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Photodetectors for Optical Fiber Communications

T. P. LeeChief Scientist and Director, Bellcore (retired)

Program Director, NSF (1997-1999)

Princeton University (2000-2003)

EE 233 Seminar

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• Principle of Photodetectorsabsorption, collection, types, responsivity, quantum efficiency,

• PIN Photodiodesstructures, Si-pin diodes, InGaAs pin diodes, rise-time and

bandwidth, quantum efficiency-bandwidth trade-off• High-speed pin diodes

small-area pin diodes, waveguide photodetector, traveling-wave photodetector, resonant cavity photodetector

• Avalanche PhotodiodesAvalanche multiplication, ionization rates, Si-APDs, InGaAs/InP

APDs, SAM-APD, SAGM-APD, gain-bandwidth product, excess noise factor of APDs.

• OEIC Receiversp-i-n/MODFET, p-i-n/HBT

• PIN and APD NoiseShot noise, thermal noise, signal-to-noise ratio

• Comparison of Receiver Sensitivities

Outline

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Types of Photodetectors

• MSM PhotodetectorsPhotoconductors

Schottky Barriers

• PIN Photodiodes• Avalanche Photodiodes (APDs)• Photo-Transistors

complexity

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Photo-detection Process

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The Photodetection Process in a PN Junction

When the incident photon energy hƲ > Eg, the photons are absorbed and electron-hole pairs are generated.

Under the influence of an electric field by the applied voltage, electrons and holes are swept across the the drift region, resulting in a flow of electric current in the load resistor.

The optical power absorbed is

Pabs = P0(1-r) [1-exp(-αd)]00

d

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Responsitivity• Photocurrent

fraction absorbed in semiconductor

( )( )dph er

hPqI α

ν−−−⎟

⎠⎞

⎜⎝⎛= 110

electronic charge

# incident photons per second

reflectivity at air-to-semiconductor interface

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Responsitivity (cont’d)• Responsitivity

)1)(1(0

dph erhq

PI

R α

ν−−−==

Photocurrent

Incident optical power

( ) ( )24.1ληνη ee hqR ==

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Quantum Efficiency• External Quantum Efficiency

• Internal Quantum Efficiency

( ) )1(1 dei er αηη −−=−=

( )[ ]dphe er

hPqI α

νη −−−== 11

0

# electrons collected

# incident photons

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Optical Absorption Coefficients

Si, Ge

• indirect-bandgap.

• slow increase in absorption near the band edge.

• α ~ 102 -103cm-1 (si)

GaAs, InGaAs

• direct–bandgap

• sharp increase in absorption near the band edge.

• α ~ 104 cm-1

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Si p-i-n Photodiode

• i-region width = 20-50 µm

• Quantum efficiency is peaked at 800 nm

• Device was used for early optical fiber transmission systems at 0.8-0.9 µm wavelength using GaAlAslasers

T.P.Lee and T. Li, Chapter 18 in Optical Fiber Communications, ed. S.E. Miller and A.G. Chynoweth, Academic Press, 1979

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Responsivity of InGaAsPhotodiode

• η = 70% no AR coating

• = 90% with AR coating

• back illuminationT.P.Lee, et al., IEEE J. Quantum Electronics, QE-15, p. 30 (1979)

T.P.Lee, Photodetectors, Chapter 5 in FiberOptics, ed. James Daly, CRC Press, (1984)

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Rise Time and Bandwidth

• The output voltage across the load R is

• The rise time is

• The bandwidth is

[ ]RCtout eVV /

0 1 −−=

( )( )

===

+=

dtr

RC

RCtrr

VdRC

T

ττ

ττ9ln

( )[ ]RCtrf ττπ +=Δ 21

Transit time

Trade off between quantum efficiency and bandwidth

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Charateristics of p-i-nPhotodiodes

