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EN1802 BasicElectronics

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Typesof

FET

How oesaFETwor

Typesof

JFET

OperatingModes

Comparison:JFETandBJT

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Similarto

a

BJT

in

appearance

Invente yWi iamS oc eyin1952

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Whatisthedifferencebetweenthese

twoFET

types?

Difference is in the way their gate terminals are

connecte to t e sem con uctor.

JFET Junction Field Effect Transistor

old: JUGFET, Junction Gate Field Effect Transistor

Gate terminal is directly connected to one of the extrinsicsemiconductor layers

MOSFET Metal Oxide Semiconductor Field Effect

Transistor

old: IGFET, Insulated Gate Field Effect Transistor

Gate terminal is electrically insulated from the

semiconductor materials using a layer of silicon dioxide

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Subtypes Categorizationaccordingto

thetype

of

channel

Pchannel Pchannel

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Water Analogy

Water control is done b s ueezin the hose.

The two ends of the pipe/hose may be named drainand source.

Hose is the channel.

Controlling water flow by squeezing the hose represents .channel width.

B this the effective cross sectional area of the hose is

varied.

When channel width is reduced by the Gate Voltage,

versa.

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semiconductor.

sourcetodrainordraintosource.

Current does not have to cross throu h a PN

junctioninthechannel.

The ate is connected to the o osite t e ofsemiconductor

to

form

a

PN

junction

for

controlpurposes.

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1 Switchin Mode

AnOn/Off

Switch

Whenthegatevoltageexceedsacertainvalue(athreshold

value)thechannelgetscompletelychokedandcurrentstops.

Knownaspinchoffcondition.

Now the FET is off. The drain current is zero.

Thisisthecutoffstate.

Whenthechannelisfullyopen,itison.

Maximum

drain

current

flows

through

the

FET.

Anexternalcircuitshouldlimitthiscurrentinordertopreventdamage

toFETfromovercurrent saturationstate

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,

reversebiased.

indifferentmodes.

T eres ou ezerooutputt roug t egate

connection.

Forthis

reason

JFET

a

voltage

controlled

device(BJT acurrentcontrolleddevice)

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Consider an Nchannel JFET.

.

open path for electrons to flow.

But we need to a l a volta e across the channel V to et

current flowing.

Current should flow via a series resistor of correct value inorder to ensure that the current limits are not exceeded.

If the gate becomes more positive the depletion region

ecreases an more curren ows. If gatetosource (VGS) voltage exceeds 0.60.7V this PN junction

.

the gate is not designed to carry currents. Therefore this

situation needs to be avoided.

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Controllingactionstartswhenweapplyanegativepotentialtothegateterminalwithrespecttothesource.

WhenavoltageisappliedassuchthisreversebiasesthePN

junctionand

the

current

flow

between

the

source

and

the

draingetsreduced.

Therefore,innormaloperationthePNJunctionisreversebiasedandonlyasmallleakagecurrentflowsthroughthe

gate.

Thedepletionregionincreaseinsizewhenthereversebiasonthejunctionincreases.

WhenthedepletionRegioncompletelyoccupiestheentire

widthof

the

channel

the

transistor

is

cut

off.

SincethisoccursatanegativeVGS ofNchannelJFETthethresholdvolta eisne ative.Thisvolta eisknownaspinchoffvoltage(VP).

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valueof

drain

voltage.

d

increaseswiththeVGS

nonlinearly.Therefore

.

MathematicalExpressionforthisbehavior

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In a commonsource circuit the output current is ID and the output

voltage is VDS.

,

applied and the DrainSource voltage is varied from 0 up to amaximum value.

.

Then ID is plotted against VDS using these readings to obtain one

curve. We can observe the drain current rising with the drain.

At a certain limit the drain current starts to level off and we cannotget any further increase in drain current by increasing the drain

This process is repeated for different values of gate voltage.

When the gate voltage is kept constant at a more negative valuean e p o s repea e we ge a curve o s m ar s ape u edraincurrent saturation occurs at a lower level.

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FET BJT

Morecompact

Consumesless

power

Utilizesasmallvolta etocontrol

esscompact

Consumesmore

power

Utilizesasmallcurrenttocontrol

alargecurrent(voltage

controlleddevice)

Threeterminals:

Gate G

alargecurrent(current

controlleddevice)

Threeterminals:

Base(B),

Source(S),Drain(D)

NonlinearDevice

Emitter(E),Collector(C)

LinearDevice

Currentflowhappensthrougha

onetype

of

semiconductor

(eitherPorN)

PNjunction

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byProf.Mrs.I.J.Dayawansa,

Eng.A.T.L.K.

, . . .

Millman andHalkias,IntegratedElectronics:

gures

ave

eenextracte

rom

var ous

e ectron c

andnonelectronicmaterialforthepurposeof

.