unit 2 diodes check average check 2

8/9/2019 UNIT 2 Diodes Check Average Check 2

1/19

Kristin Ackerson, Virginia Tech EEKristin Ackerson, Virginia Tech EESpring 2002Spring 2002


2/19

What Are Diodes Made Out Of?What Are Diodes Made Out Of?


• Silicon (Si) and Germanium (Ge) are the two mostSilicon (Si) and Germanium (Ge) are the two most

common single elements that are used to make Diodes.common single elements that are used to make Diodes.A compound that is commonl used is GalliumA compound that is commonl used is Gallium

Arsenide (GaAs)! especiall in the case of "#DsArsenide (GaAs)! especiall in the case of "#Ds$ecause of it%s large $andgap.$ecause of it%s large $andgap.

• Silicon and Germanium are $oth group & elements!Silicon and Germanium are $oth group & elements!meaning the ha'e & 'alence electrons. heirmeaning the ha'e & 'alence electrons. heirstructure allows them to grow in a shape called thestructure allows them to grow in a shape called thediamond lattice.diamond lattice.

• Gallium is a group element while Arsenide is a groupGallium is a group element while Arsenide is a group* element. When put together as a compound! GaAs* element. When put together as a compound! GaAscreates a +inc$lend lattice structure.creates a +inc$lend lattice structure.

• ,n $oth the diamond lattice and +inc$lend lattice! each,n $oth the diamond lattice and +inc$lend lattice! each

atom shares its 'alence electrons with its four closestatom shares its 'alence electrons with its four closestneigh$ors. his sharing of electrons is what ultimatelneigh$ors. his sharing of electrons is what ultimatelallows diodes to $e $uild. When dopants from groupsallows diodes to $e $uild. When dopants from groups or * (in most cases) are added to Si! Ge or GaAs it or * (in most cases) are added to Si! Ge or GaAs itchanges the properties of the material so we are a$le tochanges the properties of the material so we are a$le tomake the - and / t pe materials that $ecome themake the - and / t pe materials that $ecome thediode.diode.

SiSi0&0&

SiSi0&0&

SiSi0&0&

SiSi0&0&

SiSi0&0&

SiSi0&0&

SiSi0&0&

SiSi0&0&

SiSi0&0&

he diagram a$o'e shows thehe diagram a$o'e shows the1D structure of the Si cr stal.1D structure of the Si cr stal.

he light green lineshe light green linesrepresent the electronicrepresent the electronic$onds made when the$onds made when the

'alence electrons are shared.'alence electrons are shared.#ach Si atom shares one#ach Si atom shares oneelectron with each of its fourelectron with each of its fourclosest neigh$ors so that itsclosest neigh$ors so that its

'alence $and will ha'e a full 2'alence $and will ha'e a full 2electrons.electrons.


3/19

/ pe Material/

pe Material


/ pe Material3/

pe Material3 When e4tra 'alence electrons are introducedWhen e4tra 'alence electrons are introducedinto a material such as silicon an n t peinto a material such as silicon an n t pematerial is produced. he e4tra 'alencematerial is produced. he e4tra 'alenceelectrons are introduced $ puttingelectrons are introduced $ puttingimpurities or dopants into the silicon. heimpurities or dopants into the silicon. hedopants used to create an n t pe materialdopants used to create an n t pe materialare Group 5 elements. he most commonlare Group 5 elements. he most commonlused dopants from Group 5 are arsenic!used dopants from Group 5 are arsenic!antimon and phosphorus.antimon and phosphorus.

he 1D diagram to the left shows the e4trahe 1D diagram to the left shows the e4traelectron that will $e present when a Group 5electron that will $e present when a Group 5dopant is introduced to a material such asdopant is introduced to a material such assilicon. his e4tra electron is 'er mo$ile.silicon. his e4tra electron is 'er mo$ile.

