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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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