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901 37500 光電導論
Chapter 3 Semiconductor Science and
Light Emitting Diodes
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Outline3.1 Semiconductor Concepts and Energy Bands3.2 Direct and
Indirect Bandgap Semiconductor3.3 pn Junction Principles3.4 The pn
Junction Band Diagram3.5 Light Emitting Diodes3.6 LED Materials3.7
Heterojunction High Intensity LEDs3.8 LED Characteristics3.9 LEDs
for Optical Fiber Communications
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3-1 Semiconductor Concepts and Energy Bands
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2 s Band
Overlapping energybands
Electrons2 s2 p
3 s3 p
1 s 1sSOLIDATOM
E = 0
Free electronElectron Energy, E
2 p Band
3s BandVacuum
level
In a metal the various energy bands overlap to give a single
bandof energies that is only partially full of electrons. There are
stateswith energies up to the vacuum level where the electron is
free.?1999 S.O. Kasap, Optoelectronics (Prentice Hall)
A. Energy Band Diagrams
Li : 3 e- (1023 Li atoms)
Metal: partially filled band
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Electron energy, E
Conduction Band (CB)Empty of electrons at 0 K.
Valence Band (VB)Full of electrons at 0 K.
Ec
Ev
0
Ec+χ
(b)
Band gap = Eg
(a)
Covalent bond Si ion core (+4e)
(a) A simplified two dimensional view of a region of the Si
crystalshowing covalent bonds. (b) The energy band diagram of
electrons in theSi crystal at absolute zero of temperature.
SemiconductorValence band (VB)Conduction band (CB)Electron
affinity χBandgap (Eg)
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e–hole
CB
VB
Ec
Ev
0
Ec+χ
EgFree e–hυ > Eg
Hole h+
Electron energy, E
(a) A photon with an energy greater than Eg can excite an
electron from the VB to the CB.(b) Each line between Si-Si atoms is
a valence electron in a bond. When a photon breaks aSi-Si bond, a
free electron and a hole in the Si-Si bond is created.
hυ
(a) (b)
Effective mass:Electron in CB: me*Hole in VB: mh*
(take into account the e- in the CB interacts with a “periodic
potential” as it movers through the crystal)
viewed as free carriers with an effective mass
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B. Semiconductor Statistics
( )21
=FEf However, there may be no state.
1( )1 exp F
B
f EE E
k T
=⎛ ⎞−
+ ⎜ ⎟⎝ ⎠
(1)Probability of finding e- with energy E(Fermi-Dirac
function)
Density of States (DOS) ( ) 21)( cCB EEEg −∝
Probability of finding h+ with energy E )(1 Ef−
eVEF =Δ (2)Electric work input or output per e-
In equilibrium, no applied field, under dark environment:
FE( : uniform)0=Δ FE
Under applied potential V:
(# of e- states per unit energy per unit volume)
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E
g(E) fE)
EF
nE(E) or pE(E)
E E
Forelectrons
For holes
[1–f(E)]
nE(E)
pE(E)
Area = p
Area = � nE(E)dE = n
Ec
EvEv
Ec
0
Ec+χ
EF
VB
CB
(a) (b) (c) (d)g(E) ∝ (E–Ec)
1/2
(a) Energy band diagram. (b) Density of states (number of states
per unit energy perunit volume). (c) Fermi-Dirac probability
function (probability of occupancy of astate). (d) The product of
g(E) and f(E) is the energy density of electrons in the CB(number
of electrons per unit energy per unit volume). The area under nE(E)
vs. E isthe electron concentration.
∫ == ndEEnArea E )(
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901 37500 光電導論
( ) ( ) ( ) StatisticsBoltzmann exp ⎟⎠⎞
⎜⎝⎛ −−≈→>>− Tk
EEEfTkEEB
fBfc
g ( ) ( )cc
E
CBEn E f E dE
χ+= ∫ (3)
Electron concentration in CB
exp c FcB
E En Nk T
⎡ ⎤−= −⎢ ⎥
⎣ ⎦(4)
[ ] 2/32* /22 hTkmN Bec π=Effective DOS at the CB edge
Non-degenerate Semiconductor(# of e-h+ from Ed (B- site) into
VB
at room temperature
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e hen epσ μ μ= +2i
d e h d ed
neN e eNN
σ μ μ μ⎛ ⎞
= + ≈⎜ ⎟⎝ ⎠
(8)
(9)
C. Extrinic Semiconductorsn-type semiconductor: add pentavalent
impurity.Donor impurity: e.g. As in Si, --> As+
+e-Nd>>ni
2
, idnn N pn
≈ =
Semiconductor conductivityn-type conductivity
P-type semiconductor: add trivalent impurityAcceptor impurity:
e.g. B in Si, --> B- +h+Na>>ni
pnnNp ia
2
, ==p-type conductivity haeN μσ ≈
Electrons: majority carrier; Holes: minority carrier
nono ppnn == ,
,po pon n p p= =
(in equilibrium)
(in equilibrium)
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Ec
Ev
EFi
CB
EFp
EFnEc
Ev
Ec
EvVB
(a) (c)(b)
Energy band diagrams for (a) intrinsic (b) n-type and (c)
p-typesemiconductors. In all cases, np = ni2. Note that donor and
acceptorenergy levels are not shown.
?1999 S.O. Kasap, Optoelectronics (Prentice Hall)
The level of EF determines the electron and hole
concentrations.
⎥⎦
⎤⎢⎣
⎡ −−=
TkEENn
B
FnCC exp ⎥
⎦
⎤⎢⎣
⎡ −−=
TkEE
NpB
VFpV exp
1 1 ln2 2
cFi v g
v
NE E E kTN
⎛ ⎞= + − ⎜ ⎟
⎝ ⎠
(quasi-Fermi level: EFn & EFp)
exp Fn FiiB
E En nk T
⎡ ⎤−= ⎢ ⎥
⎣ ⎦exp Fi Fpi
B
E Ep p
k T−⎡ ⎤
= ⎢ ⎥⎣ ⎦
ni
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D. Compensation Doping
Doping of semiconductor with both donors and acceptors to
control the properties.
Electrons (from donors) recombine with holes (from
acceptors)
nnp
nNNn
i
iad2
=
>>−=
pnn
nNNp
i
ida2
=
>>−=
p-type n-type n-type p-type
n p => recombination
Finally, 2inp n=
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CB
g(E)
E
Impuritiesforming a band
(a) (b)
EFp
Ev
EcEFn
Ev
Ec
CB
VB
(a) Degenerate n-type semiconductor. Large number of donors form
aband that overlaps the CB. (b) Degenerate p-type
semiconductor.
E. Degenerate and Non-degenerate SemiconductorsNon-degenerate
semiconductor:
The electron statistics ~the Boltzmann statistics (4)Pauli
exclusion principle can be neglected.
vc NpNn
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In degenerate semiconductor:n ≠ Ndp ≠ Nanp ≠ ni2
When dopant concentration is large
dopants interact with each other
not all dopants can become ionized
Saturation of carrier concentration ~ 1020/cm3
For Si:
(heavy doping)
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F. Energy BandDiagrams in an Applied Field
V
n-Type Semiconductor
Ec
EF − eV
A
B
V(x), PE (x)
x
PE (x) = � eV
Energy band diagram of an n-type semiconductor connected to
avoltage supply of V volts. The whole energy diagram tilts
becausethe electron now has an electrostatic potential energy as
well
EElectron Energy
Ec − eV
Ev− eV
V(x)
EF
Ev
-eV(potential energy of e-)
EF uniform ΔEF=0
Applied field ΔEFe- concentration uniform⇒EC-EV constant⇒CB, VB,
and EF bend
by the same amount
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Example 3.1.1 Fermi levels in semiconductors(1) n-type doping of
Si wafer: 1016/cm3 antimony (Sb) atoms
Find: Fermi energy w. r. t. Fermi energy EFi in the intrinsic
Si
(2) Compensated doping of Si wafer: 1016/cm3 antimony (Sb) atoms
(n-type) and 2×1017/cm3 boron (B) atoms (p-type)
Find: (a) Fermi energy w. r. t. Fermi energy EFi in the
intrinsic Si at room temperature (300K)
(b) Fermi energy w. r. t. Fermi energy in the n-type case in
(1)
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901 37500 光電導論
(1) n-type doping of Si wafer: 1016/cm3 antimony (Sb) atomsFind:
Fermi energy w. r. t. Fermi energy EFi in the intrinsic Si
( )
( )[ ]( )[ ]
( )[ ]( ) ( ) ( ) eVeVnNTkEE
TkEEnNNTkEENn
TkEENncmNn
cmnNcmN
idBFiFn
BFiFnid
dBFncc
BFicci
d
id
d
348.01045.1/10ln0259.0/ln
/exp/
/exp/exp
10
1045.1
,10
1016
316
310
316
=×==−
−=
=−−=−−=
==
×=>>
=
−
−
−
exp Fn FiiB
E En nk T
⎡ ⎤−= ⎢ ⎥
⎣ ⎦
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901 37500 光電導論
(2) Compensated doping of Si wafer: 1016/cm3 antimony (Sb) atoms
(n-type) and 2×1017/cm3 boron (B) atoms (p-type)
Find: (a) Fermi energy w. r. t. Fermi energy EFi in the
intrinsic Si at room temp. (b) Fermi energy w. r. t. Fermi energy
in the n-type case in (1)
( )( )[ ]
( )[ ]( )[ ]
( ) ( ) ( ) eVeVnpTkEETkEEnp
NNTkEENpTkEENnp
cmNNpcmNcmN
iBFiFp
BFiFpi
daBvFpv
BvFivi
da
da
424.01045.1/109.1ln0259.0/ln
/exp/
/exp
/exp109.110102
10102
1017
3171617
316317
−=××−=−=−
−−=
−=−−=
−−==×=−×=−=
=>×=−
−− (P-type)
exp Fi FpiB
E Ep n
k T−⎡ ⎤
= ⎢ ⎥⎣ ⎦
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901 37500 光電導論
Example 3.1.2 ConductivityGiven: (a) N-type Si crystal with
doping 1016/cm3 phosphorus (P)
(b) Drift mobility of electrons is ~ 1350 cm2V-1s-1
Find: conductivity of the n-type Si
• Since
• The electron concentration
• We can neglect the hole concentration
• Thus
( )
( )( )( ) 1111231619
2
310316
16.21350100.1106.1
,/
1045.110
−−−−−−
−−
Ω=××==
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901 37500 光電導論
r
PE(r)PE of the electron around anisolated atom
When N atoms are arranged to formthe crystal then there is an
overlapof individual electron PE functions.
x
V(x)
x = Lx = 0 a 2a 3a
0aa
Surface SurfaceCrystal
PE of the electron, V(x), insidethe crystal is periodic with
aperiod a.
The electron potential energy (PE), V(x), inside the crystal is
periodic with the sameperiodicity as that of the crystal, a. Far
away outside the crystal, by choice, V = 0 (theelectron is free and
PE = 0).
?1999 S O Kasap Optoelectronics (Prentice Hall)
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901 37500 光電導論
0)]([2 222
=−+ ψxVEmdxψd e
( ) ( ) ; 1 , 2 , 3 ,V x V x ma m= + =
(1)
( ) ( ) exp( )k kx U x jkxψ =
(2)(3)
Schrodinger Equation
Periodic Potential
Bloch Wavefunctions
The electron in a crystal
( ) wavetraveling)(
)(on dependfunction periodic
→⋅
→− Etjjkx
k
ee
xVxU
, ( , ) exp( / ) ( )nn k n kx t jE t xψ ψ= −Overall electron
wavefunction
kn: a state in Ek
nkCrystal momentum:( ) / extd k dt F=
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901 37500 光電導論
Ek
kš /a–š /a
Ec
Ev
ConductionBand (CB)
Ec
Ev
CB
The E-k Diagram The Energy BandDiagram
Empty ψk
Occupied ψkh+
e-
Eg
e-
h+
hυ
VB
hυ
ValenceBand (VB)
The E-k diagram of a direct bandgap semiconductor such as GaAs.
The E-kcurve consists of many discrete points with each point
corresponding to apossible state, wavefunction ψk(x), that is
allowed to exist in the crystal.The points are so close that we
normally draw the E-k relationship as acontinuous curve. In the
energy range Ev to Ec there are no points (ψk(x)solutions).
E-k Diagram = 0K, all fill the lower states> 0K, from VB to
CB (thermal)
-π/a π/a
kCrystal momentumof a electron:
( ) / extd k dt F=
many discrete points=> continuous curve
Bandgap: no solution
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901 37500 光電導論
E
CB
k–k
Direct Bandgap
(a) GaAs
E
CB
VB
Indirect Bandgap, Eg
k–k
kcb
(b) Si
E
k–k
Phonon
(c) Si with a recombination center
EgEc
EvEc
Evkvb VB
CB
ErEc
Ev
Photon
VB
(a) In GaAs the minimum of the CB is directly above the maximum
of the VB. GaAs istherefore a direct bandgap semiconductor. (b) In
Si, the minimum of the CB is displaced fromthe maximum of the VB
and Si is an indirect bandgap semiconductor. (c) Recombination ofan
electron and a hole in Si involves a recombination center .
