©rlc l10-16feb20111 ideal junction theory assumptions e x = 0 in the chg neutral reg. (cnr) mb...
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©rlc L10-16Feb2011
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Ideal JunctionTheory
Assumptions
• Ex = 0 in the chg neutral reg. (CNR)
• MB statistics are applicable• Neglect gen/rec in depl reg (DR)• Low level injections apply so that
np < ppo for -xpc < x < -xp, and pn < nno for xn < x < xnc
• Steady State conditions
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Forward Bias Energy Bands
1eppkT/EEexpnp ta VV0nnFpFiiequilnon
1/exp 0 ta VV
ppFiFniequilnon ennkTEEnn
Ev
Ec
EFi
xn xnc-xpc -xp 0
q(Vbi-Va)
EFPEFNqVa
x
Imref, EFn
Imref, EFp
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Law of the junction(follow the min. carr.)
t
bia
n
p
p
na
t
bi
no
po
po
no
po
not
no
pot2
i
datbi
V
V-Vexp
n
n
pp
,0V when and
,V
V-exp
n
n
pp
get to Invert
.nn
lnVp
plnV
n
NNlnVV
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Law of the junction (cont.)
t
a
p
t
a
n
t
a
t
a
t
bi
t
bia
Vixpp
Vixnn
V
no
iVp
no
pon
Vnopo
Vpn
ennpennp
en
nep
n
np
ennaepp
V
2
V
2
V2V
VV-V
also ,
Junction theof Law the
have We
nd for So
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Law of the junction (cont.)
dnonapop
ppnn
ppopppop
nnonnnon
a
Nnn and Npp
injection level- low Assume
.pn and pn Assume
.ppp ,nnn and
,nnn ,ppp So
. 0V for nnot' eq.-non to Switched
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pt
apop
nt
anon
V
V-
pononoV
V-V
pon
t
biaponno
xx at ,1VV
expnn sim.
xx at ,1VV
exppp so
,epp ,pepp
giving V
V-Vexpppp
t
bi
t
bia
InjectionConditions
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Ideal JunctionTheory (cont.)
Apply the Continuity Eqn in CNR
ncnn
ppcp
xxx ,Jq1
dtdn
tn
0
and
xxx- ,Jq1
dtdp
tp
0
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Ideal JunctionTheory (cont.)
ppc
nn
p2p
2
ncnpp
n2n
2
ppx
nnxx
xxx- for ,0D
n
dx
nd
and ,xxx for ,0D
p
dx
pd
giving dxdp
qDJ and
dxdn
qDJ CNR, the in 0E Since
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Ideal JunctionTheory (cont.)
)contacts( ,0xnxp and
,1en
xn
pxp
B.C. with
.xxx- ,DeCexn
xxx ,BeAexp
So .D L and D L Define
pcpncn
VV
po
pp
no
nn
ppcL
xL
x
p
ncnL
xL
x
n
pp2pnn
2n
ta
nn
pp
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Excess minoritycarrier distr fctn
1eLWsinh
Lxxsinhnxn
,xxW ,xxx- for and
1eLWsinh
Lxxsinhpxp
,xxW ,xxx For
ta
ta
VV
np
npcpop
ppcpppc
VV
pn
pncnon
nncnncn
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Carrier Injection
-xp
xn-xpc 0
ln(carrier conc)ln Naln Nd
ln ni
ln ni2/Nd
ln ni2/Na
xnc
x
~Va/Vt~Va/Vt
1enxn t
aV
V
popp
1epxp t
aV
V
nonn
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Minority carriercurrents
1eLWsinh
Lxxcosh
LNDqn
xxx- for ,qDxJ
1eLWsinh
Lxxcosh
LN
Dqn
xxx for ,qDxJ
ta
p
ta
n
VV
np
npc
na
n2i
ppcdx
ndnn
VV
pn
pnc
pd
p2i
ncndxpd
pp
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Evaluating thediode current
p/nn/pp/nd/a
p/n2isp/sn
spsns
VV
spnnp
LWcothLN
DqnJ
sdefinition with JJJ where
1eJxJxJJ
then DR, in gen/rec no gminAssu
ta
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Special cases forthe diode current
nd
p2isp
pa
n2isn
nppn
pd
p2isp
na
n2isn
nppn
WN
DqnJ and ,
WND
qnJ
LW or ,LW :diode Short
LN
DqnJ and ,
LND
qnJ
LW or ,LW :diode Long
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Ideal diodeequation• Assumptions:
– low-level injection– Maxwell Boltzman statistics– Depletion approximation– Neglect gen/rec effects in DR– Steady-state solution only
• Current dens, Jx = Js expd(Va/Vt)
– where expd(x) = [exp(x) -1]
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Ideal diodeequation (cont.)• Js = Js,p + Js,n = hole curr + ele curr
Js,p = qni2Dp coth(Wn/Lp)/(NdLp) =
qni2Dp/(NdWn), Wn << Lp, “short” =
qni2Dp/(NdLp), Wn >> Lp, “long”
Js,n = qni2Dn coth(Wp/Ln)/(NaLn) =
qni2Dn/(NaWp), Wp << Ln, “short” =
qni2Dn/(NaLn), Wp >> Ln, “long”
Js,n << Js,p when Na >> Nd
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Diffnt’l, one-sided diode conductance
Va
IDStatic (steady-state) diode I-V characteristic
VQ
IQ QVa
DD dV
dIg
t
asD V
VdexpII
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Diffnt’l, one-sided diode cond. (cont.)
