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Optoelectronics (16/2) W.-Y. Choi
Lect. 22: Semiconductor Lasers
0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.2 1.3 1.4 1.5 1.6
1.7Infrared
GaAs1-yPy
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)
- Semiconductors lasers are small, cheap and very efficient
InGaN
- Lasing wavelength: Bandgap of direct semiconductor materials (III-V compound semiconductors)
Optoelectronics (16/2) W.-Y. Choi
Lect. 22: Semiconductor Lasers
0.20.40.60.8
11.21.41.61.8
22.22.42.6
0.54 0.55 0.56 0.57 0.58 0.59 0.6 0.61 0.62Lattice constant, a (nm)
GaP
GaAs
InAs
InP
Direct bandgapIndirect bandgap
In0.535Ga0.465AsX
Quaternary alloyswith direct bandgap
In1-xGaxAs
Quaternary alloyswith indirect bandgap
Eg (eV)
- AlxGa1-xAs (ternary) and In1-xGaxAs1-yPy (quaternary) material systems
Optoelectronics (16/2) W.-Y. Choi
Lect. 22: Semiconductor Lasers
Semiconductor Laser Structure: PN Junction + Mirror (Cleaved Facets)
Oxide insulator
Stripe electrode
SubstrateElectrode
Active region where J > Jth.(Emission region)
n-GaAs (Substrate)
GaAs (Active layer)
Currentpaths
L
W
Cleaved reflecting surfaceEllipticallaserbeam
p-AlxGa1-xAs (Confining layer)
n-AlxGa1-xAs (Confining layer) 12 3
Cleaved reflecting surface
Substrate
Optoelectronics (16/2) W.-Y. Choi
Lect. 22: Semiconductor Lasers
Efficient carrier confinement: PIN structure with large Eg for P, N regions
P N
Injected carriers are spread-out => smaller density
For population inversion, 2 1
1 2
1N PN P
P NI
Double heterojunction: Confinement ofInjected carriers
=> larger density
Optoelectronics (16/2) W.-Y. Choi
Lect. 22: Semiconductor Lasers
P NI
Double heterojunction
Optoelectronics (16/2) W.-Y. Choi
Lect. 22: Semiconductor Lasers
Efficient photon confinement: PIN structure with smaller n for P, N regions
Smaller Eg material has larger n (n1>n2)
Dielectric waveguide!
=> More photons interacting with injected electrons and holes in the active region
larger With <1,
th m1 1ln (mirror loss)gL R
Active(n1)
cladding(n2)
cladding(n2)
th m1 1=> lngL R
2Ln m
eff
2 ;Ln m
eff0
nk
Optoelectronics (16/2) W.-Y. Choi
Lect. 22: Semiconductor Lasers
LaserOptical Power
LEDOptical Power
th
I (mA)0 I
Spontaneousemission
Optical Power (mW)
Threshold current: Ith
Slope Efficiency:dP/dI
Optoelectronics (16/2) W.-Y. Choi
Lect. 22: Semiconductor Lasers
Analytical expression for Ith
Assume optical gain increases linearly with injected carriers:
1) Determine carrier density ( ) required for :th thN g
0( )g a N N
2) Relatioship between and I N
0 ,thth
gN Na
3) Determine from assuming steady-stateth thI N
mthg
m
0 thN Na
( : volume of active region, : carrier life time)
dN I Ndt qVV
thth
NI qV
In steady-state, NI qV
m0
1( )N qVa
th
I (mA)I
Optical Power (mW)
Optoelectronics (16/2) W.-Y. Choi
Lect. 22: Semiconductor Lasers
Analytical expression for dP/dI
Change in photon density with time
-Assume injected carriers are all converted into photons by stimulated emission when I > Ith
At steady-state,
How many photons out of laser per second?
phdndt
thI IqV ph
ph
n
m
1 ph v
thph ph
I InqV
ph
ph
n V
Output power
phout
ph
h n VP
th thph
ph
I I I IVh hqV q
thI (mA)
I
Optical Power (mW)
dP/dI
dP hdI q
Optoelectronics (16/2) W.-Y. Choi
Lect. 22: Semiconductor Lasers
More detailed model:
I
int
out/generation= m/(m+int)
Modifications: mth :g
m
1ph v
- Photons can be lost internally by impurities, scattering, … : internal loss, int
- Injected carriers are not 100% converted into photons: conversion efficiency, int
int m0
1( )thI N qVa
i t m n
: thph ph
I InqV
m int
1( )v
: phout
ph
h n VP
intth
phI I
qV
int,
( ) ph m
phth
h I Iq
,, m
with 1phph m
ph m vh n V
int,
( )thout
ph mph
I I VP hqV
mint
m int
= ( )thh I Iq
Optoelectronics (16/2) W.-Y. Choi
Lect. 22: Semiconductor Lasers
0 20 40 60 800
2
4
6
8
10
Po (mW)
I (mA)
Threshold current:
Slope Efficiency:
m int 0
1( )thI N qVa
mint
m int
dPdI
hq
mext int
m int
=
Optoelectronics (16/2) W.-Y. Choi
Lect. 22: Semiconductor Lasers
Homework:
Optoelectronics (16/2) W.-Y. Choi
Lect. 22: Semiconductor Lasers
Homework: