Optoelectronics (16/2) W.-Y. Choi - Yonsei Universitytera.yonsei.ac.kr/class/2016_2_1/lecture/Lect 22 Semiconductor... · Optoelectronics (16/2) W.-Y. Choi Lect. 22: ...1999 S.O

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)



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



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



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



P NI

Double heterojunction



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



LaserOptical Power

LEDOptical Power

th

I (mA)0 I

Spontaneousemission

Optical Power (mW)

Threshold current: Ith

Slope Efficiency:dP/dI



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)



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



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



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

=



Homework:



Homework:

Documents

Optoelectronics (16/2) W.-Y. Choi - Yonsei Universitytera.yonsei.ac.kr/class/2016_2_1/lecture/Lect 22 Semiconductor... · Optoelectronics (16/2) W.-Y. Choi Lect. 22: ...1999 S.O