0&0&0&0&

0*0*

0&0&

0&0&0&0&0&0&

0&0&0&0&


4/19

- pe Material-

pe Material


- pe Material3-

pe Material3 - t pe material is produced when the dopant- t pe material is produced when the dopantthat is introduced is from Group ,,,. Groupthat is introduced is from Group ,,,. Group,,, elements ha'e onl 'alence electrons,,, elements ha'e onl 'alence electronsand therefore there is an electron missing.and therefore there is an electron missing.his creates a hole (h0)! or a positi'e chargehis creates a hole (h0)! or a positi'e chargethat can mo'e around in the material.that can mo'e around in the material.6ommonl used Group ,,, dopants are6ommonl used Group ,,, dopants arealuminum! $oron! and gallium.aluminum! $oron! and gallium.

he 1D diagram to the left shows the holehe 1D diagram to the left shows the holethat will $e present when a Group ,,, dopantthat will $e present when a Group ,,, dopantis introduced to a material such as silicon.is introduced to a material such as silicon.his hole is 7uite mo$ile in the same wa thehis hole is 7uite mo$ile in the same wa thee4tra electron is mo$ile in a n t pe material.e4tra electron is mo$ile in a n t pe material.

0&0&0&0&

00

0&0&

0&0&0&0&0&0&

0&0&0&0&


5/19

he -/ 8unctionhe -/ 8unction


Stead StateStead State 99

--

nn

0 0 0 0 00 0 0 0 000

0 0 0 0 00 0 0 0 0

000 0 0 0 00 0 0 0 0

00

0 0 0 0 00 0 0 0 000

0 0 0 0 00 0 0 0 000

/a/a /d/dMetallurgicalMetallurgical

8unction8unction

Space 6hargeSpace 6harge:egion:egionioni+edioni+ed

acceptorsacceptorsioni+edioni+eddonorsdonors

# ;ield# ;ield

0000 < < < <

h0 drifth0 drift h0 diffusionh0 diffusion e diffusione diffusion e drifte drift== ==


6/19

he -/ 8unctionhe -/ 8unctionStead StateStead State


-- nn

0 0 0 0 00 0 0 0 0

0 0 0 0 00 0 0 0 0

0 0 0 0 00 0 0 0 0

0 0 0 0 00 0 0 0 0

/a/a /d/dMetallurgicalMetallurgical

8unction8unction

Space 6hargeSpace 6harge:egion:egionioni+edioni+ed

acceptorsacceptorsioni+edioni+eddonorsdonors

# ;ield# ;ield

0000 < < < <

h0 drifth0 drift h0 diffusionh0 diffusion e diffusione diffusion e drifte drift== ==== ==

When no e4ternal sourceWhen no e4ternal source

is connected to the pnis connected to the pn >unction! diffusion and >unction! diffusion anddrift $alance each otherdrift $alance each other

out for $oth the holesout for $oth the holesand electronsand electrons

Space 6harge :egion3

Space 6harge :egion3 Also called the depletion region. his regionAlso called the depletion region. his regionincludes the net positi'el and negati'el charged regions. he spaceincludes the net positi'el and negati'el charged regions. he spacecharge region does not ha'e an free carriers. he width of the space chargecharge region does not ha'e an free carriers. he width of the space charge

region is denoted $ W in pn >unction formula%s.region is denoted $ W in pn >unction formula%s.Metallurgical 8unction3Metall

urgical 8unction3 he interface where the p and n t pe materials meet.he interface where the p and n t pe materials meet.

/a /d3/a /d3 :epresent the amount of negati'e and positi'e doping in num$er of:epresent the amount of negati'e and positi'e doping in num$er ofcarriers per centimeter cu$ed. @suall in the range of 9carriers per centimeter cu$ed. @suall in the range of 9 9*9* to 9to 9 11 ..


7/19

he Biased -/ 8unctionhe Biased -/ 8unction


-- nn

00 < < AppliedApplied

#lectric ;ield#lectric ;ield

MetalMetal6ontact6ontact

CCOhmicOhmic6ontact6ontact

(:sE )(:sE )

00 < <

55 appliedapplied

II

he pn >unction is considered $iased when an e4ternal 'oltage is applied.he pn >unction is considered $iased when an e4ternal 'oltage is applied.here are two t pes of $iasing3 ;orward $ias and :e'erse $ias.here are two t pes of $iasing3 ;orward $ias and :e'erse $ias.

hese are descri$ed on then ne4t slide.hese are descri$ed on then ne4t slide.