?1999 S O K O l (P i H ll)
Direct bandgap semiconductor: e.g. GaAs
Indirect bandgap semiconductor: e.g. Sirecombination via a
recombination center: defect or impurityphonons (lattice
vibrations, quantized, waves)
photon momentum: negligible as compared with electron
momentum
kcrystal momentum:
( ) / extd k dt F=
Conservation: energy + momentum
(defects or impurities)
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901 37500 光電導論
3-3 pn Junction Principles
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nno
xx = 0
pno
ppo
npo
log(n), log(p)
-eNa
eNd
M
x
E (x)
B-
h+
p n
M
As+
e–
Wp Wn
Neutral n-regionNeutral p-region
Space charge region Vo
V(x)
x
PE(x)
Electron PE(x)
Metallurgical Junction
(a)
(b)
(c)
(e)
(f)
x
–WpWn
(d)
0
eVo
x (g)
–eVo
Hole PE(x)
–Eo
Eo
M
ρnet
M
Wn–Wp
ni
A. Open circuit
(abrupt discontinuity)
NdNa
NdNa
2inp n=
no applied fieldor photoexcitation
equilibrium
concentration gradient=> diffusion I
electrical field=> drift I
Equilibrium: diffusion I = drift I
ndpa WNWN =charge neutrality
(Depletion region)(layer)
(SCL)
Banddiagram
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901 37500 光電導論
ndpa WNWN = (1)
(2)21
2 2 ( )a d o
o o oa d
eN N WV E WN Nε
= − =+ (3)
Depletion widths
Built-in field
Built-in potential
2lna dB
oi
N Nk TVe n
⎛ ⎞= ⎜ ⎟
⎝ ⎠
(4)
(5)
(6)
⎟⎟⎠
⎞⎜⎜⎝
⎛=
po
noBo n
nlneTkV
⎟⎟⎠
⎞⎜⎜⎝
⎛=
no
poBo p
peTkV ln
Build-in potential
( )ε
ρ xdx
dE net=
dxdVE −=
( )TkeVpp Bopono −= exp
( )TkeVnn Bonopo −= expMinority/majority
( ) energy potential is where,exp 121
2 ETk
EEnn
B⎥⎦
⎤⎢⎣
⎡ −−=
By Boltzmann statistics
εεpand WeNWeNE −=−=0
2exp( )gc v iB
Enp N N n
k T= − =
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901 37500 光電導論
Neutral n-regionNeutral p-regionEo – E
Log (carrier concentration)
Holediffusion
Electrondiffusion
np(0)
Minute increase
pn(0)
pnonpo
pponno
V
Excess holes
Excess electrons
x′x
(a)
W
e(Vo–V)eVo M
x
Wo
Hole PE(x)
(b)
SCL
Forward biased pn junction and the injection of minority
carriers (a) Carrierconcentration profiles across the device under
forward bias. (b). The holepotential energy with and without an
applied bias. W is the width of the SCLwith forward bias
B. Forward BiasVoltage drop across SCL or depletion region W
Reduce the built-in potential against diffusion Result in the
injection of excess minority carrier and small increase in the
majority carrier
(charge neutrality)
Banddiagram
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901 37500 光電導論
(0 ) ex pn n oB
eVp pk T
⎡ ⎤= ⎢ ⎥
⎣ ⎦ (8)Law of the junction(just outside Wn)
( )(0 ) ex p on p oB
e V Vp pk T
⎡ ⎤− −= ⎢ ⎥
⎣ ⎦(7)Excess holes at x’=0
(0 ) ex pp p oB
eVn nk T
⎛ ⎞= ⎜ ⎟
⎝ ⎠ (9)Law of the junction(just outside Wp)
( ') (0 ) ex p ( '/ )n n hp x p x LΔ = Δ − (10)Excess minority
carrier concentration, : diffusion coefficient
: mean hole recomb. lifetimeh h h h
h
L D Dττ
=Hole diffusion length
(due to reduction in built-in potential barrier)
2 12 1 exp
B
E En nk T
⎛ ⎞−= −⎜ ⎟
⎝ ⎠
V=0 =>
e-
e-h+ e
-
(p and n-regions are longer than diffusion length of minority
carriers)
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901 37500 光電導論
Jelec
x
n-region
J = Jelec + Jhole
SCL
Minority carrier diffusioncurrent
Majority carrier diffusionand drift current
Total current
Jhole
Wn–Wp
p-region
J
The total currentanywhere in the device isconstant. Just outside
thedepletion region it is dueto the diffusion ofminority
carriers.
?1999 S.O. Kasap, Optoelectronics (Prentice Hall)
e-
e-h+ e
-
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901 37500 光電導論
,( ') ( ')
' 'n n
D hole h hdp x d p xJ eD eD
dx dxΔ
= − = −
,'(0) exphD hole n
h h
eD xJ pL L
⎛ ⎞ ⎛ ⎞= ⋅ Δ −⎜ ⎟ ⎜ ⎟
⎝ ⎠ ⎝ ⎠(11)
2, , exp 1h eD hole D elec i
h d e a B
eD eD eVJ J J nL N L N k T
⎡ ⎤⎛ ⎞ ⎛ ⎞= + = + −⎢ ⎥⎜ ⎟ ⎜ ⎟
⎝ ⎠⎝ ⎠ ⎣ ⎦
exp 1soB
eVJ Jk T
⎡ ⎤⎛ ⎞= −⎢ ⎥⎜ ⎟
⎝ ⎠⎣ ⎦(12)or
Hole diffusion current density
Assume electron and hole currents don’t change across the
SCL.
Shockley diode equation
⎥⎦
⎤⎢⎣
⎡−⎟⎟
⎠
⎞⎜⎜⎝
⎛⎟⎟⎠
⎞⎜⎜⎝
⎛= 1
2
TkeVexp
NLneDJ
Bdh
ihhole,DAt x’=0
At x’=0
Jso: reverse saturation current densitysoBr JJmVeTkV −=≈>−
),25/( :bias reverse
(0 ) ex pn n oB
eVp pk T
⎡ ⎤= ⎢ ⎥
⎣ ⎦
The total current density
(narrow junction and neglect e-h recombination)
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901 37500 光電導論
C
WnWp
Log (carrier concentration)
np(0)pnonpo
ppo nno
V
x
p-sideSCL
pn(0)
pM
M
nM
n-side
B
HolesElectrons
A D
Forward biased pn junction and the injection ofcarriers and
their recombination in the SCL.
