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EKT 102
CHAPTER 3
SPECIAL-PURPOSE DIODE
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3.1 Zener Diode
Zener diode is a p-n junction diode
that is designed to operate in the
reverse breakdown region.
Two things happen when the
reverse breakdown voltage (VBR) isreached:
The diode current increases
drasticall!
The reverse voltage (VR
) across
the diode re"ains relativel
constant!
#n other words$ the voltage across
a %ener diode operated in this
region is relatively constant over arange o& reverse current and nearly
equal to its %ener voltage (VZ)
rating!
3. SPECIAL-PURPOSE DIODE
'
#ZVZ
node ()
*athode (+) +
Fig.3-1: Zener diode
s"bol!
Fig.3-2: Zener diode voltage-curent (V-#) characteristic!,
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Inventor of ener Dio!e
3. SPECIAL-PURPOSE DIODE
C"#ren$e %e"vin ener was a pro&essor at *arnegie ellon
.niversit in the depart"ent o& /hsics! 0e developed the
Zener iode in 1234 and e"ploed it in "odern co"puter
circuits!
5
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3.1.1 ener &re#'!o(n
There are two tpes o& reverse breakdown:
1! valanche breakdown!
,! Zener breakdown!
3. SPECIAL-PURPOSE DIODE
Avalanche breakdown is a high-&ielde&&ect that occurs when the
electrostatic field strength associated
with the p-n junction is strong enough
to pull electrons out o& the valence
band within the depletion region!Zener breakdown is a tpe o& reverse breakdown that occurs at relativel
low reverse voltages! The n-tpe and p-tpe "aterials o& a %ener diode
are heavil doped$ resulting in a very narrow depletion region. There&ore$
the electric &ield e6isting within this region is intense enough to pull
electrons &ro" their valence bands and create current at a low reversevoltage (VR)!
)ote*Zener diodes with low VZ ratings e6perience %ener breakdown$ while
those with high VZ ratings usuall e6perience avalanche breakdown!7
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Fig.3-3: Reverse characteristic o&
a %ener diode!
VR
VZ
#Z
8VR
8#R
VBR
#Z+
#ZT
#Z
Zener knee current
Zener test current
"a6i"u" Zener current
3.1.2 &re#'!o(n C+#r#$teri,ti$,
The characteristic that indicates the abilit o&the %ener diode to keep the reverse voltage
across its ter"inals nearl constant "akes
the diode is use&ul as a voltage regulator !
9our "ain characteristics o& the %ener diode
are:Zener voltage (V Z ) is the no"inal %ener
voltage at the breakdown voltage!
Zener knee current (I ZK ) is the "ini"u"
current reuired to "aintain the diode in
breakdown &or the voltage regulation!
Zener test current (I ZT ) is the current level at
which the VZ rating o& the diode is
"easured!
Zener maximum current (I ZM ) is the"a6i"u" reverse current$ which "a not
be e6ceeded! t this current level$ the
diode can work without being da"aged or
destroed!
3. SPECIAL-PURPOSE DIODE
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3.1.3 I!e#"-#n!-Pr#$ti$#" ener Eiv#"ent Cir$it,
3. SPECIAL-PURPOSE DIODE
V9VR
#9
#R
VZ
Fig.3-/* #deal "odel and
characteristic curve o& a %ener
diode in reverse breakdown!
The constant voltage drop ;
the no"inal %ener voltage!
Fig.3-* /ractical "odel and characteristic curve o& a %ener
diode$ where the %ener i"pedance (resistance)$ ZZ is
included!
change in %ener current (8#Z) produces a s"all
changes in %ener voltage (8VZ)!<
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3. SPECIAL-PURPOSE DIODE
3.1./ Teer#tre Coeffi$ient
The %ener voltage o& a %ener diode is ver sensitive to the te"perature o& operation!
The &or"ula &or calculating the change in %ener voltage due to a change in
te"perature is
)( 01 T T x xT V V
C Z Z −=∆
where$ VZ ; no"inal %ener voltage at the re&erence te"perature o& ,3o*!
T* ; te"perature coe&&icient!
T1 ; new te"perature level!
T4 ; re&erence te"perature o& ,3o*!
3.1./ ener Po(er Di,,i#tion #n! Der#ting
The "a6i"u" current that "a be carried b a given %ener diode depends on both the
zener voltage and the maximum dc power dissipation capabilit o& the diode! The dcpower dissipation o& the %ener diode is given b the &or"ula$
Z Z D V I P =
(5-1)
(5-,)=
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3. SPECIAL-PURPOSE DIODE
The "a6i"u" power dissipation o& a %ener diode is speci&ied &or te"perature at or below
a certain value (34o*$ &or e6a"ple)!
bove the speci&ied te"perature$ the "a6i"u" power dissipation is reduced according
to a derating &actor!
