juction field transistor

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Storey: Electrical & Electronic Systems © Pearson Education Limited 2004 OHT 20.1

Field-Effect Transistors

Introduction

An Overview of ield!Effect "ransistors

Insulated!#ate ield!Effect "ransistors

$unction!#ate ield!Effect "ransistors

E" %aracteristics

Summary of E" %aracteristics

E" Am'lifiers Oter E" A''lications

%a'ter 20




Introduction

Field-effect transistors (FETs) are 'ro*a*ly te

sim'lest form of transistor

+ widely used in *ot analo,ue and di,ital a''lications

+ tey are caracterised *y a very i, in'ut resistance

and small 'ysical si-e. and tey can *e used to form

circuits wit a low 'ower consum'tion

+ tey are widely used in very large-scale integration + two *asic forms:

insulated gate FETs

junction gate FETs

20/




An Overvie of Field-Effect Transistors

1any forms. *ut *asic o'eration is te same

+ a volta,e on a control in'ut 'roduces an electric field

tat affects te current *etween two oter terminals

+ wen considerin,

am'lifiers we looed

at a circuit usin, a

3control device

+ a E" is a suita*le

control device

20/2



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"otation

+ E"s are 5 terminal devices drain (d)

source (s) ,ate(,)

+ te ,ate is te control in'ut

+ dia,ram illustrates te

notation used for la*ellin,volta,es and currents



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Insulated-$ate Field-Effect Transistors

Suc devices are sometimes called I$FETs

(insulated!,ate field!effect transistors) or sometimes

%O&FETs (metal o6ide semiconductor field!effect

transistors)

7i,ital circuits constructed usin, tese devices are

usually descri*ed as usin, %O& tec'nology

8ere we will descri*e tem as 1OSE"s

20/5



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)onstruction

+ two 'olarities: n!cannel and p-cannel



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O+eration

+ ,ate volt controls te ticness of te cannel

+ consider an n!cannel device

main, te ,ate more positive attracts electrons tote ,ate and maes te ,ate re,ion thicker +reducin, te resistance of te cannel/ "e cannelis said to *e en'anced

main, te ,ate more negative re'els electronsfrom te ,ate and maes te ,ate re,ion thinner +increasin, te resistance of te cannel/ "ecannel is said to *e de+leted



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+ te effect of varyin, te ,ate volta,e



Storey: Electrical & Electronic Systems © Pearson Education Limited 2004 OHT 20.

+ ,ates as descri*ed a*ove are termed 7e'letion!

Enancement 1OSE"s or sim'ly E %O&FETs

+ some 1OSE"s are constructed so tat in te

a*sence of any ,ate volta,e tere is no cannel suc devices can *e o'erated in an enancement mode. *ut

not in a de'letion mode (since tere is no cannel to de'lete)

tese are called En'ance/ent %O&FETs

+ *ot forms of 1OSE" are availa*le as eitern!cannel or p!cannel devices




%O&FET

circuit sy/ols




unction-$ate Field-Effect Transistors

Sometimes nown as a $FET

8ere we will use anoter common name + te FET

8ere te insulated ,ate of a 1OSE" is re'lacedwit a reverse!*iased pn 9unction

Since te ,ate 9unction is always reverse!*iased no

current flows into te ,ate and it acts as if it were

insulated

20/4




)onstruction

+ two 'olarities: n!cannel and p-cannel




O+eration

+ te reverse!*iased ,ate 9unction 'roduced a de'letionlayer in te re,ion of te cannel

+ te ,ate volt controls te ticness of te de'letionlayer and ence te ticness of te cannel

+ consider an n!cannel device te ,ate will always *e ne,ative wit res'ect to te source to

ee' te 9unction *etween te ,ate and te cannel reverse!