Some of the minority carriers will recombine in the depletion
region.
a symmetrical pn junction, at the junction C MM np =
-
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901 37500 光電導論
exp2M i B
eVp nk T
⎛ ⎞= ⎜ ⎟
⎝ ⎠
exp2 2
pi nrecom
e h B
Wen W eVJk Tτ τ
⎡ ⎤ ⎛ ⎞= + ⎜ ⎟⎢ ⎥
⎝ ⎠⎣ ⎦
exp 1oB
eVI Ik Tη
⎡ ⎤⎛ ⎞= −⎢ ⎥⎜ ⎟
⎝ ⎠⎣ ⎦
(14)
(15)
1 12 2p M n M
recome h
e W n e W pJ
τ τ≈ + (13)
Diode Equation
Recombination current
Consider a symmetrical pn junction, at the junction C,Electrons
recombine in Wp and holes recombine in Wn.
MM np =
n, pWin ion timerecombinat (electron) holemean :e h,τdt
dQJ =
η=1, diffusion controlledη=2, SCL recombination controlled
⎥⎦
⎤⎢⎣
⎡−⎟⎟
⎠
⎞⎜⎜⎝
⎛= 1
2exp
TkeVJJ
Brorecom
Diode ideality factor
(Vo-V)/2 at M
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901 37500 光電導論
nA
I
Shockley equation
Space charge layergeneration.
V
mAReverse I-V characteristics of apn junction (the positive
andnegative current axes havedifferent scales)
I = Io[exp(eV/ηkBT) − 1]
exp 1oB
eVI Ik Tη
⎡ ⎤⎛ ⎞= −⎢ ⎥⎜ ⎟
⎝ ⎠⎣ ⎦Diode Equation η=1, diffusion controlled
η=2, SCL recombination controlled
⎥⎦
⎤⎢⎣
⎡−⎟⎟
⎠
⎞⎜⎜⎝
⎛= 1
2exp
TkeVJJ
Brorecomexp 1diffusion so
B
eVJ Jk T
⎡ ⎤⎛ ⎞= −⎢ ⎥⎜ ⎟
⎝ ⎠⎣ ⎦
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901 37500 光電導論
WoWo
Neutral n-regionNeutral p-region
x
W
HolesElectrons
DiffusionDrift
x
(a)(b)
ThermallygeneratedEHP
pnonpo
Vr
Eo+E
Minority CarrierConcentration
e(Vo+Vr)eVo
W(V = –Vr)
MHole PE(x)
Reverse biased pn junction. (a) Minority carrier profiles and
the origin of thereverse current. (b) Hole PE across the junction
under reverse bias
?1999 S.O. Kasap, Optoelectronics (Prentice Hall)
C. Reversed Bias
Banddiagram
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901 37500 光電導論
g
ii
ae
e
dh
h eWnnNL
eDNL
eDJτ
+⎟⎟⎠
⎞⎜⎜⎝
⎛+= 2rev (17)
Total Reverse Current
g
ieWnJτ
=gen (16)EHP thermal generation in SCL
:gτ mean thermal generation time
Shockley diode equation
direction reverse:""
2
−
⎟⎟⎠
⎞⎜⎜⎝
⎛+−=−=
in
NLeD
NLeDJJ
ae
e
dh
hso
exp 1soB
eVJ Jk T
⎡ ⎤⎛ ⎞= −⎢ ⎥⎜ ⎟
⎝ ⎠⎣ ⎦soBr JJmVeTkV −=≈>>− ),25/( :bias reverse
(indep. of Vr)reverse saturation current density
(cf. photodetectors and solar cells: EHP generation also by
photons )
(J
-
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901 37500 光電導論
nA
I
Shockley equation
Space charge layergeneration.
V
mAReverse I-V characteristics of apn junction (the positive
andnegative current axes havedifferent scales)
I = Io[exp(eV/ηkBT) − 1]
g
ii
ae
e
dh
h eWnnNL
eDNL
eDJτ
+⎟⎟⎠
⎞⎜⎜⎝
⎛+= 2rev
exp 1oB
eVI Ik Tη
⎡ ⎤⎛ ⎞= −⎢ ⎥⎜ ⎟
⎝ ⎠⎣ ⎦Diode Equation η=1, diffusion controlled
η=2, SCL recombination controlled
⎥⎦
⎤⎢⎣
⎡−⎟⎟
⎠
⎞⎜⎜⎝
⎛= 1
2exp
TkeVJJ
Brorecomexp 1diffusion so
B
eVJ Jk T
⎡ ⎤⎛ ⎞= −⎢ ⎥⎜ ⎟
⎝ ⎠⎣ ⎦
42
901 37500 光電導論
0.002 0.004 0.006 0.0081/Temperature (1/K)
10 -16
10 -14
10 -12
10 -10
10 -8
10 -6
10 -4
Reverse diode current (A) at V = −5 V
Ge Photodiode323 K
238 K0.33 eV
0.63 eV
Reverse diode current in a Ge pnjunction as a function of
temperature ina ln( Irev ) vs. 1/ T plot. Above 238 K, Irevis
controlled by ni2 and below 238 K itis controlled by ni. The
vertical axis isa logarithmic scale with actual currentvalues.
(From D. Scansen and S.O.Kasap, Cnd. J. Physics. 70 ,
1070-1075,1992.)
