DIAGRAM SHOWING THE FORMATION OF A JFET.
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RESPONSETOWARDSBIASINGRESPONSETOWARDSBIASING
From the diagram:-
Fig-1 shows that when
biasing is not applied to
the JFET, the width of
depletion region remainssmaller into the N-
channel & thus current
flows from Drain towards
Sources.
Fig-2 shows the
spreading of depletionregion into the N-channel
as a result of REVERSE
(or negative) BIASING,
which offers a high
impedence & thus a
negligible current flowsfrom Drain to Sources.
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CHARACTERISTICSOF JFETCHARACTERISTICSOF JFET##I
NTHE
CURVE
SHO
WN, FOR
A
GIVE
NVA
LUE
OFGATEVOLTAGE, THECURRENTISNEARLYCONSTANTOVERA WIDERANGEOFSOURCE-
TO-DRAINVOLTAGES.##
When the Gate is made more negative, it
depletes the majority carriers from a larger
depletion zone .This reduces the current flow
for a given value of Source-to-Drain voltage.
Modulating the Gate voltage modulates the
current flow through the device.
**The Gate voltage at which the current
reaches zero is called the PINCH
VOLTAGE.**
##**THE TRANSFER CHARACTERISTIC FOR THEJFET IS USEFUL FORVISUALIZING THEGAINFROM THE DEVICE AND IDENTIFYING THE
REGION OF LINEARITY.**##
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APPLICATIONSOFA JFETAPPLICATIONSOFA JFET
A JFET can be used as anA JFET can be used as an electronicallyelectronically--controlledcontrolled switchswitch..
Can be used as a voltageCan be used as a voltage--controlledcontrolled resistanceresistance..
The JFET has higherThe JFET has highertransconductancetransconductance than other transistors and isthan other transistors and is
therefore used in some lowtherefore used in some low--noisenoise, high input, high input--impedanceimpedance opop--ampsamps..
JFET,having a high input impedance minimizes the "loading" of theJFET,having a high input impedance minimizes the "loading" of the
signal source when a measurement is made.signal source when a measurement is made.
The most effective use of it,is its frequently encounteredThe most effective use of it,is its frequently encountered
configuration for a JFET amplifier in the common source circuit.configuration for a JFET amplifier in the common source circuit.
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