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EE 230: Optical Fiber Communication Lecture 11 Detectors From the movie Warriors of the Net Detector Technologies Layer Structure MSM Simple, Planar, Low Capacitance Low Quantum Efficiency Semiinsulating GaAs (Metal Semiconductor Metal) InGaAsP p 5x1018 InGaAs n- 5x1014 InP n 1x1019 Contact Absorption Contact PIN Contact Multiplication Transition Absorption Contact Substrate APD Waveguide InP InP InGaAsP InGaAs InP InP Absorption Layer Absorption Layer Contact layers Trade-off Between Quantum efficiency and Speed Gain-Bandwidth: p 1x1018 120GHz n 5x1016 16 Low Noise n 1x10 14 Difficult to make n 5x10 Complex n 1x1018 Semi insulating Guide Layers Key: Features High efficiency High speed Difficult to couple into Photo Detection Principles Device Layer Structure Bias voltage usually needed to fully deplete the intrinsic “I” region for high speed operation Band Diagram showing carrier movement in E-field Light intensity as a function of distance below the surface Carriers absorbed here must diffuse to the intrinsic layer before they recombine if they are to contribute to the photocurrent. Slow diffusion can lead to slow “tails” in the temporal response. (Hitachi Opto Data Book) Current-Voltage Characteristic for a Photodiode Characteristics of Photodetectors • Internal Quantum Efficiency i Number of Collected electrons 1 e W Number of Photons *Entering* detector •External Quantum efficiency e i /q Number of Collected electrons ph 1 Rp 1 e W Number of Photons *Incident* on detector Po / h • Responsivity R i Photo Current (Amps) q ph 1 Rp 1 eW Incident Optical Power (Watts) Po h Fraction Transmitted into Detector •Photocurrent i ph P q o 1 Rp 1 e W RPo h Incident Photon Flux (#/sec) Fraction absorbed in detection region Responsivity Output current per unit incident light power; typically 0.5 A/W e R M h Photodiode Responsivity Detector Sensitivity vs. Wavelength Absorption coefficient vs. Wavelength for several materials Photodiode Responsivity vs. Wavelength for various materials (Bowers 1987) (Albrecht et al 1986) PIN photodiodes Energy-band diagram p-n junction Electrical Circuit Basic PIN Photodiode Structure Rear Illuminated Photodiode Front Illuminated Photodiode PIN Diode Structures Diffused Type (Makiuchi et al. 1990) Diffused Type (Dupis et al 1986) Etched Mesa Structure (Wey et al. 1991) Diffused structures tend to have lower dark current than mesa etched structures although they are more difficult to integrate with electronic devices because an additional high temperature processing step is required. Avalanche Photodiodes (APDs) • High resistivity p-doped layer increases electric field across absorbing region • High-energy electron-hole pairs ionize other sites to multiply the current • Leads to greater sensitivity APD Detectors q Signal Current i s M P h APD Structure and field distribution (Albrecht 1986) APDs Continued Detector Equivalent Circuits Rd Iph Id Cd PIN Rd Iph Id Cd In APD Iph=Photocurrent generated by detector Cd=Detector Capacitance Id=Dark Current In=Multiplied noise current in APD Rd=Bulk and contact resistance MSM Detectors Light Schottky barrier gate metal •Simple to fabricate •Quantum efficiency: Medium Problem: Shadowing of absorption region by contacts Semi insulating GaAs Simplest Version •Capacitance: Low •Bandwidth: High Can be increased by thinning absorption layer and backing with a non absorbing material. Electrodes must be moved closer to reduce transit time. •Compatible with standard electronic processes GaAs FETS and HEMTs InGaAs/InAlAs/InP HEMTs To increase speed decrease electrode spacing and absorption depth Absorption layer Non absorbing substrate E Field penetrates for ~ electrode spacing into material Waveguide Photodetectors •Waveguide detectors are suited for very high bandwidth applications •Overcomes low absorption limitations •Eliminates carrier generation in field free regions •Decouples transit time from quantum efficiency •Low capacitance •More difficult optical coupling (Bowers IEEE 1987) Carrier transit time Transit time is a function of depletion width and carrier drift velocity td= w/vd Detector Capacitance xp xn P N Capacitance must be minimized for high sensitivity (low noise) and for high speed operation Minimize by using the smallest light collecting area consistent with efficient collection of the incident light C A W Minimize by putting low doped “I” region between the P and N doped regions to increase W, the depletion width p-n junction w xp xn W can be increased until field required to fully deplete causes excessive dark current, or carrier transit time begins to limit speed. For a uniformly doped junction 1/ 2 A 2q C Nd 2 Vo Vbi 1/ 2 2(Vo Vbi ) W qNd Where: =permitivity q=electron charge Nd=Active dopant density Vo=Applied voltage V bi=Built in potential A=Junction area Bandwidth limit C=0K A/w where K is dielectric constant, A is area, w is depletion width, and 0 is the permittivity of free space (8.85 pF/m) B = 1/2RC PIN Bandwidth and Efficiency Tradeoff Transit time =W/vsat vsat=saturation velocity=2x107 cm/s R-C Limitation RC Rin A W Responsivity R q 1 Rp 1 e W h Diffusion =4 ns/µm (slow) Dark Current Surface Leakage Bulk Leakage Surface Leakage Bulk Leakage Ohmic Conduction Diffusion Generation-recombination via surface states Generation-Recombination Tunneling Usually not a significant noise source at high bandwidths for PIN Structures High dark current can indicate poor potential reliability In APDs its multiplication can be significant Signal to Noise Ratio i p2 M 2 S 2 N 2q I p I D M F M B 2qI L B 4k BTB / RL ip= average signal photocurrent level based on modulation index m where i 2 p 2 2 p m I 2 Optimum value of M M x2 opt 2qI L 4k BT / RL xqI p I D where F(M) = Mx and m=1 Noise Equivalent Power (NEP) Signal power where S/N=1 Units are W/Hz1/2 h 4kT x NEP 2eI D M 2 e M RL Typical Characteristics of P-I-N and Avalanche photodiodes Comparisons • PIN gives higher bandwidth and bit rate • APD gives higher sensitivity • Si works only up to 1100 nm; InGaAs up to 1700, Ge up to 1800 • InGaAs has higher for PIN, but Ge has higher M for APD • InGaAs has lower dark current