DQ
t
dQd
QDDQt
DQQd
tat
tQs
Va
DQd
tastasD
IV
g1
Vr ,resistance diode The
. VII where ,V
IVg then
, VV If . V
VVexpI
dV
dIVg
VVdexpIVVdexpAJJAI
Q
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Charge distr in a (1-sided) short diode
• Assume Nd << Na
• The sinh excess minority carrier distribution becomes linear for Wn << Lp
pn(xn)=pn0expd(Va/Vt)
• Total chg = Q’p = Q’p = qpn(xn)Wn/2x
n
x
xnc
pn(xn
)
Wn = xnc-
xn
Q’p
pn
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Charge distr in a 1-sided short diode
• Assume Quasi-static charge distributions
• Q’p = Q’p =
qpn(xn)Wn/2
• dpn(xn) = (W/2)*
{pn(xn,Va+V) -
pn(xn,Va)}xn
xxnc
pn(xn,Va)
Q’p
pn pn(xn,Va+V)
Q’p
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Cap. of a (1-sided) short diode (cont.)
p
x
x p
ntransitQQ
transitt
DQ
pt
DQQ
taaa
a
Ddx
Jp
qVV
V
I
DV
IV
VVddVdV
dVA
nc
n2W
Cr So,
. 2W
C ,V V When
exp2
WqApd2
)W(xpqAd
dQC Define area. diode A ,Q'Q
2n
dd
2n
dta
nn0nnn
pdpp
1epxp t
aV
V
nonn
nd
pisp
pa
nisn WN
DqnJ
WN
DqnJ 22 and ,
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General time-constant
np
a
nnnn
a
pppp
pnVa
pn
Va
DQd
CCC ecapacitanc diode total
the and ,dVdQ
Cg and ,dV
dQCg
that so time sticcharacteri a always is There
ggdV
JJdA
dVdI
Vg
econductanc the short, or long diodes, all For
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General time-constant (cont.)
times.-life carr. min. respective the
, and side, diode long
the For times. transit charge physical
the ,D2
W and ,
D2W
side, diode short the For
n0np0p
n
2p
transn,np
2n
transp,p
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General time-constant (cont.)
Fdd
transitminF
gC
and 111
by given average
the is time transition effective The
sided-one usually are diodes Practical
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References *Fundamentals of Semiconductor Theory and Device
Physics, by Shyh Wang, Prentice Hall, 1989. **Semiconductor Physics & Devices, by Donald A.
Neamen, 2nd ed., Irwin, Chicago. M&K = Device Electronics for Integrated Circuits, 3rd
ed., by Richard S. Muller, Theodore I. Kamins, and Mansun Chan, John Wiley and Sons, New York, 2003.
• 1Device Electronics for Integrated Circuits, 2 ed., by Muller and Kamins, Wiley, New York, 1986.
• 2Physics of Semiconductor Devices, by S. M. Sze, Wiley, New York, 1981.
• 3 Physics of Semiconductor Devices, Shur, Prentice-Hall, 1990.