The derating &actor is e6pressed in ">?o*!
The "a6i"u" derated power can be deter"ined with the &ollowing &or"ula:
T C mW P P o
Dderated D
∆−= )/((max))(
(5-5)
@
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3. SPECIAL-PURPOSE DIODE
9or an ideal "odel o& a certain %ener diode$ the minimum zener current (I ZK
is speci&ied
on datasheet! 0owever$ the maximum zener current is not given on datasheet but can
calculated &ro" the maximum diode power specification$ which is given on datasheet b
using the euation:
Z
D
ZM
V
P
I (max)=
9or the "ini"u" %ener current$ the voltage across the resistor is deter"ined b:
Z R IN V V V +=(min)
Thus$ the "ini"u" input voltage that can be regulated b the %ener diode$
R I V ZK R =
(5-7)
(5-3)
(5-
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3. SPECIAL-PURPOSE DIODE
9or the "a6i"u" %ener current$ the voltage across the resistor is deter"ined b:
Z R IN V V V +=
'
(max)
Thus$ the "a6i"u" input voltage that can be regulated b the %ener
diode$
R I V ZM R
='
(5-=)
(5-@)
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3. SPECIAL-PURPOSE DIODE
3.2.2 ener Reg"#tion (it+ # #ri#6"e Lo#!
The %ener diode "aintains a nearl constant voltage across RA as long as the %ener
current is greater than I ZK and less than I Z! !
>hen the output ter"inals o& the %ener regulator are open (RA ; ) or a no-load
condition$ the load current (#A) ; 4 and all o& the current is through the %ener!
>hen a load resistor (RA) is connected$ a part o& the total current is through the %enerand an other part through RA!
s RA is decreased$ the load current #A increases and #Z decreases! The %ener diode
continues to regulate the voltage until #Z reaches its "ini"u" value$ #Z+! t this point #A is
"a6i"u"$ and a &ull-load condition e6ists!
Fig.3-7: Zener voltage regulation
with a variable load
1,
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3. SPECIAL-PURPOSE DIODE
B using "athe"aticall &or"ula$ when #Z is "a6i"u"$ we obtain:
)(0(min) ∞== L L R A I
R
V V I I Z IN
T Z
−==(max)
thus$
>hen #Z is "ini"u" (#Z ; #Z+)$ so
ZK T L I I I −=(max)
(max)
(min)
L
Z
L
I
V R =
(5-2)
(5-14)
(5-11)
(5-1,)
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3. SPECIAL-PURPOSE DIODE
#n addition to voltage regulation applications$
%ener diode can be used in ac applications to
li"it voltage swings to desired levels!
Fig.3-8.
/art (a) shows a %ener used to li"it the
positive peak o& a signal voltage to the
selected %ener voltage!
uring the negative alternation$ the %ener acts
as a &orward-biased diode and li"its the
negative voltage to -4!= V!
>hen the %ener is turned around$ as in part
(b)$ the negative peak is li"ited b %ener action
and the positive voltage is li"ited to '4!= V!
Two back-to-back %eners li"it both peaks to
the %ener voltage C4!= V$ as shown in part (c)!
uring the positive alternation$ , is
&unctioning as the %ener li"iter and 1 is
&unctioning as a &orward-biased diode! uring
the negative alternation$ the roles are
reversed!
3.2.3 ener Reg"#tion (it+ # #ri#6"e Lo#!
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3. SPECIAL-PURPOSE DIODE
3.3 Varactor Diode
Varactor is a tpe o& p-n junction diode thatoperates in reverse bias! The capacitance o& the
junction is controlled b the a"ount o& reverse
bias!
Varactor diodes are also re&erred to as varicaps
or tuning diodes and the are co""onl used in
co""unication sste"s!
3.3.1 ,i$ Oer#tion
The capacitance o& a reverse-biased varactor
junction is &ound as:
Fig.3-10: Reverse-biased varactor
diode acts as a variable capacitor!
Fig.3-9: Varactor diode s"bol
d
AC
ε =
where$ * ; the total junction capacitance! ; the plate area!
D ; the dielectric constant (per"ittivit)!
d ; the width o& the depletion region
(plate separation)!
(5-15)
13
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3. SPECIAL-PURPOSE DIODE
The abilit o& a varactor to act as a voltage-controlled capacitor is de"onstrated in 9ig!
5-14!
Fig.3-10: Varactor diode capacitance varies with reverse voltage!
s the reverse-bias voltage increases$ the depletion region widens$ increasing the plateseparation$ thus decreasing the capacitance!
>hen the reverse-bias voltage decreases$ the depletion region narrows$ thus increasing
the capacitance! 1<
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3. SPECIAL-PURPOSE DIODE
3.3.2 #r#$tor A"i$#tion
"ajor application o& varactor is in tuning circuits$ &or e6a"ple$ V09$ .09$ and satelite
receivers utili%e varactors! Varactors are also used in cellular co""unications!
>hen used in a parallel resonant circuit$ as shown in 9ig! 5-11$ the varactor acts as a
variable capacitor " thus allowing the resonant &reuenc to be adjusted b a variable
voltage level!
Fig.3-11: resonant
band-pass &ilter!
1=
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3. SPECIAL-PURPOSE DIODE
*1 prevents a dc path &ro" potentio"eter wiper back to the ac source through the
inductor and R1!
*, prevents a dc path &ro" cathode to the anode o& the varactor through the inductor!
*5 prevents a dc path &ro" the wiper to a load on the output through the inductor!
*7 prevents a dc path &ro" the wiper to ground!
R,$ R5$ R7 and R3 &unction as a variable dc voltage divider &or biasing the varactor!
The parallel resonant &reuenc o& the A* circuit is
LC f r
π 21≅
where$ A ; the inductance o& an inductor (0)
* ; the capacitance o& a capacitor (9)!
(5-17)
1@
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3. SPECIAL-PURPOSE DIODE
3.4 Optical Diodes
There are two popular tpes o& optoelectronic devices: light#emitting diode ($%& and
photodiode.
3./.1 T+e Lig+t-Eitting Dio!e :LED;
AE is diode that e"its light when biased in the &orward direction o& p-n junction!
node *athode
Fig.3-12: The sche"atic s"bol and construction &eatures!
(b) (c)
12
2 DIODES
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2. DIODES
Fig.3-13: AE that are produced in an arra o& shapes and si%es!
AE characteristics:
characteristic curves are ver si"ilar
to those &or p-n junction diodes
higher &orward voltage (V9)
lower reverse breakdown voltage (VBR)!,4
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3. SPECIAL-PURPOSE DIODE
The basic operation o& AE is as illustrated in 9ig!
5-17:
“When the device is forward-biased, electronscross the p-n junction from the n-type materialand recombine with holes in the p-type material.These free electrons are in the conduction bandand at a higher energy than the holes in thevalence band.
When recombination takes place, therecombining electrons release energy in theform photons .
A large eposed surface area on one layer of
the semiconductive material permits thephotons to be emitted as visible light.!
This process is called electroluminescence.
Various i"purities are added during the doping
process to establish the wavelength o& the e"itted
light! The wavelength deter"ines the color o&
visible light! Fig.3
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TBAE 5-1: *o""on AEs
3. SPECIAL-PURPOSE DIODE
L! "emiconductor Materials
The color e"itted b a given AE depends on the co"bination o& ele"ents used to
produce the co"ponent! Fo"e co""on ele"ent co"binations are identi&ied in Table
5-1!
*o"pound 9orward Voltage (V9) *olor E"itted
Gas 1!3 V #n&rared (invisible)
lGas 1!@ V Red
Ga/ ,!7 V Green
Gas/ ,!4 V Hrange
GaI 7!1 V >hite
lGa#n/ ,!4 V "ber (ellow)
lGa#nI 5!< V Blue
V9 is "easured at #9 ; ,4 " in each case!
,,
3 SPECIAL PURPOSE DIODE
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3. SPECIAL-PURPOSE DIODE
#urrent$Limiting %esistor
>hen used in "ost practical applications$ AE reuire the use o& a series current-
li"iting resistor$ as shown in 9ig! 5-1< (a)! The resistor ensures that the "a6i"u"
current rating o& the AE can not be e6ceeded b the circuit current!
The a"ount o& power output translated into light is directly proportional to the &orward
current$ as indicated in 9ig! 5-1< (b)
Fig.3-15: Basic operation o& a AE! ,5
3 SPECIAL PURPOSE DIODE
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3. SPECIAL-PURPOSE DIODE
The li"iting resistor (RA##T) is deter"ined using the &ollowing uestion:
F
F Bias
LIMIT
I
V V R
−
=
,7
3 SPECIAL PURPOSE DIODE
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3. SPECIAL-PURPOSE DIODE
A&&lication
The seven segment display is an example of !Ds "se for display ofdecimal digits.