?1999 S.O. Kasap, Optoelectronics (Prentice Hall)
( )
( )
2
238 K1ln v.s. 0.63 eV~ 0.66 eV ,
238 K11ln v.s. 0.33 eV~2
rev g rev i
rev g rev i
T
I E I nTT
I E I nT
>
= = → ∝
<
= → ∝
exp( )2
gi c v
B
En N N
k T= −
⎥⎥⎦
⎤
⎢⎢⎣
⎡+⎟⎟
⎠
⎞⎜⎜⎝
⎛+=
g
ii
ae
e
dh
h eWnnNL
eDNL
eDAIτ
2rev
43
901 37500 光電導論
1/ 2
1/ 2
( )( ) 2( )
a ddep
o a d
e N NA ACW V V N N
εε ⎡ ⎤= = ⎢ ⎥− +⎣ ⎦
(20)
D. Depletion Layer Capacitance
d e pd QCd V
= (18)Depletion Layer Capacitance
1/ 22 ( )( )a d o
a d
N N V VWeN N
ε⎡ ⎤+ −= ⎢ ⎥
⎣ ⎦(19)SCL width
and voltage
212 2 ( )
a d oo o o
a d
eN N WV E WN Nε
= − =+ (3)
pF
44
901 37500 光電導論
/p p p thermaln t Bn p G∂Δ ∂ = − +
( )p p p po pon
B n p n pt
∂Δ= − −
∂p p
e
n nt τ
∂Δ Δ= −
∂
/p a pn t BN n∂Δ ∂ = − Δ
(21)
(22)
(23)
(24)
E. Recombination Lifetime
1/e aBNτ =
2/ ( )p p p pn t B p n B n∂Δ ∂ = Δ Δ = Δ
(25)
(26)
B: direct recombination capture coefficient
Rate of change due to recombination
Excess minority carrier recombination time (lifetime)
In weak injection, apoppppop Nppnnpn ≈≈Δ≈→Δ
majority ) (
minority
ppppop
ppop
npnppnnn
Δ=ΔΔ+=
Δ+=Forward bias
steady state
Banddiagram
-
45
901 37500 光電導論
Example 3.3.1: A direct bandgap pn junctionSymmetric GaAs pn
junction with a cross section A = 1 mm2, given
(1) Na (p-side doping) = Nd (n-side doping) = 1023 m-3
(2) B = 7.21 x 10-16 m3s-1
(3) ni = 1.8 x 1012 m-3
(4) εr = 13.2(5) μh (in the n-side) = 250 cm2V-1s-1
(6) μe (in the p-side) = 5000 cm2V-1s-1
(7) Diffusion coefficients (Einstein relation)Dh = μhkBT/e &
De = μekBT/e
(8) Forward voltage = 1 V
Find: (a) diode current due to the minority carrier diffusion at
300K
(assume direct recombination)(b) Estimate the recombination
component of the current
(if the mean minority carrier recombination time in the
depletion region is ~ 10ns) 46
901 37500 光電導論
• Assuming weak injection, we can readily calculate the
recombination times τe and τh for electrons and holes recombining
in the neutral p and n-regions.
( )( )( )( )( )( )
[ ] ( )( )[ ][ ] ( )( )[ ]
( ) ( )( ) ( )( ) ( )( )
( ) AV
VATk
eVII
A
msm
msm
enNL
DNL
DAI
mssmDL
mssmDL
smeTkDsmeTkD
smsmBN
VeTk
Bsodiff
iae
e
dh
hso
eee
hhh
Bee
Bhh
aeh
B
421
21
21219235
122
236
1246
2
55.081225.0
65.081245.0
1224
1244
83231316
109.302585.0
0.1exp1013.6exp
1013.6
108.1106.1101034.1
1029.1101000.3
1046.610
1034.11039.11029.1
1000.31039.11046.6
1029.11050002585.0/
1046.6102502585.0/
1039.1100.11021.7
1102585.0/
−−
−
−−
−−
−
−−−
−−−−
−−−−
−−−
−−−
−−−−
×=⎟⎠⎞
⎜⎝⎛×=⎟⎟
⎠
⎞⎜⎜⎝
⎛=
×=
××⎥⎦
⎤⎢⎣
⎡××
+××
=
⎟⎟⎠
⎞⎜⎜⎝
⎛+=
×=××==
×=××==
×=×==
×=×==
×=××
===
=
τ
τ
μ
μ
ττ
47
901 37500 光電導論
( ) ( )( )( )
( )( )( )( )( )( )
( )( )( )
( ) ( ) AVVA
TkeVII
ACWAenWWAenI
nsWWW
mmmmC
VmFm
NeNVVNNW
VVnNN
eTkV
Brorecom
r
i
h
n
e
piro
rhe
np
da
oda
i
daBo
412
129
812196
85.0
32332319
32323112
5.0
212
2323
2
103.302585.02
0.1exp103.12
exp
103.11010109
2108.1106.110
22
10
5.0
09.0100.91010106.1
128.110101085.82.132
2
28.1108.11010ln02585.0ln
−−
−−
−−−
−−−−
−−−
×=⎟⎟⎠
⎞⎜⎜⎝
⎛×=⎟⎟
⎠
⎞⎜⎜⎝
⎛=
×=⎟⎟⎠
⎞⎜⎜⎝
⎛××××
⎟⎟⎠
⎞⎜⎜⎝
⎛=⎟⎟
⎠
⎞⎜⎜⎝
⎛+=
≈==
==
=×=⎥⎦
⎤⎢⎣
⎡×
−+××=
⎥⎦
⎤⎢⎣
⎡ −+=
=⎟⎟⎠
⎞⎜⎜⎝
⎛
×=⎟⎟
⎠
⎞⎜⎜⎝
⎛=
τττ
τττ
μ
ε
In this example, the diffusion and recombination components are
about the same order of magnitude.
48
901 37500 光電導論
3-4 The pn Junction Band Diagram
-
49
901 37500 光電導論
EcEv
Ec
EFp
M
EFn
eVo
p nEo
Evnp
(a)
VI
np
Eo–E
e(Vo–V)
eV
EcEFn
Ev
Ev
Ec
EFp
(b)
(c)
Vr
np
e(Vo+Vr)
EcEFn
Ev
Ev
Ec
EFp
Eo+E (d)
I = Very SmallVr
np
Thermalgeneration Ec
EFnEv
Ec
EFp
Ev
e(Vo+Vr)
Eo+E
Energy band diagrams for a pn junction under (a) open circuit,
(b) forwardbias and (c) reverse bias conditions. (d) Thermal
generation of electron holepairs in the depletion region results in
a small reverse current.
SCLdiffusion efor
eV barrier potentialpotentialin -built variationh and e
:bended is Banduniform is
-0
-
→
+
FE( )
TkeV
Be
VVe
∝
→
−
current forwardionconcentratcarrier minority
barrier potential 0
( )
current reversesmallEHP of generation thermalthin carrier
wiminority small
barrier potential
or
0
→+
+
he
r
LVVe
50
901 37500 光電導論 Introduction of optoelectronics 50
Forward bias
hhhhhhh hh h
Apply positive bias at lefthand side
hhhhhhh hh h
hhhhhhh h
h hhhhhh hhhh
+V
V0
⎥⎦
⎤⎢⎣
⎡−⎟⎟
⎠
⎞⎜⎜⎝
⎛=⎥
⎦
⎤⎢⎣
⎡−⎟⎟
⎠
⎞⎜⎜⎝
⎛⎟⎟⎠
⎞⎜⎜⎝
⎛+ 1exp1exp~ 2
TkeVJ
TkeVn
NLeD
NLeDJ
Bso
Bi
ae
e
dh
h
If electron and hole do not recombine (Si), the current can be
expressed as :
lifetimeion recombinat : t coefficiendiffusion :
ττ DDL =
51
901 37500 光電導論 Introduction of optoelectronics 51
hhhhhhh hh h h hhhhhh hhhh
+V
1 12 2p M n M
recome h
e W n e W pJ
τ τ≈ +
Recombination Current
Consider a symmetrical pn junction, at the junction C,Electrons
recombine in Wp and holes recombine in Wn.
