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 Transistor has 2 main draw backs

  a)Low input impedance

  b)Noise level

 These drawbacks can be overcome to a great extentin FET

FET is a 3 terminal device and the conduction is by

only one type o charge carries

 The another name or FET is Electric !eld"voltage)controlled device

FIELD EFFECTTRANSISTORS(FET)

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 The 3 terminals o FET are#

  a) $%&'N"$)  b) (*%+E"()

  c) ,&TE",)

FET is classi!ed in to 3 types

  a)-unction Field E.ect Transistor"-FET)

  b)/etal semiconductor FET"/(FET()  c) /etal xide semiconductor

FET"/(FET)

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&n N0type silicon bar reerred to as+1&NNEL# has 2 similar pieces o 0typesilicon material di.used on opposite sides omiddle part orming N unction

 The N unctions are internally connectedthrough a common terminal called as ,&TE

hmic contacts are made at two ends o the

channel is called 4(*%+E and $%&'N

 JUNCTION FIELD EFFECTTRANSISTOR(JFET)

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 -FET is again classi!ed in to 2 types

  a)N0+hannel -FET

  b) 0+hannel -FET

(ource"()5/aority charge carriers enter the channel"() and current

is "'s)

$rain"$)5The maority carriers are leave the channel is called drain"$) and current is 'd6

,ate",)5 2 internally connected heavily dopped impurity regions bydi.usion is called ,ate

+hannel5The region between source and drain sandwitched

between 2 gates is called +hannel

 JUNCTION FIELD EFFECTTRANSISTOR(JFET)

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  0+hannel -FET

Schematic symbols:

$%&'N"$)

(*%+E"()

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 The voltage between gate and source is such thatgate is reverse biased

$rain and (ource terminals re interchangeable6

For N0channel -FET ( is connected to the negativeterminal w6r6t to $

For 0+hannel -FET# ( is connected to the positiveterminal w6r6t $

For N0channel -FET 'd and 7ds are positive and 7gs isNegative

For 0channel -FET 'd and 7ds are negative and 7gs ispositive6

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8hen neither any bias is applied to

gate"7gs9:)#and 7ds9: the depletionregion around N unctions are e;ualthickness and symmetrical

8hen positive voltage is applied at $ withrespect to (

 The electrons will <ow rom ( to $

  The conventional +urrent "'d) <ows rom $to (

OPERATION OF NC!ANNEL JFET

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 There is voltage drop across channelresistance rom $ to (

  This voltage drop reverse bias the diode

  The gate is more negative with respect to $

than to ( 

$epletion region penetrates more deeply into

  the channel at points closer to $ than (

  8E$,E shaped depletion regions are

ormed

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N channel JFET: (a) Depletion at gate diode. (b) Reverse biased gate diode increasesdepletion region.

(c) Increasing reverse bias enlarges depletion region. (d) Increasing reverse bias pinches-of the -D channel.

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  8hen gate is reverse biased

  N unctions are reverse biased and

depletion regions are ormed

  regions are dopped heavily comparedto N0+hannel

 $epletion regions penetrate deeply into

the channel"this is act as insulator)

"a#iatio$ o% &e'letio$ #eio$ i&thith *a#iatio$ o% +ate *oltae:

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$ue to this channel becomes narrrowed#theresistance is increases and drain current isreduced

' the negative voltage is urtherincreased#depletion region meet at the

center and drain current is cut o.completely

  The gate source voltage at which the drain

current 'd is +ut0o. completely is calledpinch0o. voltage "7p)

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 -FET

C!ARACTERSTICS OF JFET:

$rain +haracteristics Transer

+haracteristics

$rain chara withshorted gate

$rain chara withexternal bias

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$rain current in the pinch0o. region is givenby shockleys e;uation is#

 '$9'$((">07,(?7)2

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&s 7gs is increases#

a)/aximum saturation drain current becomessmaller because conducting channelbecomes narrower

b)inch0o. voltage is reached at lower valueo drain current"'d)

c)The ohmic region portion decreases

d)7alue o 7ds or the avalanche breakdowno the gate unction is reduced6

DRAIN C!ARACTERSTICS ,IT!E-TERNAL .IAS:

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TRANSFERC!ARACTERSTICS:

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$rain current is decreases with the increasein 4ve gate source bias

$rain current 'd9'dss when 7gs9:

$rain +urrent 'd9:#when 7gs97p

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't is a unipolar device 't is simple to abricate#smaller in si@e 't has high input impedance 't is called voltage driven circuit ,ate terminal is used to control the drain current This gain is characterised by T%&N(+N$*+T&N+E 't is immune to radiation 't has 4ve temp coeAcient so it has better

thermal stability

/e#its a$& Deme#its o% JFET:

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't has high power gain#low o. set voltageand @ero drain current

't has high re;uency response

co$01#atio$

I$'1tTe#mi$al

O1t'1tte#mi$al

.JTco$01#ato$

+ommon,ate

(ource $rain +B

+ommonsource

,ate $rain +E

+ommon$rain

,ate (ource ++