Fig.3-17: The =-seg"ent AE displa!,3
3 SPECIAL PURPOSE DIODE
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3. SPECIAL-PURPOSE DIODE
3./.2 T+e P+oto!io!e
/hotodiode is a p-n junction that can convert
light energ into electrical energ!
#t operates in reverse bias voltage (VR)$ as
shown in 9ig! 5-1@$ where I= is the reverse light
current!
#t has a s"all transparent window that allows
light to strike the p-n junction!
The resistance o& a photodiode is calculated b
the &or"ula as &ollows:
λ I
V R
R R =
Fig.3-18: /hotodiode!
,<
3 SPECIAL PURPOSE DIODE
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3. SPECIAL-PURPOSE DIODE
>hen its p-n junction is e6posed to light$ the reverse current increases with the light
intensit as shown b the graph in 9ig! 5-12 e6pressed as irradiance (">?c",)!
>hen there is no incident light$ the reverse current is al"ost negligible and is called
the dark current !
Fig.3-19: Tpical photodiode characteristics!,=
3 SPECIAL PURPOSE DIODE
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3. SPECIAL-PURPOSE DIODE
9ig! 5-,4 illustrates that the photodiode is placed in the circuit in reverse bias! s with
"ost diodes when in reverse bias$ no current &lows when in reverse bias$ but when lightstrikes the e6posed junction through a tin window$ reverse current increases
proportional to light intensit!
Fig.3-20: Hperation o& photodiode! ,@
3 SPECIAL-PURPOSE DIODE
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3. SPECIAL-PURPOSE DIODE
3.# Other Types of Diodes
3..1 T+e S$+ott'4 Dio!e
Fchottk diode s"bol is shown in 9ig! 5-,1(a)! The Fchottk diodeJs signi&icant
characteristic is its fast switching speed ! This is use&ul &or high &reuencies and digital
applications! #t is not a tpical diode in that it does not have a p-n junction! #nstead$ itconsists o& a doped se"iconductor (usuall n-tpe) and "etal bound together$ as
shown in 9ig! 5-,1(b)!
Fig.3-21: (a) Fchottk diode s"bol and (b) basic internal construction
o& a Fchottk diode!,2
3 SPECIAL-PURPOSE DIODE
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3. SPECIAL-PURPOSE DIODE
3..2 T+e L#,er Dio!e
The laser diode ("ight #"pli&ication b ,ti"ulated e"ission o& r adiation) produces a
"onochro"atic (single color) light! Aaser diodes in conjunction with photodiodes are
used to retrieve data &ro" co"pact discs!
Fig.3-22: Basic laser diode construction and operation!54
3 SPECIAL-PURPOSE DIODE
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3. SPECIAL-PURPOSE DIODE
3..3 T+e PI) Dio!e
The pin diode is also used in "ostl "icrowave &reuenc applications! #ts variable
&orward series resistance characteristic is used &or attenuation$ "odulation$ and
switching! #n reverse bias it e6hibits a nearl constant capacitance!
Fig.3-23: /#I diode
51
3 SPECIAL-PURPOSE DIODE
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3. SPECIAL PURPOSE DIODE
3../ Crrent Reg"#tor Dio!e
*urrent regulator diodes keeps a constant current value over a speci&ied range o&
&orward voltages ranging &ro" about 1!3 V to < V!
Fig.3-2/: F"bol &or a current regulator diode!
5,
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3. SPECIAL PURPOSE DIODE
3.. T+e Ste-Re$over4 Dio!e
The step-recover diode is also used &or &ast switching applications! This is achievedb reduced doping at the junction!
3..5 T+e Tnne" Dio!e
The tunnel diode has negative resistance! #t will actuall conduct well with low &orward
bias! >ith &urther increases in bias it reaches the negative resistance range where
current will actuall go down! This is achieved b heavil-doped p and n "aterials that
creates a ver thin depletion region!
Fig.3-2: Tunnel diode s"bol and characteristic curve!55
3. SPECIAL-PURPOSE DIODE
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3. SPECIAL PURPOSE DIODE
3.6 Troubleshooting
lthough power suppl outputs generall use #* regulators$ %ener diodes can be usedas a voltage regulator when less precise regulation and low current is acceptable!
Fig.3-2* Zener-regulated power suppl test!
The "eter readings
o& 13!3 V &or no-load
check and 17!@ V &or
&ull-load test indicateappro6i"atel the
e6pected output
voltage o& 13 V!
properl &unctioning
%ener will work to
"aintain the outputvoltage within certain
li"its despite
changes in load!
57
3. SPECIAL-PURPOSE DIODE
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3 S C U OS O
C#,e-1* ener Dio!e Oen
#n no-load check$ output voltage is ,7 V as shown in 9ig! 5-,
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