P-n Junction Diode and Zener Diode

8/9/2019 P-n Junction Diode and Zener Diode

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P-N JUNCTION DIODE ANDZENER DIODE

N U R U L Z U H A D A B I N T I A H M A D

N U R A I N A B I N T I A B D U L L A H

N U R H A F I Z A B I N T I J U N I

N U R M A L I S A B T M D S A R I B

N U R U L N O O R A S Y I K I N B T M O H A M A DN A S H A R U D D I N


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PN JUNCTIONS : OVERVIEW

The most important device is a junctionet!een a p-t"pe re#ion and an n-t"pere#ion

$hen the junction is %rst &ormed' due to theconcentration #radient' moi(e char#estrans&er near junction

E(ectrons (eave n-t"pe re#ion and ho(es (eave p-t"pe re#ion

These moi(e carriers ecome minorit" carriersin ne! re#ion )can*t penetrate &ar due torecomination+

Due to char#e trans&er' a vo(ta#e di,erence

occurs et!een re#ionsThis creates a %e(d at the junction that causes

dri&t currents to oppose the di,usion current

In therma( eui(irium' dri&t current anddi,usion must a(ance


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C U R R E N

T . I N

P N

J U N C

T I O N .


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DIODE UNDER THERMAL EQUILIBRIUM

Di,usion sma(( since &e! carriers have enou#h ener#" to penetrate arrier

Dri&t current is sma(( since minorit" carriers are &e! and &ar et!een/ On("minorit" carriers #enerated !ithin a di,usion (en#th can contriute current

Important Point/ 0inorit" dri&t current independent o& arrier1

Di,usion current stron# )e2ponentia(+ &unction o& arrier

p-type -type-

-

-

-

-

-

-

-

-

-

-

-

-

!

!!!!!!!!!

!!!

"

"

!

!

"

"

T#e$%&'

(ee$&t)*

Re+*%,)&t)*C&$$)e$ )t# ee$.y,e'* ,&$$)e$ #e).#t

M)*$)ty C&$$)e$ C'*/e t* J0+t)*

D N A

N

0 E

biqφ

, p diff J

, p drift J

,n diff J

,n drift J


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REVERSE BIAS

Re1e$/e B)&/ +&0/e/ & )+$e&/e/ ,&$$)e$t* 2)30/)* D)30/)* +0$$et )/ $e20+e2 e4p*et)&''y

D$)5t +0$$et 2*e/ *t +#&.e Net $e/0't: S%&'' $e1e$/e

+0$$et

p-type -type-

--

-

-

-

-

!!

!!!!!

!"

D N A N

( )bi Rq V φ +


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FORWARD BIAS

F*$&$2 ,)&/ +&0/e/ & e4p*et)&'

)+$e&/e ) t#e 0%,e$ *5 +&$$)e$/ )t#/06+)et ee$.y t* peet$&te ,&$$)e$

D)30/)* +0$$et increases e4p*et)&''y

D$)5t +0$$et 2*e/ *t +#&.e

Net $e/0't: L&$.e 5*$&$2+0$$et

p-type -type-

-

-

-

-

-

-

!

!!!!!!

!"

D N A N

( )bi Rq V φ +


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P-N JUNCTION DIODE

Free electrons on the n-side and free holes on the p-

side can initially diffuse across the junction. Uncovered

charges are left in the neighbourhood of the junction.

This region is depleted of mobile carriers and is

called the DEPLETION REGION (thickness 0.5 !.0

"m#.


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P-N JUNCTION DIODE

$n i t s o%n a p- type or n - typesemiconduc to r i s no t ve ry use fu l .

&o%ever %hen combined very use fu l

dev ices can be made.

The p -n junc t ion can be fo rmed by

a l lo%ing a p - type mater ia l to d i f fuse

in to a n - type reg ion a t h ightempera tu res .


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The p-n junction has led to manyinventions like the diode' transistors

and integrated circuits.


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P-N JUNCTION DIODE

The diffusion of electrons and holes stopdue to the barrier p.d (p.d across the

junction# reaching some critical value.

The barrier p.d (or the contact potential#depends on the type of semiconductor'

temperature and doping densities.

t room temperature' typical values ofbarrier p.d. are)

*e + 0., 0.

/i + 0. 0.1


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232 4

P-N junction diode

V

I


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232 4

P-N junction diode

V

I

I = I0(eqV/ηkT -1)

pn v

Ip0 = q(ni2/ND) (Lp/τp)


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P-N JUNCTIONS - EQUILIBIU!

