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Photodiodes_for_Technicians.php    6326 Bytes    08-02-2023 09:38:28

Photodiodes for Technicians

That's the Fast-Bleach-Version of that subject

✈ What's all this Photodiode stuff, anyhow ?

A Photodiode can be considered as a Photon (or other High Energy Particle) to Electron converter. Any Photon which manages to reach the depletion region of the Diode, generates a hole - electron pair, which contributes to the Photocurrent. Photons arriving in the outer areas (outside the window) do not contribute to the Photocurrent.

a low voltage makes a narrow depletion region
a higher voltage widens the depletion region

Applying a voltage (reverse Bias) is one trick to make the depletion region wider and therefore increase the Sensitivity, as the probability is increased, that the Photons hit this "window". This also reduces the capacity, as the distance of the "plates" of the capacitor is increased. Therefore the Photodiode gets faster, i.e. can handle higher frequencies.

✈ The Responsivity, R in A/W

The Responsivity (= R) of a photodiode is the ratio of generated photocurrent and incident optical power, determined in the linear region of response. It depends on the wavelength. The datasheet should provide you with a graph. If you have absolutely no clue, start with R = 0.5

PDB210A, Silicium, courtesy of Thorlabs
PDB210C, InGaAs, courtesy of Thorlabs

The graphs above show two Responsivity curves of two diodes by Thorlabs. The Responsivity can be seen as a function of the wavelength. One can also see, that for larger wavelengths, the InGaAs types are more suiteable than Silicium. The Bandwidth can also be seen, as the range between the two Wavelengths, where the Responsivity is e.g. 10 %.

✈ The Dark Current, I in A

The Dark current is a measure for the current, when no Photons (or other High Energy Particles) can reach the Photodiode. This current is caused by thermal effects. This value is always mentioned with respect to a certain reverse Bias Voltage, e.g. BPX-65 has a Dark Current of typ. 1 nA @ 20 V Reverse Bias.

✈ The Quantum Efficiency, η

This is a measure to describe the fraction of incident photons which are absorbed by a photoconductor and generated electrons which are collected at the detector terminal in a specific timespan. This value can be greater than 100% if you consider the internal current gain mechanism in Avalanche Photodiodes. For a BPX-65, this is η = 0.80 Electrons / Photon.

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t1 = 6586 d

t2 = 181 ms

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