Parameter Unit Si InGaAsWavelength (λ) µm 0.4 – 1.1 1.0 – 1.7Responsivity (R) A/W 0.4 – 0.6 0.6 – 0.9Quantum eff. (η) % 75 – 90 60 – 70Dark current (Id) nA 1 – 10 1 – 20Rise time (Tr) ns 0.5 – 1* 0.02 – 0.5Bandwidth (Δf) GHz 0.3 – 0.6* 1 – 10Bias voltage (Vb) V 50 – 100* 5 – 10

* For 0.8 to 0.9 µm wavelength region

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Quantum Efficiency and Bandwidth Trade-off

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Methods for Increased Bandwidth and Quantun Efficiency

• Reduction of RC time-constant by- small diode diameter or area- integrated bias tee- waveguide photodetector- traveling wave photodetector

• Increasing quantum efficiency by- resonant cavity photodetector

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A High-Speed InGaAs pin Photodiode

• Area = 25 µm2

• Q.E. = 31%

• Δf = 42 GHz

Crawford et al., IEEE Photonic Technology Letters, 2, p.647 (1990)

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InGaAs photodiode with integrated Bias Tee and Matched Resistor

Y.-G. Wey et al., IEEE Photonic Technology Letters, 5, p.1310 (1993)

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InGaAs Waveguide Photodetector

Wake et al., Electronic Letters, 27, p.1073 (1991)

Kato et al., IEEE Photonics Tech. Lett. 6, p.719 (1994)

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InGaAs Traveling Wave Photodetector

K.S. Giboney et al., IEEE Photonic Tech. Lett., 7, p.412 (1995)

η = 44%

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InP/InGaAsP/InGaAs Resonant Cavity Photodiode

(R2)

(R1)

A.G.Dentai et al., Electronic Letters, 27, p.2125 (1991)

I.-H. Tan et al., IEEE Photonics Tech. Lett., 6, p. 811 (1994)

Dentai, η = 82%, Tan, η = 93%,

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Transit Time and RC Bandwidth

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Avalanche Photodiodep ni

E

• limited gain-bandwidth product

• higher avalanche noise

(c) Electron ionization rate is larger than hole ionization rate

• large gain-bandwidth product

• lower avalanche noise

(a) p-i-n diode in high electric field (105V/cm) results in impact ionization.

(b) Electron and hole ionization rates are almost equal

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Ionization Rate – Si & Ge

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Ionization Rate - InGaAs

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Avalanche Multiplication for an Uniform E-field

• β = 0,

• α = β,

• β << α,

( )wdxMw

e αα expexp0

=⎥⎦⎤

⎢⎣⎡= ∫

( )wdxMMw

he αα −=⎥⎦⎤

⎢⎣⎡ −== ∫ 1111

0

( )[ ]αβ

α

=

−−−−

=

eff

effeff

effe

kkwk

kM

1exp1

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Gain & Bandwidth of APD

Mo < α/β

Bandwidth is almost independent of gain

Mo > α/β

Gain-Bandwidth product is limited

GB=(α/β)/NTav

N = 1/3 to 2

Tav= ave. transit time

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Excess Noise Factor

F = keffMe+[2-1/Me](1-keff)

keff = β/α

Electron ionization only, β=0

F = 2

Both e & h ionization, β/α=1

F = M

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Si Reach-through APD Structure

• p+-π-p-n+ reach-through structure

• high field appears at the pn+ junction

• low field in the π-(nearly intrinsic) drift region

• electrons drift toward pn+ junction initiates impact ionization

• holes drifting in the low-field π-region toward p+ result in no ionization

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Si Avalanche Photodiode

•λ= 825 nm

• G >100

• gain reduces at high temperature

T.P.Lee and T. Li, Chapter 18, Optical Fiber Communications, ed. S. E. Miller and A. G. Chynoweth, Academic Press (1979)

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Si – APD Excess Noise Factor

T.P.Lee and T. Li, Chapter 18, Optical Fiber Communications, ed. S. E. Miller and A. G. Chynoweth, Academic Press (1979)

