Optical Fiber CommunicationDetectorsBy:

Mr. Gaurav VermaAsst. Prof.ECE dept.NIEC

Detector TechnologiesMSM(Metal Semiconductor Metal)

PIN

APD

WaveguideContact InP p 1x1018Multiplication InP n 5x1016Transition InGaAsP n 1x1016Absorption InGaAs n 5x1014Contact InP n 1x1018Substrate InP Semi insulatingSemiinsulating GaAsContact InGaAsP p 5x1018Absorption InGaAs n- 5x1014Contact InP n 1x1019Absorption LayerGuide LayersSimple, Planar, Low CapacitanceLow Quantum EfficiencyTrade-off Between Quantum efficiency and SpeedHigh efficiencyHigh speedDifficult to couple intoGain-Bandwidth: 120GHzLow NoiseDifficult to makeComplexKey:Absorption Layer

Contact layersLayer StructureFeatures

Photo Detection Principles(Hitachi Opto Data Book)Device Layer StructureBand Diagramshowing carriermovement in E-fieldLight intensity as a function of distance below the surfaceCarriers 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.Bias voltage usually needed to fully deplete the intrinsic I region for high speed operation

Current-Voltage Characteristic for a Photodiode

Characteristics of Photodetectors Internal Quantum Efficiency

External Quantum efficiency

Responsivity

PhotocurrentIncident Photon Flux (#/sec)Fraction Transmitted into DetectorFraction absorbed in detection region

ResponsivityOutput current per unit incident light power; typically 0.5 A/W

Photodiode Responsivity

Detector Sensitivity vs. WavelengthAbsorption coefficient vs. Wavelength for several materials (Bowers 1987)Photodiode Responsivity vs. Wavelength for various materials (Albrecht et al 1986)

PIN photodiodesEnergy-band diagramp-n junctionElectrical Circuit

Basic PIN Photodiode StructureFront Illuminated PhotodiodeRear Illuminated Photodiode

PIN Diode StructuresDiffused Type (Makiuchi et al. 1990)Etched Mesa Structure(Wey et al. 1991)Diffused Type(Dupis et al 1986)Diffused structures tend to have lower dark current than mesa etched structures although they aremore 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 regionHigh-energy electron-hole pairs ionize other sites to multiply the currentLeads to greater sensitivity

APD DetectorsSignal CurrentAPD Structure and field distribution (Albrecht 1986)

APDs Continued

Detector Equivalent CircuitsPINIph=Photocurrent generated by detectorCd=Detector CapacitanceId=Dark CurrentIn=Multiplied noise current in APDRd=Bulk and contact resistance

MSM DetectorsSemi insulating GaAsSimple to fabricate

Quantum efficiency: MediumProblem: Shadowing of absorption region by contacts

Capacitance: Low

Bandwidth: HighCan 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 processesGaAs FETS and HEMTs InGaAs/InAlAs/InP HEMTsTo increase speeddecrease electrode spacingand absorption depthAbsorptionlayerNon absorbing substrateE Fieldpenetrates for ~ electrode spacinginto materialSimplest VersionSchottky barriergate metalLight

Waveguide Photodetectors(Bowers IEEE 1987)Waveguide detectors are suited for very high bandwidth applicationsOvercomes low absorption limitationsEliminates carrier generation in field free regionsDecouples transit time from quantum efficiencyLow capacitanceMore difficult optical coupling

Carrier transit timeTransit time is a function of depletion width and carrier drift velocity

td= w/vd

Detector Capacitancep-n junctionxpxnPNCapacitance 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

Minimize by putting low doped I region between the P and N doped regions to increase W, the depletion width

W can be increased until field required to fully deplete causes excessive dark current, or carrier transit time begins to limit speed.

Bandwidth limitC=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 TradeoffTransit time

=W/vsat

vsat=saturation velocity=2x107 cm/s

R-C Limitation

Responsivity

Diffusion

=4 ns/m (slow)

Dark CurrentSurface LeakageBulk LeakageSurface Leakage

Ohmic Conduction

Generation-recombinationvia surface statesBulk Leakage

Diffusion

Generation-Recombination

TunnelingUsually not a significant noise source at high bandwidths for PIN StructuresHigh dark current can indicate poor potential reliabilityIn APDs its multiplication can be significant

Signal to Noise Ratio

ip= average signal photocurrent levelbased on modulation index m where

Optimum value of M

where F(M) = Mx and m=1

Noise Equivalent Power (NEP)Signal power where S/N=1Units are W/Hz1/2

Typical Characteristics of P-I-N and Avalanche photodiodes

ComparisonsPIN gives higher bandwidth and bit rateAPD gives higher sensitivitySi works only up to 1100 nm; InGaAs up to 1700, Ge up to 1800InGaAs has higher for PIN, but Ge has higher M for APDInGaAs has lower dark current