opto-electronics and photonics - yonseitera.yonsei.ac.kr/class/2020_2_1/lecture/lecture 24... ·...

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



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 …



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)



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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

=



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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



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



8

L

n, p

d

w

+ v -I

Photoconductor

- Large gain

- Very easy to make

- Speed limited by t

- Dark currents can be large



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



10


è Solar (Photovoltaic) Cell

Theoretically, max. conversion efficiency: 32.33%

Commercial solar cell conversion efficiency: < 20%



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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



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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)



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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)



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Applications: è LIDAR (Light Detection And Ranging)



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Homework: (Due 12/6)



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● 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

opto-electronics and photonics - yonseitera.yonsei.ac.kr/class/2020_2_1/lecture/lecture 24... ·...

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