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6.973 Semiconductor OptoelectronicsLecture 7: p-i-n PhotodetectorsRajeev J. RamElectrical EngineeringMassachusetts Institute of TechnologyReading:• Chuang, 14.2 & 14.3Outline:• Velocity Saturation• Ramo’s Theorem and Displacement Currents• Quantum Efficiency•BandwidthDiode Under IlluminationDiode Under Illuminationdiffusionpp--typetypeGaAsGaAsNN--typetypeAlGaAsAlGaAslightCapacitance of Capacitance of HomojunctionHomojunctionDiodesDiodes• Wider depletion region gives lower capacitance, faster devicesHomojunctionIntroduction to Introduction to pp--ii--nnDiodesDiodespp--typetypenn--typetypeintrinsicintrinsic• Wider depletion width, so more absorption in high-field region• Same built-in potential as p-n junction• Lower peak electric field than p-n junctiondiffusiondiffusiondriftIntroduction to Introduction to pp--ii--nnDiodesDiodes• primary transport by diffusion• slow carrier movement by diffusion• small depletion with - high capacitance• can’t change bandgap to reduceparasitic absorption• primary transport by drift• low capacitance• can engineer bandgap so absorption is only in depletion regionDrift in Depletion RegionsDrift in Depletion Regions• This is not the same as transport through a quasi-neutral region (resistor) electronhole• Current is a response to carrier movement, not carrier collection • As soon as charge starts to move, a current is observed at the terminals of the deviceVelocity SaturationVelocity Saturation• For large fields, the electron velocity is independent of E-field• In GaAs, the electron velocity saturates at E-fields near 1 V/µmRamo’sRamo’sTheoremTheorem• As soon as charge starts to move, a current is observed at the terminals of the device • Work done by field to move charge• Work done by external circuitRamo’sRamo’sTheoremTheoremtime- currentTotal collected chargenot equal to !!Drift PhotocurrentDrift PhotocurrentEach electron-hole pair generates the equivalent current of a single charge:Quantum efficiency…efficiency of converting input photons into collected chargeResponsivityDesign for high quantum efficiency:• Long intrinsic absorbing region• Low reflectivity surface• Small doped p-regionnn--typetypeintrinsicintrinsicppAR AR coatingcoatingPhotodetectorPhotodetectorQuantum Quantum EfficieincyEfficieincydiffusiondriftdriftdrift+ diffusionEffects of Absorption in QuasiEffects of Absorption in Quasi--Neutral RegionsNeutral Regionstimecurrent• Diffusion in the p-region slows down the speed of the photodetectorBandwidth of Bandwidth of PhotodetectorPhotodetectorConsider a sinusoidal modulation of the incident light…Since the current is a liner response to the light, we can assume that it also varies sinusoidally…(look at hole limited transport)The total current includes the photocurrent from the entire depletion region…forBandwidth of Bandwidth of PhotodetectorPhotodetectorThe total current includes the photocurrent from the entire depletion region…Bandwidth of Bandwidth of PhotodetectorPhotodetectorSo, for a transit time limited device, the bandwidth is…Including the RC limit due to the depletion capacitance…Jang, J.-H. et al.Device Research Conference, 200000.20.40.60.811.210 100 10003 dB Bandwidth (GHz)Equivalent Device Slope Efficiency at λ = 1300 nm (A/W)UCSB / Colorado State, 1993PIN – 1300 nmOrtel, 1994UCSB / Colorado State, 1995GainGain--Bandwidth Product of Bandwidth Product of PhotodetectorPhotodetectorBandwidth of Bandwidth of


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MIT 6 973 - Velocity Saturation

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