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Dark Current of InGaAs PIN PD

( )[ ]kTVqAwqnI effirg 2exp1)( −−=− τ

[ ]mgtun EqEmAI h/exp 23210θγ −=

Idiff+Ig-r

Itun

The dark current of InGaAs p-i-ndiode is dominated by the tunneling current at high voltages:

( )( ) 21

0*

2321*2

mm

hVEqEm mg

κθ

γ

=

=

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InGaAs Separate Absorption and Multiplication APD (SAM-APD)

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SAM APD Boundary Conditions

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InGaAs SAM-APD Structures

Mesa Structure Planar Structure

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SAM-APD Dark Current

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Pulse response of SAM-APD

The long tail is due to holes piling up at the InP/InGaAs interface

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SAGM APD Band StructureGraded Layer

InP InGaAs

InGaAsP

A graded layer is added to reduce trapped holes

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Frequency Response: SAGM APD vs SAM APD

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Gain-Bandwidth Product of InGaAs SAGM-APD

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Improved GxB Product SAGM APD

Gain x Bandwidth = 122 GHz

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Excess Noise Factor of InGaAs SAGM APD

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Multiple Quantum Well APD

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InAlGaAs/InAlAs Multiple Quantum Well APD

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Resonant-Cavity SAM APD

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Resonant-Cavity SAM APD

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Bandwidth vs Multiplication

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A p-i-n/MODFET OEIC Receiver

• Single epitaxial growth on recessed substrate

• High yield OEIC

• 3-dB bandwidth of 6 GHz

T.P.Lee and S. Chandrasekhar, Chapter 7 in Modern Semiconductor Device Physics, ed. S. M. Sze, John Wiley and sons, 1998

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A p-i-n/HBT OEIC Receiver

• Both p-i-n and HBT are grown on a single planar substrate.

• then they are separated by wet chemical etching.

• 3-dB bandwidth of 20 GHz achieved.

T.P.Lee and S. Chandrasekhar, Chapter 7 in Modern Semiconductor Device Physics, ed. S. M. Sze, John Wiley and sons, 1998

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Photodetector Noise• Shot Noise – the photo

current is consisted of a stream of electron-hole pairs that are generated randomly in response to the optical signal. The current flucturationproduces shot noise.

• The current fluctuation follows Poisson statistics.

• The spectral density of shot noise is constant

• The total shot noise

( ) ps qIfs =

( ) ( )tiItI sp +=

( ) ( ) fqIdffsti psss Δ=== ∫∞

∞−222σ

( )dps IIq += 22σ

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Photodetector Noise (cont’d)• Thermal Noise –due to

the random motion of electrons in a conductor

• Modeled with Gaussian statistics

• Spectral density is independent of frequency

• Total photodetectornoise

( ) ( ) ( )titiItI Tsp ++=

( ) LBT RTkfs 2=

( )

( ) ( ) fRTkdffs

ti

LBT

TT

Δ==

=

∫∞

∞−4

22σ

( ) fRTkIIq LBdpTs Δ++=+= ]42[222 σσσ

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P-i-n Receiver Noise• Singal-to-Noise

Ratio

• Thermal-Noise Limit

• Shot-Noise Limit

( ) ( ) fRTkfIRPqPR

ISNR

LBdin

in

p

Δ+Δ+=

=

42

22

22 σ

fTkPRRSNR

B

inL

Δ=

4

22

fqRPSNR in

Δ=

2

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APD Receiver Noise

( )( ) ( )( )

( )( ) ( ) fRTkfIRPFqM

MRPSNR

MkMkMFfIRPFqM

MRPI

LBdin

in

effeff

dins

inp

Δ+Δ+=

−−+=Δ+=

=

/42

1212

2

2

22σ

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Photoreciever Sensitivities vs Bit Rate

T.P.Lee and S. Chandrasekhar, Chapter 7 in Modern Semiconductor Device Physics, ed. S. M. Sze, John Wiley and Sons, 1998