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O-K-State BAE 5413 - Application of photodiodes

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Application of photodiodesQuantum devicesPhotodiode structurePhotodiode fundamentalsPhotodiode characteristicsPhotodiode current/voltage characteristicsTrans-impedance amplifier functionDiode operating modesFor the photovoltaic modeCircuit Optimization3/26/2003 BAE 5413 1 of 10Application of photodiodesA brief overview3/26/2003 BAE 5413 2 of 10Quantum devices•Absorption of a photon of sufficient energy elevates an electron into the conduction band and leaves a hole in the valence band.•Conductivity of semi-conductor is increased.•Current flow in the semi-conductor is induced.3/26/2003 BAE 5413 3 of 10Photodiode structure3/26/2003 BAE 5413 4 of 10Photodiode fundamentals•Based on PN or PIN junction diode–photon absorption in the depletion region induces current flow•Spectral sensitivityMaterial Band gap (eV)Spectral sensitivitysilicon (Si) 1.12 250 to 1100 nmindium arsenide (InGaAs) ~0.35 1000 to 2200 nmGermanium (Ge) .67 900 to 1600 nm3/26/2003 BAE 5413 5 of 10Photodiode characteristics•Circuit model–I0 Dark current (thermal)–Ip Photon flux related current•Noise characterization–Shot noise (signal current related)–q = 1.602 x 10–19 coulombs–I = bias (or signal) current (A)–is = noise current (A rms)–Johnson noise (Temperature related)–k = Boltzman’s constant = 1.38 x 10–23 J/K–T = temperature (°K)–B = noise bandwidth (Hz)–R = feedback resistor (W)–eOUT = noise voltage (Vrms)qiis2kTBReout43/26/2003 BAE 5413 6 of 10Photodiode current/voltage characteristics3/26/2003 BAE 5413 7 of 10Trans-impedance amplifier function•Current to voltage converter (amplifier)•Does not bias the photodiode with a voltage as current flows from the photodiode (V1 = 0)•Circuit analysissfII 0oI01VsffffIRIRV sffoutIRVV –Note: current to voltage conversion3/26/2003 BAE 5413 8 of 10Diode operating modes•Photovoltaic mode–Photodiode has no bias voltage–Lower noise–Lower bandwidth–Logarithmic output with light intensity•Photoconductive mode–Higher bandwidth–Higher noise–Linear output with light intensity3/26/2003 BAE 5413 9 of 10For the photovoltaic mode•I = thermal component + photon flux related current•where I = photodiode currentV = photodiode voltageI0 = reverse saturation current of diodee = electron chargek = Boltzman's constantT = temperature (K) = frequency of lighth = Plank’s constantP = optical power = probability that hv will elevate an electron across the band gaphePeIIkTeV 103/26/2003 BAE 5413 10 of 10Circuit Optimization•Burr-Brown recommendations (TI)•Photodiode capacitance should be as low as possible.•Photodiode active area should be as small as possible so that CJ is small and RJ is high.•Photodiode shunt resistance (RJ ) should be as high as possible.•For highest sensitivity use the photodiode in a “photovoltaic mode”.•Use as large a feedback resistor as possible (consistent with bandwidth requirements) to minimize noise.•Shield the photodetector circuit in a metal housing.•A small capacitor across RF is frequently required to suppress oscillation or gain peaking.•A low bias current op amp is needed to achieve highest

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