EE 233 Seminar

Photodetectors for Communications

T. P. Lee Chief Scientist and Director, Bellcore (retired) Program Director, NSF (1997-1999) Princeton University (2000-2003)

EE233 3/9/06 TPLee Outline • Principle of Photodetectors absorption, collection, types, responsivity, quantum efficiency, • PIN structures, 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 Photodiodes Avalanche multiplication, ionization rates, Si-APDs, InGaAs/InP APDs, SAM-APD, SAGM-APD, gain-bandwidth product, excess noise factor of APDs. • OEIC Receivers p-i-n/MODFET, p-i-n/HBT • PIN and APD Noise , thermal noise, signal-to-noise ratio • Comparison of Receiver Sensitivities

EE233 3/9/06 TPLee Types of Photodetectors

• MSM Photodetectors Photoconductors Schottky Barriers complexity • PIN Photodiodes • Avalanche Photodiodes (APDs) • Photo-Transistors

EE233 3/9/06 TPLee Photo-detection Process

EE233 3/9/06 TPLee 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

d P = P (1-r) [1-exp(-αd)] 00 abs 0

EE233 3/9/06 TPLee Responsitivity

• Photocurrent

⎛ P0 ⎞ −αd ph = qI ⎜ ⎟()()11 −− er ⎝ hν ⎠

reflectivity at # incident electronic air-to- fraction absorbed photons per charge in semiconductor second interface

EE233 3/9/06 TPLee Responsitivity (cont’d)

• Responsitivity

Photocurrent

I q R ph −−== er −αd )1)(1( P0 hν

Incident optical power

=ηe ( hqR ν )=ηe (λ 24.1 )

EE233 3/9/06 TPLee Quantum Efficiency

• External Quantum Efficiency # electrons collected

ph qI −αd ηe ()[]11 −−== er 0 hP ν

# incident photons • Internal Quantum Efficiency

−αd ηηei ( ) −=−= er )1(1

EE233 3/9/06 TPLee 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

EE233 3/9/06 TPLee EE233 3/9/06 TPLee EE233 3/9/06 TPLee Si p-i-n

• 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 GaAlAs lasers

T.P.Lee and T. Li, Chapter 18 in Optical Fiber Communications, ed. S.E. Miller and A.G. Chynoweth, Academic Press, 1979 EE233 3/9/06 TPLee Responsivity of InGaAs Photodiode

• η = 70% no AR coating • = 90% with AR coating • back illumination T.P.Lee, et al., IEEE J. Quantum Electronics, QE-15, p. 30 (1979)

T.P.Lee, Photodetectors, Chapter 5 in Fiber EE233 3/9/06 TPLee Optics, ed. James Daly, CRC Press, (1984) Rise Time and Bandwidth

• The output voltage − / RCt across the load R is out 0 [1−= eVV ]

• The rise time is Tr = ( 9ln )(τ +τ RCtr )

τ RC = RC

τ tr Vd d == Transit time

• The bandwidth is Δf = [21 π (τ +τ RCtr )]

Trade off between quantum efficiency and bandwidth

EE233 3/9/06 TPLee Charateristics of p-i-n Photodiodes

Parameter Unit Si InGaAs Wavelength (λ) µm 0.4 – 1.1 1.0 – 1.7 Responsivity (R) A/W 0.4 – 0.6 0.6 – 0.9 Quantum eff. (η) % 75 –90 60 –70

Dark current (Id) nA 1 – 10 1 – 20 * Rise time (Tr)ns0.5 –10.02 – 0.5 Bandwidth (Δf) GHz 0.3 – 0.6* 1 – 10 * Bias voltage (Vb) V 50 – 100 5 – 10

* For 0.8 to 0.9 µm wavelength region EE233 3/9/06 TPLee Quantum Efficiency and Bandwidth Trade-off

EE233 3/9/06 TPLee 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

EE233 3/9/06 TPLee 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)

EE233 3/9/06 TPLee InGaAs photodiode with integrated Bias Tee and Matched Resistor

Y.-G. Wey et al., IEEE Photonic Technology EE233 3/9/06 TPLee Letters, 5, p.1310 (1993) InGaAs Waveguide Photodetector

Wake et al., Electronic Letters, 27, p.1073 (1991) Kato et al., IEEE Photonics Tech. Lett. 6, p.719 (1994)EE233 3/9/06 TPLee InGaAs Traveling Wave Photodetector

η = 44%

K.S. Giboney et al., IEEE Photonic Tech. Lett., 7, p.412 (1995) EE233 3/9/06 TPLee InP/InGaAsP/InGaAs Resonant Cavity Photodiode

(R1)

(R2)

Dentai, η = 82%, Tan, η = 93%,

A.G.Dentai et al., Electronic Letters, 27, p.2125 (1991) I.-H. Tan et al., IEEE Photonics Tech. Lett., 6, p. 811 (1994)EE233 3/9/06 TPLee Transit Time and RC Bandwidth