*iased main, te ,ate more negative increases te ticness of te

de'letion layer. reducin, te widt of te cannel + increasing te resistance of te cannel/



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+ te effect of varyin, te ,ate volta,e



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FET circuit sy/ols



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FET )'aracteristics

ile 1OSE"s and $E"s o'erate in different

ways. teir caracteristics are ;uite similar

In+ut c'aracteristics

+ in *ot 1OSE"s and $E"s te ,ate is effectively

insulated from te remainder of te device

Out+ut c'aracteristics

+ consider n!cannel devices

+ usually te drain is more 'ositive tan te source

+ te drain volta,e affects te ticness of te cannel

20/<



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FET out+ut c'aracteristics




Transfer c'aracteristics

+ similar sa'e for all forms of E" + *ut wit a different

offset

+ not a linear res'onse. *ut over a small re,ion mi,t *econsidered to a''ro6imate a linear res'onse




"or/al o+erating ranges for FETs




en o'eratin, a*out its o+erating +oint we can

descri*e te transfer caracteristic *y te change in

out'ut tat is caused *y a certain change in te in'ut

+ tis corres'onds to te slo'e of te earlier curves + tis ;uantity as units of current=volta,e. wic is te

reci'rocal of resistance (tis is conductance)

+ since tis ;uantity descri*ed te transfer

caracteristics it is called te transconductance. g m

>ote:

GS

Dm

V

I

∆

∆= g

GS

Dm

V

I ≠ g




&/all-signal euivalent circuit of a FET

+ models te *eaviour of te device for small variations

of te in'ut a*out te o'eratin, 'oint




&u//ary of FET )'aracteristics

E"S ave tree terminals: drain. source and ,ate

"e ,ate is te control in'ut

"wo 'olarities of device: n!cannel and p!cannel "wo main forms of E": 1OSE" and $E"

In eac case te drain current is controlled *y te

volta,e a''lied to te ,ate wit res'ect to te source

?eaviour is caracterised *y te transconductance

"e o'eratin, 'oint differs *etween devices

20/@



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FET circuit

sy/ols4



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FET A/+lifiers

A si/+le E %O&FET a/+lifier

+ R G is used to 3ias te

,ate at its correct o'eratin,

'oint (wic for a7E 1OSE" is 0 )

+ C is a cou+ling ca+acitor

and is used to cou'le te

A% si,nal wile 'reventin,e6ternals circuits from

affectin, te *ias

+ tis is an A)-cou+led a/+lifier

20/B



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A)-cou+led a/+lifier

+ in'ut resistance + e;ual to R G

+ out'ut resistance + a''ro6imately e;ual to R D

+ ,ain + a''ro6imately +g mR D (te minus si,n sows tattis is an invertin, am'lifier)

+ C 'roduced a low!fre;uency cut!off at a fre;uency f c,iven *y

were R is te in'ut resistance of te am'lifier (wic intis case is e;ual to R G)

CR c

π 2

0= f



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"egative feedac5 a/+lifier

+ reduces 'ro*lems of varia*ility

of active com'onents

+ volta,e across R s is'ro'ortional to drain current.

wic is directly 'ro'ortional

to te out'ut volta,e

+ tis volta,e is su*tractedfrom in'ut volta,e to ,ate

+ ence negative feedac5



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&ource folloer

+ similar to earlier circuit.

*ut out'ut is now taen

from te source + feed*ac causes te

source to follow te in'ut

volta,e

+ 'roduces a unity-gaina/+lifier

+ also called a source folloer




Ot'er FET A++lications

A voltage controlled attenuator

+ for small drain!to!source

volta,es E"s resem*le

voltage-controlled resistors

+ te ,ate volta,e V G is used

to control tis resistance and

ence te ,ain of te 'otential

divider

+ used. for e6am'le. in auto/atic

gain control in radio receivers

20/C




A FET as an analogue sitc'




A FET as a logical sitc'



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6ey 7oints

E"s are widely used in *ot analo,ue and di,ital circuits

"ey ave i, in'ut resistance and small 'ysical si-e

"ere are two *asic forms of E": 1OSE"s and $E"s

1OSE"s may *e divided into 7E and Enancement ty'es

In eac case te ,ate volta,e controls te current from tedrain to te source

"e caracteristics of te various forms of E" are similar

e6ce't tat tey re;uire different *ias volta,es "e use of cou'lin, ca'acitors 'revents te am'lification of

7% and 'roduced A% am'lifiers

E"s can *e used to 'roduce various forms of am'lifier anda ran,e of oter circuit a''lications

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juction field transistor