principles of semiconductor devices-l23

8/8/2019 Principles of Semiconductor Devices-L23

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NCN

Lecture23:[email protected]

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TopicMap

E uilibrium DC Small Lar e Circuitssignal Signal

Diode

Schottky

BJT/HBT

MOSFET

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WhyshouldwestudyACResponse?

SeriesResistance

ConductanceJunction

Capacitance

www.sc -toy.com

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DiffusionCapacitance


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Outline

1) Conductanceand

series

resistance

2) Majoritycarrierjunctioncapacitance

3) Minoritycarrierdiffusioncapacitance

4 Conclusion

. ,

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ForwardBiasConductance2

3ln(I)

( )( )/ 1A Sq V R I mo I I e = V6,7

G 5RG, diff, ambiploar

0

ln ( )o A Sq V R I

I m

= CJ

m dV

Cdiff

0( )FBS

q IgR

I+ +=( )S

oq I I dI = +

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ReverseBiasConductance

2

3ln(I)

V6,7

( ) /1A S

q V R I m

o I I e

=

0i

bi A

qn B V V

5

0

I 02

i

bi A

qn B V V

01 iqn B=RSCJ

2RB bi Ag V V C

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Outline

1) Conductanceand

series

resistance

2) Majoritycarrierjunctioncapacitance3) Minoritycarrierdiffusioncapacitance

4 Conclusion

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JunctionCapacitance

Conductance

VDC

JunctionCapacitance


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MajorityCarrierJunctionCapacitance

Conductance

JunctionCapacitance

x


x

VA>0

Measure

0 0

0 02 2 ( )

s sJ

n ps s

bi A

K A K ACW W K K

V V

= =+

+

Va

0

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D A


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MeasurementofBuiltinPotential

1 2V V

0( ) J D sC qN x K A

(Assumesinglesidedp+njunction)

plot CJ

measureCJ

VAVA

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Vbi


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DielectricRelaxationTime(majorityside)

1n

n n

dn dJ R G= +n N N J qn E qD n= +

t q x

( ) ( )1 Nd n d qn d = =E E

VDC

D Ndt q dx dx

d qE0

0

D A

Sdx k =

Nd n n

0 0S Sndt k k = =

0t t

0 0.1 pssdK

=

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0

0 0( ) S d

kn t n e n e

= =


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Outline

1) Conductanceandseriesresistance

2) Majoritycarrierjunctioncapacitance

3) Minoritycarrierdiffusioncapacitance4 Conclusion

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or nor ty arr ers

dn

np

N N N qn q

dx

= +

1 nN N

n dJr g

t q dx

= +

DC

( ) ( )20 02

j t j t j tdc ac dc ac dc ac

N

n n n e d n n n e n n eD

t dx

+ + + + + =

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DiffusionCapacitanceforMinorityCarriers

( ) ( )20 0dc dj t j t

acc dc

j t

ac acn e nn d nD

n n e n en + + + + +

=

nt dx

2 2

ac dc acdc j t j t j t d dn n n n

n D ee e

= +

n nd dxx

2n n

x x

dc dc L Ld n n + 2N

n

dcdx

2* * *

AC : 0 1 n n n x x x

L L Lac acd n n

D n Ce De Ce +

= + + = ann

c

dx

* *

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n n n n n n n


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ACBoundaryConditions

2

( 0) 1cdqV

i kTdc

nn x e

N

= =

( )2

1

j t

acdc V eqkTj t

ac j t

A ac

Vi

dcn een

p e

n

N

+ + = +

n

2

1

dcj t

acqV q

i kT kT dc a

V e

cj t nn n ee e

+ A

2 dcqV

acki

t

T

jqVn e

xn

AN kT

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( 0)dc

ac kTac

i

A

qVn x e C

kT

n

N

= = =


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ACCurrentandImpedance

2

( 0)dcqV

ac i kTac

qV nn x e

kT NC = = =

* * *n n n

x x x

L L Ln x D

+

= +

2 dcqV

ac n i kTacVd n qD q n

ac

*

0

ac n

x n Adx L kT N =

2 2

0*1

dcq

n

n

Vac n i kT

ac

ac A

J q D nY e G

V kTj

NL+= =

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DiffusionConductanceandCapacitance

1Y G C G= + +

DG

ac n

Separateinreal&

1/ 2

imaginaryparts

2 20 1 12

D nG = + +

1/ 22 20 1 1

2D n

GC = +

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D


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Conclusion

1) Smallsignalresponserelevantformanyanalog

applications.2) SmallsignalparametersalwaysrefertotheDC

operatingconditions,assuchtheparameterchanges

.

3) Importanttodistinguishbetweenmajorityand

m nor ycarr ercapac ance. e rre a veimportancedependsonspecificapplications.

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principles of semiconductor devices-l23

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