8/19

he Biased -/ 8unctionhe Biased -/ 8unction


;orward Bias3;orward Bias3 ,n forward $ias the depletion region shrinks slightl in,n forward $ias the depletion region shrinks slightl inwidth. With this shrinking the energ re7uired forwidth. With this shrinking the energ re7uired forcharge carriers to cross the depletion region decreasescharge carriers to cross the depletion region decreasese4ponentiall . herefore! as the applied 'oltagee4ponentiall . herefore! as the applied 'oltageincreases! current starts to flow across the >unction.increases! current starts to flow across the >unction.he $arrier potential of the diode is the 'oltage athe $arrier potential of the diode is the 'oltage atwhich apprecia$le current starts to flow through thewhich apprecia$le current starts to flow through thediode. he $arrier potential 'aries for differentdiode. he $arrier potential 'aries for differentmaterials.materials.

:e'erse Bias3:e'erse Bias3 @nder re'erse $ias the depletion region widens. his@nder re'erse $ias the depletion region widens. hiscauses the electric field produced $ the ions to cancelcauses the electric field produced $ the ions to cancelout the applied re'erse $ias 'oltage. A small leakageout the applied re'erse $ias 'oltage. A small leakage

current! ,s (saturation current) flows under re'erse $iascurrent! ,s (saturation current) flows under re'erse $iasconditions. his saturation current is made up ofconditions. his saturation current is made up ofelectron hole pairs $eing produced in the depletionelectron hole pairs $eing produced in the depletionregion. Saturation current is sometimes referred to asregion. Saturation current is sometimes referred to asscale current $ecause of it%s relationship to >unctionscale current $ecause of it%s relationship to >unction

temperature.temperature.

55 appliedapplied F F

55

appliedapplied


9/19

-roperties of Diodes-ro perties of Diodes


;igure 9.9 H he Diode ransconductance 6ur'e;igure 9.9 H he Diode ransconductance 6ur'e 11

• 55 DD = Bias 5oltage= Bias 5oltage

• ,,DD = 6urrent through= 6urrent throughDiode. ,Diode. , DD is /egati'eis /egati'efor :e'erse Bias andfor :e'erse Bias and-ositi'e for ;orward-ositi'e for ;orward

BiasBias• ,,SS = Saturation= Saturation

6urrent6urrent

• 55 B:B: = Breakdown= Breakdown

5oltage5oltage• 55 = Barrier Potential= Barrier Potential

VoltageVoltage

55 DD

,,DD (mA)(mA)

(nA)(nA)

55 B:B:

E5E5

,,SS


10/19

-roperties of Diodes-ro perties of Diodeshe Shockle #7uationhe Shockle #7uation


• he transconductance cur'e on the pre'ious slide is characteri+ed $he transconductance cur'e on the pre'ious slide is characteri+ed $

the following e7uation3the following e7uation3

,,DD = ,= , SS (e(e 55 DD I Iη 55 H 9) H 9)• As descri$ed in the last slide! ,As descri$ed in the last slide! , DD is the current through the diode! ,is the current through the diode! , SS isis

the saturation current and 5the saturation current and 5 DD is the applied $iasing 'oltage.is the applied $iasing 'oltage.• 55 is the thermal e7ui'alent 'oltage and is appro4imatel 1J m5 at roomis the thermal e7ui'alent 'oltage and is appro4imatel 1J m5 at room

temperature. he e7uation to find 5temperature. he e7uation to find 5 at 'arious temperatures is3at 'arious temperatures is3

55 == kk

77 k = 9. 2 4 9k = 9. 2 4 9 11 8IK = temperature in Kel'in 7 = 9.J 4 98IK = temperature in Kel'in 7 = 9.J 4 9 9L9L 66

η is the emission coefficient for the diode. ,t is determined $ the wais the emission coefficient for the diode. ,t is determined $ the wathe diode is constructed. ,t somewhat 'aries with diode current. ;or athe diode is constructed. ,t somewhat 'aries with diode current. ;or asilicon diodesilicon diode η is around 1 for low currents and goes down to a$out 9 atis around 1 for low currents and goes down to a$out 9 athigher currentshigher currents


11/19

Diode 6ircuit ModelsDiode 6ircuit Models


he ,deal Diodehe ,deal DiodeModelModel

he diode is designed to allow current to flow inhe diode is designed to allow current to flow inonl one direction. he perfect diode would $e aonl one direction. he perfect diode would $e a

perfect conductor in one direction (forward $ias)perfect conductor in one direction (forward $ias)and a perfect insulator in the other directionand a perfect insulator in the other direction(re'erse $ias). ,n man situations! using the ideal(re'erse $ias). ,n man situations! using the idealdiode appro4imation is accepta$le.diode appro4imation is accepta$le.

#4ample3 Assume the diode in the circuit $elow is ideal. Determine the#4ample3 Assume the diode in the circuit $elow is ideal. Determine the'alue of ,'alue of , DD if a) 5if a) 5 AA = * 'olts (forward $ias) and $) 5= * 'olts (forward $ias) and $) 5 AA = * 'olts (re'erse= * 'olts (re'erse$ias)$ias)

00

< < 55 AA

,,DD

:: SS = *= * a) With 5a) With 5 AA F the diode is in forward $iasF the diode is in forward $iasand is acting like a perfect conductor so3and is acting like a perfect conductor so3

,,DD = 5= 5 AA I: I: SS = * 5 I *= * 5 I * = 9 mA= 9 mA

$) With 5$) With 5 AA the diode is in re'erse $ias the diode is in re'erse $iasand is acting like a perfect insulator!and is acting like a perfect insulator!therefore no current can flow and ,therefore no current can flow and , DD = .= .


12/19

Diode 6ircuit ModelsDiode 6ircuit Models


he ,deal Diode withhe ,deal Diode withBarrier -otentialBarrier -otential

his model is more accurate than the simplehis model is more accurate than the simpleideal diode model $ecause it includes theideal diode model $ecause it includes the

appro4imate $arrier potential 'oltage.appro4imate $arrier potential 'oltage.:emem$er the $arrier potential 'oltage is the:emem$er the $arrier potential 'oltage is the'oltage at which apprecia$le current starts to'oltage at which apprecia$le current starts toflow.flow.

#4ample3 o $e more accurate than >ust using the ideal diode model#4ample3 o $e more accurate than >ust using the ideal diode model

include the $arrier potential. Assume 5include the $arrier potential. Assume 5 = . 'olts (t pical for a= . 'olts (t pical for agermanium diode) Determine the 'alue of ,germanium diode) Determine the 'alue of , DD if 5if 5 AA = * 'olts (forward $ias).= * 'olts (forward $ias).

00

< < 55 AA

,,DD

:: SS = *= * With 5With 5 AA F the diode is in forward $iasF the diode is in forward $iasand is acting like a perfect conductorand is acting like a perfect conductorso write a K5" e7uation to find ,so write a K5" e7uation to find , DD33

= 5 = 5 AA H , H ,DD:: SS 5 5

,,DD = 5= 5 AA 5 5 = &. 5 = L& mA= &. 5 = L& mA:: SS **

55 00

55 00


13/19

Diode 6ircuit ModelsDiode 6ircuit Modelshe ,deal Diodehe ,deal Diode

with Barrierwith Barrier-otential and-otential and

"inear ;orward"inear ;orward:esistance:esistance

his model is the most accurate of the three. ,t includes ahis model is the most accurate of the three. ,t includes alinear forward resistance that is calculated from the slope oflinear forward resistance that is calculated from the slope of

the linear portion of the transconductance cur'e. Nowe'er!the linear portion of the transconductance cur'e. Nowe'er!this is usuall not necessar since the :this is usuall not necessar since the : ;; (forward(forwardresistance) 'alue is prett constant. ;or low powerresistance) 'alue is prett constant. ;or low powergermanium and silicon diodes the :germanium and silicon diodes the : ;; 'alue is usuall in the'alue is usuall in the1 to * ohms range! while higher power diodes ha'e a :1 to * ohms range! while higher power diodes ha'e a : ;; 'alue closer to 9 ohm.'alue closer to 9 ohm.