MM np =
dtdQJ =
n, pWin ion timerecombinat (electron) holemean :e h,τ
V0-V
52
901 37500 光電導論 Introduction of optoelectronics 52
Reverse bias
hhhhhhh hh hhhhhh h hhhhhhhh hh hhhhh hhh-VR
h
g
ieWnJτ
=gen
EHP thermal generation in SCL
g
ii
ae
e
dh
h eWnnNL
eDNL
eDJτ
+⎟⎟⎠
⎞⎜⎜⎝
⎛+= 2rev
Total Reverse Current
h
1exp , and 0when
~1exp~ 22
-
53
901 37500 光電導論
53
3-5 Light Emitting Diodes
54
901 37500 光電導論
Introduction of optoelectronics 54
An actual configuration of a LED
55
901 37500 光電導論
LED = 半導體?什麼是半導體?導電性介於導體與非導體之間
半導體的組成
週期表上第四族,三五或二六族化合物週期性排列的結構
依據參入不同特定之雜質又分為p型與 n型半導體
(a) (b) (c)
帶電的載子n型 – electronp型 – hole
55 56901 37500 光電導論
半導體如何發光?
• Absorption
• Electron transition
• Light emission
56
-
57
901 37500 光電導論
57
hυ - Eg
Eg (b)
V
(a)
p n+
EgeVo
EF
p n+
Electron in CBHole in VB
Ec
Ev
Ec
Ev
EF
eVo
Electron energy
Distance into device
(a) The energy band diagram of a p-n+ (heavily n-type doped)
junction without any bias.Built-in potential Vo prevents electrons
from diffusing from n+ to p side. (b) The appliedbias reduces Vo
and thereby allows electrons to diffuse, be injected, into the
p-side.Recombination around the junction and within the diffusion
length of the electrons in thep-side leads to photon emission.?1999
S.O. Kasap, Optoelectronics (Prentice Hall)
LED: pn junction diodedirect band gap semiconductor, e.g.
GaAsEHP recombination photon emission (injection
electroluminescence)
spontaneous emission in active region (SCL & within Le in p
region)
A. Principles
(Random directions)
58
901 37500 光電導論
58
Light output
Insulator (oxide)p
n+Epitaxial layer
A schematic illustration of typical planar surface emitting LED
devices. (a) p-layergrown epitaxially on an n+ substrate. (b) First
n+ is epitaxially grown and then p regionis formed by dopant
diffusion into the epitaxial layer.
Light output
pEpitaxial layers
(a) (b )
n+
Substrate Substrate
n+
n+
Metal electrode
B. Device Structuresn+ region: heavily doped, electron
injection
photon absorbed or reflectionp region: light emitting, narrow (a
few microns) to avoid photon absorption
Epitaxial (a) or diffused (b) junction planar LED Substrate-n+
region: Lattice matching
Lattice mismatch radiationless EHP recombination
59
901 37500 光電導論
59
Light output
p
Electrodes
LightPlastic dome
Electrodes
Domedsemiconductor
pn Junction
(a) (b) (c)
n+n+
(a) Some light suffers total internal reflection and cannot
escape. (b) Internal reflectionscan be reduced and hence more light
can be collected by shaping the semiconductor into adome so that
the angles of incidence at the semiconductor-air surface are
smaller than thecritical angle. (b) An economic method of allowing
more light to escape from the LED isto encapsulate it in a
transparent plastic dome.
Substrate
?1999 S.O. Kasap, Optoelectronics (Prentice Hall)
TIR low e.g. GaAs-air, θc~16o
Domed semiconductorDifficult process in fabrication
Plastic Domeinexpensive and widely used
Cf. overcome TIR by roughness and photonic crystals
60
901 37500 光電導論
60
3-6 LED Materials
-
61
901 37500 光電導論
什麼材料 - 發什麼光1λ
2λ
‧藍綠光: 氮化銦鎵
‧紅黃光: 磷化鋁銦鎵 (四元)
61
InGaN
AlInGaP
62
901 37500 光電導論
Introduction of optoelectronics 62
The Material Properties – Zinc Blend structure
63
901 37500 光電導論
Introduction of optoelectronics 63
Wurtzite Nitrides
Wu et. Al. Solid State Comm., 127:411-414, 2003.
The bandgap of nitrides cover the whole visible light
spectrum.
64
901 37500 光電導論
64
Ec
Ev
EN
(b) N doped GaP
Eg
(a) GaAs1-yPyy < 0.45
(a) Photon emission in a direct bandgap semiconductor. (b). GaP
is anindirect bandgap semiconductor. When doped with nitrogen there
is anelectron trap at EN. Direct recombination between a trapped
electron at ENand a hole emits a photon. (c) In Al doped SiC, EHP
recombination isthrough an acceptor level like Ea.
Ec
EvEa
(c) Al doped SiC
?1999 S O Kasap Optoelectronics (Prentice Hall)
Direct bandgap:
Indirect Bandgap:
1
1
0.49 0.51
1 1
0.45, 630)
0.43, 640)0.17)
( 3.4 )
(0 870 nm
(0 870 nm (0.058
y y
x x
x x
x x y y
g
GaAs P yAl Ga As xIn Al Ga P xIn Ga As PGaN E eV
λ
λ−
−
−
− −
≤ < > >
≤ < > >
≤ <
=
ternary (3 elements)
AlSNyPGaAs yy
with doped , iC with doped ),45.0( 1 >−
(isoelectronic impurities,nucleus of N is less shielded)
acceptor type localized energy level
quarternary (4 elements)(High intensity)
(Optical communication)
(EHP recomb. thrurecomb. centers and Involve lattice
vibrations)
GaAs1-yPyN doped indirect Egwidely used in inexpensive green,
yellow, and orangeLEDs
Blue LED: InGaN (Eg = 2.7 eV)
(GaN, Eg = 3.4 eV)Al-SiCZnSe
-
65
901 37500 光電導論
65
0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.2 1.3 1.4 1.5 1.6λ
1.7Infrared
GaAs1-yPy
InP
In1-xGaxAs1-yPyAlxGa1-xAs
x = 0.43
Indirectbandgap
Free space wavelength coverage by different LED materials from
the visible spectrum to theinfrared including wavelengths used in
optical communications. Hatched region and dashedlines are indirect
Eg materials.
