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8/8/2019 Principles of Semiconductor Devices-L23
1/19
www.nanohub.org
NCN
Lecture23:[email protected]
Alam ECE606S09 1
8/8/2019 Principles of Semiconductor Devices-L23
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TopicMap
E uilibrium DC Small Lar e Circuitssignal Signal
Diode
Schottky
BJT/HBT
MOSFET
Alam ECE606S09 2
8/8/2019 Principles of Semiconductor Devices-L23
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WhyshouldwestudyACResponse?
SeriesResistance
ConductanceJunction
Capacitance
www.sc -toy.com
Alam ECE606S09 3
DiffusionCapacitance
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Outline
1) Conductanceand
series
resistance
2) Majoritycarrierjunctioncapacitance
3) Minoritycarrierdiffusioncapacitance
4 Conclusion
. ,
Alam ECE606S09 4
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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 = +
Alam ECE606S09 5
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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
DiffusionCapacitance
Alam ECE606S09 8
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MajorityCarrierJunctionCapacitance
Conductance
JunctionCapacitance
x
DiffusionCapacitance
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
Alam ECE606S09 9
D A
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MeasurementofBuiltinPotential
1 2V V
0( ) J D sC qN x K A
(Assumesinglesidedp+njunction)
plot CJ
measureCJ
VAVA
Alam ECE606S09 10
Vbi
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8/8/2019 Principles of Semiconductor Devices-L23
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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
=
Alam ECE606S09 12
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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DiffusionCapacitance
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
* *
Alam ECE606S09 15
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
Alam ECE606S09 16
( 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+= =
Alam ECE606S09 17
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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 = +
Alam ECE606S09 18
D
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Conclusion
1) Smallsignalresponserelevantformanyanalog
applications.2) SmallsignalparametersalwaysrefertotheDC
operatingconditions,assuchtheparameterchanges
.
3) Importanttodistinguishbetweenmajorityand
m nor ycarr ercapac ance. e rre a veimportancedependsonspecificapplications.
Alam ECE606S09 19