"inear -ortion of"inear -ortion oftransconductancetransconductance

cur'ecur'e

55 DD

,,DD

55DD

,,DD:: ;; == 55 DD

,,DD


0055 :: FF


14/19

Diode 6ircuit ModelsDiode 6ircuit Modelshe ,deal Diodehe ,deal Diode

with Barrierwith Barrier-otential and-otential and

"inear ;orward"inear ;orward:esistance:esistance


#4ample3 Assume the diode is a low power diode#4ample3 Assume the diode is a low power diodewith a forward resistance 'alue of * ohms. hewith a forward resistance 'alue of * ohms. he$arrier potential 'oltage is still3 5$arrier potential 'oltage is still3 5 = . 'olts (t pical= . 'olts (t picalfor a germanium diode) Determine the 'alue of ,for a germanium diode) Determine the 'alue of , DD ifif55 AA = * 'olts.= * 'olts.

00

< < 55 AA

,,DD

:: SS = *= *

55 00

:: ;;

Once again! write a K5" e7uationOnce again! write a K5" e7uationfor the circuit3for the circuit3

= 5 = 5 AA H , H ,DD:: SS 55 , , DD:: ;;

,,DD = 5= 5 AA 5 5 = * H . = 2*.* mA = * H . = 2*.* mA

:: SS 0 :0 : ;; * 0 * * 0 *


15/19

he -ointhe -oint


he operating point or point of the diode is the 7uiescent or nohe operating point or point of the diode is the 7uiescent or nosignal condition. he point is o$tained graphicall and is reall onlsignal condition. he point is o$tained graphicall and is reall onl

needed when the applied 'oltage is 'er close to the diode%s $arrierneeded when the applied 'oltage is 'er close to the diode%s $arrierpotential 'oltage. he e4amplepotential 'oltage. he e4ample $elow that is continued on the ne4t$elow that is continued on the ne4t

slide! shows how the point is determined using theslide! shows how the point is determined using thetransconductance cur'e and the load line.transconductance cur'e and the load line.

00

< < 55 AA

= J5= J5

,,DD

:: SS = 9= 9

55 00

;irst the load line is found $ su$stituting in;irst the load line is found $ su$stituting in

different 'alues of 5different 'alues of 5 into the e7uation for ,into the e7uation for , DD usingusingthe ideal diode with $arrier potential model for thethe ideal diode with $arrier potential model for thediode. With :diode. With : SS at 9 ohms the 'alue of :at 9 ohms the 'alue of : ;; wouldn%t ha'e much impact on the results.wouldn%t ha'e much impact on the results.

,,DD = 5= 5 AA H 5 H 5

:: SS

@sing 5@sing 5 'alues of 'olts and 9.& 'olts we o$tain'alues of 'olts and 9.& 'olts we o$tain,,DD 'alues of J mA and &.J mA respecti'el . /e4t'alues of J mA and &.J mA respecti'el . /e4t

we will draw the line connecting these two pointswe will draw the line connecting these two pointson the graph with the transconductance cur'e.on the graph with the transconductance cur'e.

his line is the load line.his line is the load line.


16/19

he -ointhe -oint,,DD (mA)(mA)

55 DD (5olts)(5olts)

11

&&

JJ

22

99

9191

.1.1 .&.& .J.J .2.2 9.9. 9.19.1 9.&9.&

hehetransconductancetransconductance

cur'e $elow is for acur'e $elow is for a

Silicon diode. heSilicon diode. he point in this point in thise4ample is locatede4ample is located

at . 5 and *. mA.at . 5 and *. mA.

&.J&.J


..

*.*.

