Chapter 1- Semiconductor Diodes

7/23/2019 Chapter 1- Semiconductor Diodes

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Chapter 1:

Semiconductor Diodes



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DiodesDiodes

The diode is a 2-terminal device.

A diode ideally conducts in only one direction.



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Diode CharacteristicsDiode Characteristics

Conduction RegionConduction Region

• The voltage across the diode is 0 V

• The current is infinite

• The forward resistance is defined as

R F = V

F/ I

F

• The diode acts like a short

Non-Conduction RegionNon-Conduction Region

•

All of the voltage is across the diode• The current is 0 A

• The reverse resistance is defined as

R R = VR / IR

• The diode acts like open



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Semiconductor MaterialsSemiconductor Materials

Materials commonly used in the

development o semiconductor

devices:

• ilicon !i"ilicon !i"

• #alliu$ nitride !#a%"#alliu$ nitride !#a%"

• &inc o'ide !&n("&inc o'ide !&n("

• #er$aniu$ !#e"#er$aniu$ !#e"

• #alliu$ Arsenide !#aAs"#alliu$ Arsenide !#aAs"



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DopingDoping

The electrical characteristics o silicon and germanium areimproved !y adding materials in a process called doping.

There are "ust t#o types o doped semiconductor materials:

nn)type)type

p p)type)type

• n)type $aterials contain an e'cess of conduction *and

electrons.

• p)type $aterials contain an e'cess of valence *and holes.



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p-n p-n $unctions$unctions

(ne end of a silicon or ger$aniu$ crystal can *e doped

as a p)type $aterial and the other end as an n)type

$aterial.

The result is a p-n p-n +unction +unction.



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p-n p-n $unctions$unctions

• %t the p-n "unction& the e'cess

conduction-!and electrons on the n-

type side are attracted to the

valence-!and holes on the p-type

side.

The electrons in the n-type materialmigrate across the "unction to the p-

type material (electron lo#).

• The electron migration results in a

negativenegative charge on the p-type side

o the "unction and a positivepositivecharge on the n-type side o the

"unction.

The result is the for$ation of a

depletion regiondepletion region around the

+unction.



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Diode *perating ConditionsDiode *perating Conditions

% diode has three operating conditions:

• %o *ias

•

Forward *ias• Reverse *ias



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No +iasNo +ias

• %o e'ternal voltage is applied, V D

= 0 V

• %o current is flowing, I D = 0 A

• (nly a $odest depletion region e'ists



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Reverse +iasReverse +ias

-'ternal voltage is applied across the p-n +unction in

the opposite polarity of the p) and n)type $aterials.

• The reverse voltage causes the

depletion region to widen.

• The electrons in the n)type $aterial

are attracted toward the positiveter$inal of the voltage source.

• The holes in the p)type $aterial areattracted toward the negativeter$inal of the voltage source.



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,or#ard +ias,or#ard +ias

-'ternal voltage is applied across the p-n +unction in

the sa$e polarity as the p) and n)type $aterials.

• The forward voltage causes the depletionregion to narrow.

• The electrons and holes are pushedtoward the p-n +unction.

• The electrons and holes have sufficient

energy to cross the p-n +unction.



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%ctual Diode Characteristics%ctual Diode Characteristics

%ote the regions for no

*ias reverse *ias and

forward *ias conditions.

arefully note the scale

for each of these

conditions.



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Ma"ority and Minority CarriersMa"ority and Minority Carriers

T#o currents through a diode:

a+ority arriersa+ority arriers

• The $a+ority carriers in n)type $aterials are electrons.

• The $a+ority carriers in p)type $aterials are holes.

inority arriersinority arriers

• The $inority carriers in n)type $aterials are holes.

• The $inority carriers in p)type $aterials are electrons.



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ener Regionener Region

• The ener region is in thediodes reverse-!ias region.

• %t some point the reverse!ias voltage is so large the

diode !rea/s do#n and thereverse current increasesdramatically.

• The $a'i$u$ reverse voltage that won1t take a diode

into the 2ener region is called the peak inverse voltagepeak inverse voltage

or peak reverse voltagepeak reverse voltage.

• The voltage that causes a diode to enter the 2ener

region of operation is called the 2ener voltage !V2ener voltage !V&&"".



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,or#ard +ias 0oltage,or#ard +ias 0oltage

The point at #hich the diode changes rom no-!iascondition to or#ard-!ias condition occurs #hen theelectrons and holes are given suicient energy tocross the p-n "unction. This energy comes rom the

e'ternal voltage applied across the diode.

