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Opto-Electronics and
Photonics
Woo-Young Choi
Dept. of Electrical and Electronic EngineeringYonsei University
Lecture 24 : Semiconductor Photodetectors
Opto-Electronics and Photonics (2020/2) W.-Y. Choi
Lecture 24: Semiconductor Photodetectors
2
Many different types of photodetecting devices
CMOS Image Sensor (CIS) X-ray sensor
Photomultiplier Semiconductor-based Photodetectors
Opto-Electronics and Photonics (2020/2) W.-Y. Choi
Lecture 24: Semiconductor Photodetectors
3
Photodetection in Semiconductor:
hu Currents
Materials for photodetection: Eg < hn
Absorption è Current Generation
Various methods for generating currents with photo-generated carriers:
Photoconductors, photodiodes, avalanche photodiodes …
Opto-Electronics and Photonics (2020/2) W.-Y. Choi
Lecture 24: Semiconductor Photodetectors
4
0.2 0.4 0.6 0.8 1.2 1.4 1.6 1.8
Wavelength (mm)
In0.53Ga0.47As
Ge
Si
In0.7Ga0.3As0.64P0.36
InPGaAs
a-Si:H
12345 0.9 0.8 0.7
1´103
1´104
1´105
1´106
1´107
1´108
Photon energy (eV)
Absorption coefficient (a) vs. wavelength (l) for various semiconductors(Data selectively collected and combined from various sources.)
a (m-1)
1.0
Figure 5.3
Sharp decrease in absorptionfor photon energy < Eg
Pout/Pin = exp(gL) g < 0 è a = -g
Indirect bandgap semiconductor can absorb
Direct Indirect
Indirect cannot efficiently satisfy k-conservation for emission process
Indirect can satisfy k-conservation for absorption process
è Indirect semiconductors used for photodetectors (solar cells, image sensors)
Opto-Electronics and Photonics (2020/2) W.-Y. Choi
Lecture 24: Semiconductor Photodetectors
5
Photodetection efficiency (Responsivity)R
Responsivity (R) vs. wavelength (l) for an idealphotodiode with QE = 100% (h = 1) and for a typicalcommercial Si photodiode.
0 200 400 600 800 1000 12000
0.10.20.30.40.50.60.70.80.9
1
Wavelength (nm)
Si Photodiode
lg
Responsivity (A/W)
Ideal PhotodiodeQE = 100% ( h = 1)
?1999 S.O. Kasap, Optoelectronics (Prentice Hall)
(Quantum Efficiency)h
qRh
hn
= × [ m]~ [1/V]1.24
qh
l mn
[ ]~ [1/ ]1.24
um Vlh
= I
qP
hn
hRqn
= ×
IP
=
Opto-Electronics and Photonics (2020/2) W.-Y. Choi
Lecture 24: Semiconductor Photodetectors
6
Photoconductor
n0, p0
d
w
+ v -
L,
Conductivity: ( : electron, hole mobility)
e h
e h
q n q ps m mm
= +
Without light,
I
With light,
0 0, n n n p p p= + D = + D
J Es=VI wdL
s=
0( ) ( )e hq n n q p ps s m m+ D = + D + + D
( ) e hV VI wd wd q n q pL L
s m mD = ×D × = × D + D ×
?R =
Light è Change in R
Opto-Electronics and Photonics (2020/2) W.-Y. Choi
Lecture 24: Semiconductor Photodetectors
7
L
n, p
d
w
+ v -I
With light,
( )
0 0
0
, ( ) ( )
e h
e h
n n n p p pq n n q p p
VI wd q n qL
s s m m
m m
= + D = + D
+ D = + D + + D
D = D + D
ID =
IRP
=
intqR Gh
hn
= ×
( ) inte hP Vwd qh wLd L
tm m hn
× + × × ( ) int 2= e hPq Vh L
tm m hn
+ × × × ×
( ) int 2e hqR Vh L
tm m hn
= + × × ×IPD;
( ) 2where e hG VLtm m= + × ×
Responsivity?
n pD = D intPh wLd
thn
= × × (Assume , are uniform)n pD D
(Assume dark current is small)
Gain possible because photogenerated carriers go through photoconductor several times before disappear
t : carrier lifetime
Opto-Electronics and Photonics (2020/2) W.-Y. Choi
Lecture 24: Semiconductor Photodetectors
8
L
n, p
d
w
+ v -I
Photoconductor
- Large gain
- Very easy to make
- Speed limited by t
- Dark currents can be large
Opto-Electronics and Photonics (2020/2) W.-Y. Choi
Lecture 24: Semiconductor Photodetectors
9
Photodiodes: PN junction
exp( 1)sqVI IkT
= -With light
phI-
Iph depends on where the light is incident
è Larger if closer to depletion region
Opto-Electronics and Photonics (2020/2) W.-Y. Choi
Lecture 24: Semiconductor Photodetectors
10
Photodiodes: PN junction
è Solar (Photovoltaic) Cell
Theoretically, max. conversion efficiency: 32.33%
Commercial solar cell conversion efficiency: < 20%
Opto-Electronics and Photonics (2020/2) W.-Y. Choi
Lecture 24: Semiconductor Photodetectors
11
Photodiodes: PN junction
Larger Iph if closer to depletion region
Typically, the depletion width is very small ( < 1mm)
Iph max. if all photons incident in depletion regiondue to built-in field
è Use PIN structure
phI = P qhn
è PIN PD
intP qh
hn
Opto-Electronics and Photonics (2020/2) W.-Y. Choi
Lecture 24: Semiconductor Photodetectors
12
PIN PD
0.2 0.4 0.6 0.8 1.2 1.4 1.6 1.8
Wavelength (mm)
In0.53Ga0.47As
Ge
Si
In0.7Ga0.3As0.64P0.36
InPGaAs
a-Si:H
12345 0.9 0.8 0.7
1´103
1´104
1´105
1´106
1´107
1´108
Photon energy (eV)
Absorption coefficient (a) vs. wavelength (l) for various semiconductors(Data selectively collected and combined from various sources.)
a (m-1)
1.0
Figure 5.3InGaAs, Ge: Long-distance optical fiber comm.
GaAs: Short-distance optical fiber comm.
Si: Visible light detection
Galaxy Note 20 Ultra:
12,000 x 900, each pixel 0.8mm
CMOS Image Sensor (CIS)
Opto-Electronics and Photonics (2020/2) W.-Y. Choi
Lecture 24: Semiconductor Photodetectors
13
Avalanche Photodiode (APD)
Under high E-field, electrons and holes can have sufficiently high kinetic energies breaking bonds and creating new e-h pairs(Impact Ionization)
PIN PD with gain?
è Gain by multiplying electrons and/or holes
(avalanche: a large mass of snow, ice, earth, rock, or other material in swift motion down a mountainside)
è Very high sensitivity
(Single photon detection possible)
Opto-Electronics and Photonics (2020/2) W.-Y. Choi
Lecture 24: Semiconductor Photodetectors
14
Applications: è LIDAR (Light Detection And Ranging)
Opto-Electronics and Photonics (2020/2) W.-Y. Choi
Lecture 24: Semiconductor Photodetectors
15
Homework: (Due 12/6)
Opto-Electronics and Photonics (2020/2) W.-Y. Choi
Lecture 24: Semiconductor Photodetectors
16
● Goals:
Learn basic properties of light
Learn how to control the light property for useful applications
- Final Exam: Dec. 9, Wed., 10-12 am
- Discussion Session: Dec. 7, Mon., 10 am
- Final Exam Review: Dec. 14, Mon., 10 am