EE233 3/9/06 TPLee Avalanche Photodiode pni (a) p-i-n diode in high electric field 5 E (10 V/cm) results in impact ionization. (b) Electron and hole ionization rates are almost equal • limited gain-bandwidth product • higher avalanche noise

(c) Electron ionization rate is larger than hole ionization rate • large gain- bandwidth product • lower avalanche noise

EE233 3/9/06 TPLee Ionization Rate – Si & Ge

EE233 3/9/06 TPLee Ionization Rate - InGaAs

EE233 3/9/06 TPLee Avalanche Multiplication for an Uniform E-field

⎡ w ⎤ • β = 0, M e = exp αdx = exp()αw ⎣⎢∫0 ⎦⎥

⎡ w ⎤ • α = β, MM he 11 −== αdx ()11 −= αw ⎣⎢ ∫0 ⎦⎥

1− keff • β << α, M e = []()1exp eff α −−− kwk eff

keff = αβ EE233 3/9/06 TPLee 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

EE233 3/9/06 TPLee 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

EE233 3/9/06 TPLee 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 EE233 3/9/06 TPLee Si Avalanche Photodiode

•λ= 825 nm • G >100 • gain reduces at high temperature

T.P.Lee and T. Li, Chapter 18, Optical Fiber Communications, EE233 3/9/06 TPLee ed. S. E. Miller and A. G. Chynoweth, Academic Press (1979) 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) EE233 3/9/06 TPLee Dark Current of InGaAs PIN PD

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

exp[−= θγ 2321 / EqEmAI ] Itun tun 0 g h m

= ( AwqnI τ [ − (− 2exp1) kTVq )] − irg eff * 21 3 2 Idiff+Ig-r γ = (2 g ) m hVEqEm

* 21 = κθ()mm 0

EE233 3/9/06 TPLee InGaAs Separate Absorption and Multiplication APD (SAM-APD)

EE233 3/9/06 TPLee SAM APD Boundary Conditions

EE233 3/9/06 TPLee InGaAs SAM-APD Structures

Mesa Structure Planar Structure

EE233 3/9/06 TPLee SAM-APD Dark Current

EE233 3/9/06 TPLee Pulse response of SAM-APD

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

EE233 3/9/06 TPLee SAGM APD Band Structure

Graded Layer

InGaAsP

InP InGaAs

A graded layer is added to reduce trapped holes EE233 3/9/06 TPLee Frequency Response: SAGM APD vs SAM APD

EE233 3/9/06 TPLee Gain-Bandwidth Product of InGaAs SAGM-APD

EE233 3/9/06 TPLee Improved GxB Product SAGM APD

Gain x Bandwidth = 122 GHz EE233 3/9/06 TPLee Excess Noise Factor of InGaAs SAGM APD

EE233 3/9/06 TPLee Multiple Quantum Well APD

EE233 3/9/06 TPLee InAlGaAs/InAlAs Multiple Quantum Well APD

EE233 3/9/06 TPLee Resonant-Cavity SAM APD

EE233 3/9/06 TPLee Resonant-Cavity SAM APD

EE233 3/9/06 TPLee Bandwidth vs Multiplication

EE233 3/9/06 TPLee 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 EE233 3/9/06 TPLee 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 EE233 3/9/06 TPLee Photodetector Noise

• Shot Noise – the photo current is consisted of a stream of electron-hole pairs that are ()= + sp (tiItI ) generated randomly in response to the optical signal. The current flucturation produces shot noise. s ( )= qIfs p • The current fluctuation

follows Poisson ∞ 22 σ ss ()== s () 2 pΔ= fqIdffsti statistics. ∫ ∞− • The spectral density of shot noise is constant 2 • The total shot noise σ s 2 ( += IIq dp )

EE233 3/9/06 TPLee Photodetector Noise (cont’d)

• Thermal Noise –due to ()= + ( )+ (titiItI ) the random motion of Tsp electrons in a conductor • Modeled with Gaussian T ()= 2 RTkfs LB statistics 22 • Spectral density is σ = TT (ti )

independent of ∞ frequency = T ()= ()4 LB ΔfRTkdffs ∫ ∞− • Total photodetector noise

222 σσσTs 2[ ( dp )++=+= LB ]4 ΔfRTkIIq EE233 3/9/06 TPLee P-i-n Receiver Noise

22 • Singal-to-Noise = ISNR p σ Ratio PR 22 = in 2 ()din +Δ+ 4()LB ΔfRTkfIRPq

PRR 22 • Thermal-Noise SNR = inL Limit 4 B ΔfTk

RP • Shot-Noise SNR = in Limit 2 Δfq

EE233 3/9/06 TPLee APD Receiver Noise

p = MRPI in 2 2 σ s = 2 ()din Δ+ fIRPFqM

()eff ()eff ()−−+= 121 MkMkMF 2 ()MRPin SNR = 2 2 ()()din +Δ+ /4 LB ΔfRTkfIRPFqM

EE233 3/9/06 TPLee 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 EE233 3/9/06 TPLee