The forward *ias voltage re3uired for a,

• galliu$ arsenide diode 4.5 V

• silicon diode 0.6 V

• ger$aniu$ diode 0.7 V



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Temperature ectsTemperature ects

• %s temperature increases it adds energy tothe diode.

• t reduces the re3uired or#ard !ias voltageor or#ard-!ias conduction.

• t increases the amount o reverse current inthe reverse-!ias condition.

• t increases ma'imum reverse !iasavalanche voltage.

• 4ermanium diodes are more sensitive totemperature variations than silicon orgallium arsenide diodes.



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Resistance 5evelsResistance 5evels

• Semiconductors react dierently to DC

and %C currents.

• There are three types o resistance:

6 DC (static) resistance6 %C (dynamic) resistance

6 %verage %C resistance



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DC (Static) ResistanceDC (Static) Resistance

,or a speciic applied DC voltage V D& the diode

has a speciic current I D& and a speciic

resistance R D.

D

DD

I

VR =



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%C (Dynamic) Resistance%C (Dynamic) Resistance

n the or#ard !ias region: B

D

d r I

r +=′$V58

• The resistance depends on the a$ount of current ! I D

" in the diode.

• The voltage across the diode is fairly constant !58 $V for 59°".

• r B

ranges fro$ a typical 0.4 for high power devices to 5 for low

power general purpose diodes. In so$e cases r B

can *e ignored.

In the reverse *ias region,∞ rd

The resistance is effectively infinite. The diode acts like an open.



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%verage %C Resistance%verage %C Resistance

pt.topt. d

d

av

ΔI

ΔV r =

A resistance can *e

calculated using thecurrent and voltage

values for two points on

the diode characteristic

curve.



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Diode 3uivalent CircuitDiode 3uivalent Circuit



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Diode CapacitanceDiode Capacitance

In reverse *ias the depletion layer is very large. The diode1s strong positive and

negative polarities create capacitance T. The a$ount of capacitance depends

on the reverse voltage applied.

In forward *ias storage capacitance or diffusion capacitance !:" e'ists as the

diode voltage increases.



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Reverse Recovery Time (trr)Reverse Recovery Time (trr)

Reverse recovery ti$eReverse recovery ti$e is the ti$e re3uired for a diode to

stop conducting once it is switched fro$ forward *ias to

reverse *ias.



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Diode Speciication SheetsDiode Speciication Sheets

:ata a*out a diode is presented unifor$ly for $any different diodes. This$akes cross)$atching of diodes for replace$ent or design easier.

4. Forward Voltage !VF" at a specified current and te$perature

5. a'i$u$ forward current !IF" at a specified te$perature

7. Reverse saturation current !IR " at a specified voltage and

te$perature

;. Reverse voltage rating or V!<R" at a specified te$perature

9. a'i$u$ power dissipation at a specified te$perature8. apacitance levels

6. Reverse recovery ti$e , t rr

. (perating te$perature range



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Diode Chec/er Diode Chec/er

any digital $ulti$eters have a diode checking function.

The diode should *e tested out of circuit.

A nor$al diode e'hi*its its forward voltage,

• #alliu$ arsenide 4.5 V

• ilicon diode 0.6 V

• #er$aniu$ diode 0.7 V



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*hmmeter *hmmeter An oh$$eter set on a low (h$s scale can *e used to test a diode.

The diode should *e tested out of circuit.



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Curve Tracer Curve Tracer

A curve tracer displays the characteristic curve of a diode in thetest circuit. This curve can *e co$pared to the specifications of

the diode fro$ a data sheet.



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*ther Types o Diodes*ther Types o Diodes

• ener diode

• 5ight-emitting diode

•Diode arrays



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ener Diodeener Diode

• % ener is a diode operated in reverse

!ias at the ener voltage (0).

• Common ener voltages are !et#een

1.7 0 and 288 0



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5ight-mitting Diode (5D)5ight-mitting Diode (5D)

An >-: e$its photons when it is forward *iased.

These can *e in the infrared or visi*le spectru$.

The forward *ias voltage is usually in the range of 5 V to 7 V.



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Diode %rraysDiode %rrays

ultiple diodes can *e packaged togetherin an integrated circuit !I".

Common %nodeCommon %node

Common CathodeCommon CathodeA variety of co$*inations

e'ist.

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Chapter 1- Semiconductor Diodes