Lect. 10: Photodetectors - Yonseitera.yonsei.ac.kr/class/2015_2_2/lecture/Lect 10... · 2015. 12....

Lect. 10: Photodetectors

W.-Y. ChoiSi Photonics (2015/2)

Photodetection: Absorption => Current Generation

h Currents

Materials for photodetection: Eg < hVarious methods for generating currents with photo-generated carriers:photoconductors, photodiodes, avalanche photodiodes



0.2 0.4 0.6 0.8 1.2 1.4 1.6 1.8

Wavelength (m)

In0.53Ga0.47As

Ge

Si

In0.7Ga0.3As0.64P0.36

InPGaAs

a-Si:H

12345 0.9 0.8 0.7

1103

1104

1105

1106

1107

1108

Photon energy (eV)

(m-1)

1.0

- Sharp decrease in for >Eg

- Photodetection for indirectbandgap materials?



- Photodetection for indirect bandgap materials?

E

CB

VB

k–k

Direct Bandgap Eg Photon

Ec

Ev

(a) GaAs (Direct bandgap)

E

k–k

(b) Si (Indirect bandgap)

VB

CB

Ec

Ev

Indirect Bandgap, Eg

Photon

Phonon

Unlike emission, absorption in indirect bandgap semiconductor is highly probable



Photodetection efficiency

(Responsivity) IRP

0 200 400 600 800 1000 12000

0.10.20.30.40.50.60.70.80.9

1

Wavelength (nm)

Si Photodiode

g

Responsivity (A/W)

Ideal PhotodiodeQE = 100% ( = 1)

(Quantum Efficiency) = I

qP

h

qRh

1.24[eV][ m]

h

[ ] [1/ ]1.24[ ] 1.24

R q C VeV



Photoconductor

n0, p0

d

w

+ v -

L,

Conductivity: ( : electron, hole mobility)

e h

e h

q n q p

Without light,

I

With light,

0 0, n n n p p p

J EVI wdL

0( ) ( )e hq n n q p p

e hV VI wd wd q n q pL L

?R



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

V VI wd wd q n q pL L

int (Assume , are uniform)Pn p n ph wLd

VI wdL

IRP

intqR Gh

inte hP Vwd qh wLd L

int 2= e hPq Vh L

int 2e hq Vh L

(Assume dark current is small)IP

2where e hG VL



2Gain: e hG VL

Iph

Photoconductore–

h+

With h

2Assuming , e h eG VL

e

e

LVL

==> electrons circulate many time before recombination e

Time for travelling distance L

2 = ( ) ehe h

GL

V

( )( )eh

e h e he h

L L LV E v vL

1 11 1 ( )

e h

e h e h

e h

v vL

2

e

LV

=

e

;Lv

e

LE



L

n, p

d

w

+ v -I

Photoconductors:

- Very easy to make- Large gain - But slow (speed limited by - Can have significant dark currents



Faster, less dark-current photodetectors?

B-

h+

p n

As+

e–

E (x)

x0

–Eo

M

Wn–Wp

PN junction in reverse bias

photodiode

- No significant current flow=> small dark currents

- Photo-generated carriers are removedby built-in field in depletion region (space charge region)



E (x)

x0

–Eo

M

Wn–Wp

- Photo-generated carriers drift into P (holes)and N (electrons) regions generating currents

P N

intPI qh

- One photon creates a pair of electron and hole

- Problem: depletion region is very thin (< 1 m) int is very small

=> Use PIN structure



p+

i-Si n+

SiO2Electrode

net

–eNa

eNd

x

x

E (x)

R

Eo

E

e–h+

Iph

h > Eg

W

(a)

(b)

(c)

(d )

Vr

Vout

Electrode

PIN Photodiode



Avalanche Photodiode (APD)(avalanche: a large mass of snow, ice, earth, rock, or other material in swift motion down a mountainside)

Achieve gain by multiplying electrons and/or holes.

Impact Ionization: Under high E-field, electrons and holes can have sufficiently high kinetic energies breaking bonds and creating new e-h pairs.

PD with gain?

h+E

šn+ p

e–

Avalanche region

e–

h+

Ec

Ev

EIt is preferred only one type of carrier (either electron or hole) causes impact Ionization

: ratio of ionization coefficients (= hole/electron)



Metal-Semiconductor-Metal (MSM) Ge PD– Low responsivity due to metal shadow (surface illuminated type)– Large dark current (low schottky barrier, quality of Ge grown on Si)– Electrode distance photodetection bandwidth

Type Responsivity OE bandwidth Dark current Ge thickness

WG MSM 0.14 A/W @ -1V 40 GHz @ -2V 90 uA @ -1V 100 nm

Ref) 2010, OE, CMOS-integrated high-speed MSM germanium waveguide photodetector, IBM



Vertical PIN Ge PD– Thickness of intrinsic Ge: tradeoff between transit time and junction cap– RC time and transit time photodetection bandwidth

Ref) 2015, JLT, High-responsivity low-voltage 28-Gb/s Ge p-i-n photodetector with silicon contacts, IMEC


WG Vertical PIN 0.5 A/W @ -1V 50 GHz @ -1V 50 nA @ -1V 400 nm

E-field



Lateral PIN Ge PD– Lower minority carrier diffusion length increase photodetection bandwidth

Ref) 2015, OE, High bandwidth, high responsivity waveguide-coupled germanium p-i-n photodiode, IHP


WG Lateral PIN > 1 A/W @ -1V > 70 GHz @ -1V 100 nA @ -1V 500 nm



Separate-Absorption-Charge-Multiplication (SACM) PD (Ge/Si APD) – Si’s low noise property & Ge’s strong absorption near 1.55 μm wavelength– Low keff (k ~ 0.09, ratio of ionization coefficients of electrons and holes) high gain-bandwidth products, low noise

– Need large reverse bias for avalanche high dark current

Ref) 2013, OFC, High speed waveguide-integrated Ge/Si avalanche photodetector, IME


WG SACM APD 22 A/W @ -27V 20 GHz @ -27V 10 μA @ -27V 1 μm

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Lect. 10: Photodetectors - Yonseitera.yonsei.ac.kr/class/2015_2_2/lecture/Lect 10... · 2015. 12....

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