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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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