In0.49AlxGa0.51-xP
?1999 S.O. Kasap, Optoelectronics (Prentice Hall)
= internal efficiency of radiative recombination process and
photon extraction
(Generally, ext. efficiency < 1% for indirect Eg
materials)
( ) 100%outexternalP optical
IVη = ×External efficiency
66
901 37500 光電導論
66
, 100%out opticalexternal internal extractionP
IVη η η= ⋅ = ⋅
67
901 37500 光電導論
67
3-7 Heterojunction High Intensity LEDs and Quantum
Well LEDs
68
901 37500 光電導論
68
2 eV
2 eVeVo
Holes in VB
Electrons in CB1.4 eV
No bias
Withforwardbias
Ec
EvEc
Ev
EFEF
(a)
(b)
(c)
(d)
pn+ p
ΔEc
GaAs AlGaAsAlGaAs
ppn+
~ 0.2 μm
AlGaAsAlGaAs
(a) A doubleheterostructure diode hastwo junctions which
arebetween two differentbandgap semiconductors(GaAs and AlGaAs)
(b) A simplified energyband diagram withexaggerated features.
EFmust be uniform.
(c) Forward biasedsimplified energy banddiagram.
(d) Forward biased LED.Schematic illustration ofphotons
escapingreabsorption in theAlGaAs layer and beingemitted from the
device.
?1999 S.O. Kasap, Optoelectronics (Prentice Hall)
GaAs
Homojunction: narrow p region EHP recombine through surface
defectslong p region re-absorption of photons increases
Heterojunction: a junction between two diff. Eg
semiconductors
Double heterojunction device:Thin p-GaAs layer between p and
nAlGaAs layers: a confining layer
Photon will not be absorbed with the wide bandgapmaterial
AlGaAs.
∆Ec: potential energy barrier against electron in CB (p-GaAs)to
CB of p-AlGaSa
Additional advantage of AlGaAs/GaAs hetero-junction:small
lattice mismatch between two crystal materials
negligible strain induced defects(Homo-junction: defects at the
surface )
large nsmall n small n
-
69
901 37500 光電導論 Introduction of optoelectronics 69
Double Heterojunction LEDsHomojunction: narrow p region EHP
recombine through surface defects
long p region re-absorption of photons increasesHeterojunction:
a junction between two diff. Eg semiconductors
Eg1 Eg2
Heterojunction
Type I semiconductor
Eg1Eg2
Type II
Eg1
Eg2
Type IIIvaccum
sχ
70
901 37500 光電導論 Introduction of optoelectronics 70
Alloy -> Using two materials with different composition to
change the bandgap.
GaAsInAs
InxGa1-xAs
( ) ( )( ) ( )xInGaExGaAsEAsGaInE ggxxg +−− 1~1
( ) ( ) ( )InAsmx
GaAsmx
AsGaInm xx**
1*
1~1 +−
−
71
901 37500 光電導論 Introduction of optoelectronics 71
Double Heterojunction LEDs
Eg1 Eg2 Eg1
~0.1-0.2um
Eg1
Eg1
n
Eg2hhhhhhh hh hhhhh hhhhhhh hhhhhhhh
p
Eg2 Eg1
p
Eg1n
+v
hhhhhh h
hhh
hhh
hhh
h
hhh
hh
h
hh
72
901 37500 光電導論 Introduction of optoelectronics 72
Band diagrams of different designs of Band diagrams of different
designs of heterojunctionsheterojunctions
-
73
901 37500 光電導論
Carrier loss in double Carrier loss in double
heterostructuresheterostructures
Leakage increases exponentially with temperatureRadiative
recombination decreases with the increase of temperature.
74
901 37500 光電導論 Introduction of optoelectronics 74
Eg2 Eg1
p
Eg1n
+v
hhhhhh h
hhh
hhh
hhh
h
hhh
hh
h
hh
1. Due to the potential barrier, carrier is much harder to
overflow.2. Weaker carrier reabsorption. The material on both with
larger bandgap will not re-absorb the emitted light.
1. The active layer is still to thick that the electron and hole
are still free to move. electron might concentrate at left side
while hole might accumulate at right hand side
75
901 37500 光電導論 Introduction of optoelectronics 75
Quantum Well LED A electron or a hole is a wave, and the typical
wave length of electron or hole wave in the semiconductor is around
1nm~10nm
3nm~20nm
e
h
1. Electron and hole are confined in the quantum well and cannot
move freely. Electron and hole are staying in the quantum well and
enhance radiative recombination chance.
2. Since the quantum well is very thin, it cannot hold to many
electrons and holes, not good for high current cases.
76
901 37500 光電導論 Introduction of optoelectronics 76
The confined electric wave will have corresponding eigenEnergy
level, where the effective band gap will change.
Quantum Well LED
W
Effective bandgap
2*
222
2*
222
22~ effective
Wmn
WmnEE
hegg
ππ++
By changing the quantum well width, we can fine tune the
effective bandgap.
-
77
901 37500 光電導論 Introduction of optoelectronics 77
Multiple Quantum Well LED e
h
1. Ef1. n-type
1. i-type
1. p-type
1. Blocking layer for electron
Opt
ical
pow
ercurrent
1 well
3wells2wells
+ ++ ++-- - -- - - --
78
901 37500 光電導論 Introduction of optoelectronics 78
Multiple Quantum Well LED e
h
eee eee
hhh h h h
1. Using a multiple quantum well, the total carrier staying in
the active layer will enhance.
2. More quantum well, the total internal resistant from the
barrier between two quantum wells will increase, so the electron
may not be able to the last quantum well. So the numbers of quantum
well is limited.
79
901 37500 光電導論 Introduction of optoelectronics 79
Optical emission with different number of quantum wellsOptical
emission with different number of quantum wells
80
901 37500 光電導論 Introduction of optoelectronics 80
Electron blocking layerElectron blocking layer
-
81
901 37500 光電導論
81
3-8 LED Characteristics
5/07/2009
82
901 37500 光電導論
82
E
Ec
Ev
Carrier concentrationper unit energy
Electrons in CB
Holes in VBhυ
1
0
Eg
hυ1
hυ2
hυ3
CB
VBλ
Relative intensity
1
0λ
1λ
2λ
3
ΔλΔhυ
Relative intensity
(a) (b) (c) (d)
Eg + kBT
(2.5-3)kBT
1/2kBT
Eg1 2 3
2kBT
(a) Energy band diagram with possible recombination paths. (b)
Energy distribution ofelectrons in the CB and holes in the VB. The
highest electron concentration is (1/2)kBT aboveEc . (c) The
relative light intensity as a function of photon energy based on
(b). (d) Relativeintensity as a function of wavelength in the
output spectrum based on (b) and (c).