"#$% #$ppen& '#en %#e&e $n&%*+,%+*e& ,..ie

e*i ene*3 +&% e ,n&%$n% $% eq+i.i*i+4 & $n& +&% en ne$* in%e*5$,e $* 5* %#e in%e*5$,e4 $n&%*+,%+*e& +&% *eve*%

67 8-type' lo% 2F

- 7 fi9ed ioni:ed acceptors

; 7 mobile holes' p

67


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FORWARD BIAS P-N JUNCTION

=hen an e9ternal voltage is applied to

the 8-< junction making the 8 side

positive %ith respect to the < side the

diode is said to be for%ard biased

(F.>#.

The barrier p.d. is decreased by the

e9ternal applied voltage. The depletion

band narro%s %hich urges majoritycarriers to flo% across the junction.

F.>. diode has a very lo% resistance.


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REVERSE BIAS P-N JUNCTION

= h e n a n e 9 t e r n a l v o l t a g e i s a p p l i e d

t o t h e 8 < j u n c t i o n m a k i n g t h e 8

s i d e n e g a t i v e % i t h r e s p e c t t o t h e <

s i d e t h e d i o d e i s s a i d t o b e ? e v e r s e

> i a s e d ( ? . > . # .

T h e b a r r i e r p . d . i n c r e a s e s . T h e

d e p l e t i o n b a n d % i d e n s p r e v e n t i n g

t h e m o v e m e n t o f m a j o r i t y c a r r i e r s

a c r o s s t h e j u n c t i o n .

? . > . d i o d e h a s a v e r y h i g h

r e s i s t a n c e .


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REVERSE BIAS P-N JUNCTION

$ n l y t h e r m a l l y g e n e r a t e d m i n o r i t y c a r r i e r sa r e u r g e d a c r o s s t h e p - n j u n c t i o n . T h e r e f o r e t h e

m a g n i t u d e o f t h e r e v e r s e s a t u r a t i o n c u r r e n t ( o r

r e v e r s e l e a k a g e c u r r e n t # d e p e n d s o n t h e

t e m p e r a t u r e o f t h e s e m i c o n d u c t o r . = h e n t h e 8 < j u n c t i o n i s r e v e r s e d b i a s e d t h e

% i d t h o f t h e d e p l e t i o n l a y e r i n c r e a s e s ' h o % e v e r

i f t h e r e v e r s e v o l t a g e g e t s t o o l a r g e a

p h e n o m e n o n k n o % n a s d i o d e b r e a k d o % n o c c u r s .


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ZERO BIASED PN JUNCTION DIODE


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W#e & 2)*2e )/ +*e+te2 ) & Zero3ias +*2)t)*8 * e4te$&' p*tet)&' ee$.y )/&pp')e2 t* t#e PN 90+t)* H*e1e$ )5 t#e 2)*2e/te$%)&'/ &$e /#*$te2 t*.et#e$8 & 5e #*'e/;%&9*$)ty +&$$)e$/< ) t#e P-type %&te$)&' )t#e*0.# ee$.y t* *1e$+*%e t#e p*tet)&' ,&$$)e$

)'' %*1e &+$*// t#e 90+t)* &.&)/t t#)/ ,&$$)e$p*tet)&' T#)/ )/ =* &/ t#e >4or!ardCurrent?

L)=e)/e8 #*'e/ .ee$&te2 ) t#e N-type %&te$)&';%)*$)ty +&$$)e$/


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Zener

Diode


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INTRODUCTIONThe zener diode is a silicon pn junction devices that differs from rectifier

diodes because it is designed for operation in the reverse-breakdownregion. The breakdown voltage of a zener diode is set by carefully

controlling the level during manufacture. The basic function of zener

diode is to maintain a specific voltage across its terminals within given

limits of line or load change. Typically it is used for providing a stable

reference voltage for use in power supplies and other e!uipment.


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CON.TRUCTION O4 ZENER"ener diodes are designed to operate in reverse breakdown. Two types of reverse

breakdown in a zener diode are avalanche and zener . The avalanche break down

occurs in both rectifier and zener diodes at a sufficiently high reverse voltage. Zenerbreakdown occurs in a zener diode at low reverse voltages.

# zener diode is heavily doped to reduced the breakdown voltage.

This causes a very thin depletion region.

The zener diodes breakdown characteristics are determined by the

doping process

"eners are commercially available with voltage breakdowns of 1.8 V

to 200 V.


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$OR5IN6 O4 ZENER

# zener diode is much like a normal diode. The e$ception being is that it

is placed in the circuit in reverse bias and operates in reverse breakdown.This typical characteristic curve illustrates the operating range for a zener.

%ote that its forward characteristics are just like a normal diode.


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3REA5DO$N C7ARACTERI.TIC.

&igure shows the reverse portion of a zener diodes characteristic

curve. #s the reverse voltage (' ) is increased, the reverse current ( )remains e$tremely small up to the *knee+ of the curve. The reverse

current is also called the zener current, ". #t this point, the breakdown

effect begins the internal zener resistance, also called zener impedance

(""), begins to decrease as reverse current increases rapidly.