-oint3 -oint3 he intersection of thehe intersection of theload line and theload line and thetransconductance cur'e.transconductance cur'e.


17/19

D namic :esistanceD namic :esistance


he d namic resistance of the diode is mathematicall determinedhe d namic resistance of the diode is mathematicall determinedas the in'erse of the slope of the transconductance cur'e.as the in'erse of the slope of the transconductance cur'e.

herefore! the e7uation for d namic resistance is3herefore! the e7uation for d namic resistance is3r r ;; == η 55

,,DD

he d namic resistance is used in determining the 'oltage drophe d namic resistance is used in determining the 'oltage dropacross the diode in the situation where a 'oltage source isacross the diode in the situation where a 'oltage source issuppl ing a sinusoidal signal with a dc offset.suppl ing a sinusoidal signal with a dc offset.

he ac component of the diode 'oltage is found using thehe ac component of the diode 'oltage is found using thefollowing e7uation3following e7uation3

'' ;; = '= ' acac r r ;;

r r ;; 0 :0 : SShe 'oltage drop through the diode is a com$ination of the ac andhe 'oltage drop through the diode is a com$ination of the ac and

dc components and is e7ual to3dc components and is e7ual to3== 0


18/19

D namic :esistanceD namic :esistance


#4ample3#4ample3 @se the same circuit used for the point e4ample $ut@se the same circuit used for the point e4ample $utchange the 'oltage source so it is an ac source with a dc offset. hechange the 'oltage source so it is an ac source with a dc offset. hesource 'oltage is now! 'source 'oltage is now! ' inin = J 0 sin(wt) 5olts. ,t is a silicon diode so the= J 0 sin(wt) 5olts. ,t is a silicon diode so the$arrier potential 'oltage is still . 'olts.$arrier potential 'oltage is still . 'olts.

00

'' inin

,,DD

:: SS = 9= 9

55 00

he D6 component of the circuit is thehe D6 component of the circuit is thesame as the pre'ious e4ample andsame as the pre'ious e4ample and

therefore ,therefore , DD == J5 H . 5J5 H . 5 = *. mA= *. mA

99r r ;; == η 55 == 9 P 1J m59 P 1J m5 = &.L= &.L

,,DD *. mA*. mA

η = 9 is a good appro4imation if the dc= 9 is a good appro4imation if the dc

current is greater than 9 mA as it is in thiscurrent is greater than 9 mA as it is in thise4ample.e4ample.

'' ;; = '= ' acac r r ;; = sin(wt) 5 &.L= sin(wt) 5 &.L = &.22 sin(wt) m5 = &.22 sin(wt) m5

r r ;; 0 :0 : SS &.L&.L 0 90 9

herefore! 5herefore! 5 DD = 0 &.L sin (wt) m5 (the 'oltage drop across the= 0 &.L sin (wt) m5 (the 'oltage drop across the


19/19


pes of Diodes and heir @sespes of Diodes and heir @ses

-/ 8unction-/ 8unction

Diodes3Diodes3

Are used to allow current to flow in one directionAre used to allow current to flow in one directionwhile $locking current flow in the oppositewhile $locking current flow in the oppositedirection. he pn >unction diode is the t pical diodedirection. he pn >unction diode is the t pical diodethat has $een used in the pre'ious circuits.that has $een used in the pre'ious circuits.

AA KK

Schematic S m$ol for a -/Schematic S m$ol for a -/8unction Diode8unction Diode

-- nn

:epresentati'e Structure for:epresentati'e Structure fora -/ 8unction Diodea -/ 8unction Diode

Qener Diodes3Qener Diodes3 Are specificall designed to operate under re'erseAre specificall designed to operate under re'erse$reakdown conditions. hese diodes ha'e a 'er$reakdown conditions. hese diodes ha'e a 'eraccurate and specific re'erse $reakdown 'oltage.accurate and specific re'erse $reakdown 'oltage.

AA KK

Schematic S m$ol for aSchematic S m$ol for a

Qener DiodeQener Diode

unit 2 diodes check average check 2

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