Wavelengths of photon emission depends on the e- and h+
distribution.The min. photon energy = EgThe max. emission at photon
energy ~ Eg+kBT= Eg+0.5kBT+0.5kBTThe linewidth ~ 2.5-3 kBT
For heavily doped semiconductor narrow impurity band overlaps
CBthe minimum photon energy < Eg
83
901 37500 光電導論
83
V
2
1
(c)
0 20 40I (mA)0
(a)
600 650 7000
0.5
1.0
λ
Relativeintensity
24 nm
Δλ
655nm
(b)
0 20 40I (mA)0
Relative light intensity
(a) A typical output spectrum (relative intensity vs wavelength)
from a red GaAsP LED.(b) Typical output light power vs. forward
current. (c) Typical I-V characteristics of ared LED. The turn-on
voltage is around 1.5V.
Example: GaAsP LED (center at 655 nm)- Spectrum: less asymmetric
than ideal spectrum- Linewidth ~24 nm (2.7kBT)- The output light
power is not linear with LED current as in (b)- The turn-on or the
cut-in voltage ~ 1.5V (depends on material, and increases as Eg
increases e.g. blue 3.5~4.5V, yellow ~2V)
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Example 3.8.1: LED output spectrumGiven: the width of the
relative light intensity vs. photon energy spectrum of an
LED is ~ 3kBTFind: the linewidth Δλ1/2 in the output spectrum in
terms of wavelength?
( )
nmnmnmnm
nmnmhc
TkTkhvE
EEhc
dEd
EEhc
dEd
Ehcvc
ph
phph
phphphph
ph
149,1550105,1300
47,870
3,3
,
//
2
2
2
=Δ==Δ=
=Δ=
≈Δ≈Δ=Δ
Δ≈Δ
≈ΔΔ
−=
==
λλλλ
λλ
λλ
λ
λλλ
λ
BB
We note that the emitted wavelength λ is related to the photon
energy Eph by
The linewidths are typical values and the exact values depend on
the LED structure.
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Example 3.8.2: GaAs LED output wavelength variationsGiven: GaAs
Eg (300K) = 1.42 eV
dEg / dT = -4.5 x 10-4 eV K-1 (decreases with temp.)Find: the
change in the emitting wavelength if the temperature change is
10°C
( )( )( ) ( )
( )( ) nmKmnKTdTd
nmKormKdTd
dTdE
Ehc
dTd
Ehctaking
TkNeglecting
g
g
g
B
87.210277.0
.277.0,1077.2
106.1105.4106.142.1
10310626.6
/
1
1110
194219
834
2
≈=Δ=Δ
×=
×××−××
××−=⎟⎟
⎠
⎞⎜⎜⎝
⎛−=
=
−
−−−
−−
−
−
λλ
λ
λ
λ
The wavelength increases with temperature. This calculated
change is within 10% of typical values for GaAs LEDs quoted in data
books.
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Example 3.8.3: InGaAsP on InP substrateBackground: ternary alloy
In1-xGaxAsyP1-y on InP substrate
for infrared wavelength LED and laser diodeRequirement: InGaAsP
is lattice matched to In P
(avoid crystal defects in InGaAsP)y ≈ 2.2 x
Given: Eg of the ternary alloy (empirical relationship)Eg ≈ 1.35
– 0.72y + 0.12 y2 0 ≤ x ≤ 0.47
Find: the compositions of InGaAsP ternary alloys for peak
emission at a wavelength of 1.3 μm
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• We first note that we need the required bandgap Eg at the
wavelength of interest. The photon energy at peak emission is
hc/λ=Eg+kBT. Then in electron volts,
( )( )( )( )
34.066.03.07.0
2
619
348
6
3.02.2/66.066.0
12.072.035.1928.0
928.00259.0103.1106.110626.6103
300,103.1
/
PAsGaInxy
yy
eVeVE
KTm
eTkehcE
g
Bg
⇒==
=+−=
=−××××
=
=×=
−=
−−
−
−λ
λ
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3-9 LED for Optical Fiber Communications
-
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(a) Surface emitting LED (b) Edge emitting LED
Doubleheterostructure
Light
Light
?1999 S.O. Kasap, Optoelectronics (Prentice Hall)
Short haul communication: use LEDsimpler to drive, more
economic, longer lifetime, wider output spectrum
Long haul and wide bandwidth communication: use LDnarrow
linewidth, high output power, higher signal BW capacity
Surface emitting LED (SLED)Edge emitting LED (ELED)
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ElectrodeSiO2 (insulator)
Electrode
Fiber (multimode)
Epoxy resin
Etched wellDouble heterostructure
Light is coupled from a surface emitting LEDinto a multimode
fiber using an index matchingepoxy. The fiber is bonded to the
LEDstructure.
(a)
Fiber
A microlens focuses diverging light from a surfaceemitting LED
into a multimode optical fiber.
Microlens (Ti2O3:SiO2 glass)
(b)
?1999 S.O. Kasap, Optoelectronics (Prentice Hall)
Burrus type device
Coupling of SLED to a fiber
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Schematic illustration of the the structure of a double
heterojunction stripecontact edge emitting LED
InsulationStripe electrode
SubstrateElectrode
Active region (emission region)
p+-InP (Eg = 1.35 eV, Cladding layer)
n+-InP (Eg = 1.35 eV, Cladding/Substrate)
n-InGaAs (Eg = 0.83 eV, Active layer)
Currentpaths
L
60-70 μm
Light beam
p+-InGaAsP (Eg = 1 eV, Confining layer)
n+-InGaAsP (Eg = 1 eV, Confining layer) 12 3200-300 μm
© 1999 S.O. Kasap, Optoelectronics (Prentice Hall)
InGaAs: active regionInGaAsP: confining layerInP: cladding
layer
Edge emitting LED: greater light intensity, more collimated
beam
Op. @ 1.5μm
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Multimode fiberLens
(a)
ELED
Active layer
Light from an edge emitting LED is coupled into a fiber
typically by using a lens or aGRIN rod lens.
GRIN-rod lens
(b)
Single mode fiberELED
?1999 S.O. Kasap, Optoelectronics (Prentice Hall)
Coupling of ELED to a fiber
Output spectra from SLED and ELED are not necessarily the
samebecause (a) different doping levels,
(b) self-absorption guided along active layer in ELED
Typical linewidth of ELED < linewidth of SLEDe.g. InGaAsP
operated at 1300 nm
linewidth (ELED) = 75 nm linewidth (SLED) = 125 nm
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