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PD Photocurrent rise/fall time an unexpectedly strong function of wavelength?!

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evb149:
This is a mere curiosity since it has been a while since I read about / did design with photodiodes and I was surprised to see a data sheet which indicated that a reverse biased silicon photodiode would have a LOT (50:1?) longer rise & fall time across an external 50R load resistor depending on wavelength. 

The device in question "typically" has the slowest rise/fall times at around 940nm, its peak sensitivity wavelength, whereas it
has much faster (1/50th) rise/fall times around 540nm where the photocurrent sensitivity is about 50% of the peak value.
So only one octave difference in wavelength, one octave difference in photocurrent sensitivity, but a 50x worse transition time at the peak sensitivity wavelength.

I don't recall ever seeing anything like that empirically listed in a photodiode data sheet nor do I recall any theoretical reason why
it'd be so in a case such as the above.

I don't think there are any phosphors or odd wavelength dependent factors about the device besides being a generic Si PIN photodiode.

I didn't ask the OEM or look at wherever they detail their test conditions specification for enlightenment, and it's of no real importance in this ad hoc case, I'm just wondering if I'm not thinking of some general truth that'd extend this to other devices and situations which I should know.

EDIT: The only things that comes to mind (and I doubt it's relevant or even the case to a significant degree here) is if the actual photoelectrons are 'hotter' / faster when a short wavelength generates them than a long wavelength and may have more mobility / speed; also it'd be possible the short wavelength phototons are generally absorbed at a different depth than the long wavelength ones and that could lead to a different diffusion time until they're externalized and contribute to the photo-current.   Interesting ideas, but somehow they don't feel satisfactory here and if they were significant to this degree I would think I would remember hearing about this level of time dispersion before since the effect if true is HUGE.

741:
That is interesting and likely not "well known" - would you be able to post a link to the datasheet please? I don't think A.D. do photodiodes, but if they did I'd post an RAQ (Rarely Asked Question).
Cheers
Stephen

Kleinstein:
The wavelength effects on how deep the light goes into to bulk matrial. At 940 nm (and even more with longer wavelengths liek 980 nm) much of the light is absorbt deep in the bulk material, while the active PIN structure is close to the surface of maybe a few µm. With PIN diodes this depth depends on the type, but often the low dopded zone is still relatively thin and not making up the bulk. The electron hole pairs have to first move (in a relatively weak field, gradient) to the junction before they actually contribute to the photo current.
With 540 nm most of the light is absorbed in front or inside the junction. This makes the photocurrent to get effective nearly instantaneous.  The front layer (often N doped) is usually quite thin.

For the different wavelengths the relevant effect is changing the absorbtion in the silicon. The absorbtion goes down quite a lot when approaching the band edge.

jonpaul:
please post diode ID or spec sheet

All photo sensitive devices have a curve of current per unit illumination vs wavelength

Risetime into a capacitive load dv/dt= I/C

As excitation wavelength changes, so will Conversion to current, thus low current gives slow Risetime.


Suggest you read a few papers or a textbook, EGG, UDT, Tektronix have great light measurement notes

Jon

evb149:
Thank you all very much for the responses!

Here's the spec sheet which motivated my question attached (hopefully) to this.

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