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ZENER BREAKDOWN

Zener and avalance e!!ec"# are re#$%n#&'le !%r #(c adra)a"&c &ncrea#e &n "e val(e %! c(rren" a" "e

'rea*d%+n v%l"a,e

I! "e &)$(r&". c%ncen"ra"&%n ver. &,/ "en "e +&d" %!de$le"&%n re,&%n ver. le## Le## +&d" %! de$le"&%n

re,&%n +&ll ca(#e &, &n"en#&". %! elec"r&c !&eld "%

devel%$ &n "e de$le"&%n re,&%n a" l%+ v%l"a,e#


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A8A9ANC7E 3REA5DO$N

Zener e,ect predominates on diodes !hoserea:do!n vo(ta#e is e(o! ; 8< Therea:do!n vo(ta#e can e otained at a (ar#eva(ue " reducin# the concentration o&

impurit" atom<

$e :no! that ver" (itt(e amount o& current>o!s in the reverse iased diode< This currentis due to the >o! o& minorit" char#e carriersi


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The !idth o& dep(etion re#ion is (ar#e !hen theimpurit" concentration is (ess<

$hen a reverse ias vo(ta#e is app(ied acrossthe termina(s o& the diode' the e(ectrons &romthe p t"pe materia( and ho(es &rom the n-t"pemateria(s acce(erates throu#h the dep(etionre#ion<

This resu(ts in co((ision o& intrinsic partic(es)e(ectrons and ho(es+ !ith the ound e(ectronsin the dep(etion re#ion< $ith the increase inreverse ias vo(ta#e the acce(eration o&e(ectrons and ho(es a(so increases<

No! the intrinsic partic(es co((ides !ith ound

e(ectrons !ith enou#h ener#" to rea: itscova(ent ond and create an e(ectron-ho(e pair<This is sho!n in the %#ure<


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AVALANCHE BREAKDOWN MECHANISM


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• T#e +*'')/)* *5 e'e+t$*/ )t# t#e &t*%+$e&te/ & e'e+t$*-#*'e p&)$

• T#)/ e'y +$e&te2 e'e+t$* &'/* .et/&++e'e$&te2 20e t* e'e+t$)+ @e'2 &2 ,$e&=/%&y %*$e +*1&'et ,*2 t* 50$t#e$ +$e&te%*$e e'e+t$*-#*'e p&)$

•

T#)/ p$*+e// =eep/ * $epe&t). &2 )t )/+&''e2 carrier mu(tip(ication• T#e e'y +$e&te2 e'e+t$*/ &2 #*'e/

+*t$),0te t* t#e $)/e ) $e1e$/e +0$$et•

T#e p$*+e// *5 +&$$)e$ %0't)p')+&t)* *++0$/1e$y 0)+='y &2 ) 1e$y '&$.e 0%,e$/ t#&tt#e$e )/ apparent(" an ava(anche o&char#e carriers T#0/ t#e ,$e&=2* )/

+&''e2 &1&'&+#e ,$e&=2*


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DIFFERENCE BETWEEN ZENER

AND AVALANCHE BREAKDOWN

Z E N E R B R E A K D O W N

?< This occurs at junctions !hich

ein# heavi(" doped have narro!dep(etion 9a"ers

@< This rea:do!n vo(ta#e sets aver" stron# e(ectric %e(d acrossthis narro! (a"er<

< 7ere e(ectric %e(d is ver" stron#to rupture the cova(ent ondsthere" #eneratin# e(ectron-ho(epairs< .o even a sma(( increase inreverse vo(ta#e is capa(e o&producin# 9ar#e numer o&

current carriers<

B< Zener diode e2hiits ne#ativetemp/ coecient< Ie< rea:do!nvo(ta#e decreases as temperatureincreases<

AVA L A N C H E

B R E A K D O W N?< This occurs at junctions !hich ein# (i#ht("doped have !ide dep(etion (a"ers<

@< 7ere e(ectric %e(d is not stron# enou#h toproduce Zener rea:do!n<

< 7er minorit" carriers co((ide !ith semi conductoratoms in the dep(etion re#ion' !hich rea:s thecova(ent onds and e(ectron-ho(e pairs are#enerated< Ne!(" #enerated char#e carriers areacce(erated " the e(ectric %e(d !hich resu(ts inmore co((ision and #enerates ava(anche o& char#ecarriers< This resu(ts in ava(anche rea:do!n<

B< Ava(anche diodes e2hiits positive temp/coecient< i


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ZENER DIODE APPLICATIONS –

ZENER RE6U9ATION $IT7 A 8ARIN6 INPUT8O9TA6E


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ZENER 9I0ITIN6

"ener diodes can used in ac applications to limit voltage swings to

desired levels.

VZ- zener voltage

Vd: iode voltage

Vd = 0./


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P-n Junction Diode and Zener Diode

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