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PD Photocurrent rise/fall time an unexpectedly strong function of wavelength?!
Kleinstein:
In the red range photodiodes can reach a quantum efficiency quite close to 1. The main point there is reflection and shading from the contact areas. For the blue range the refraction index of silicon changes quite a bit when the direct band-gap is reached. This makes the anti reflective coating less effective and makes most solar cells and photodiodes look blue. The surface layer may also have nore recombination that the bulk material, which can also slightly reduce the QE in the deep blue.
For the NIR side the QE goes down as the lighte enters ever deeper into the buld and thus a slightly increased properbility for reconbination (and thus loss in current). The design and purity of the material effects how fast to QE drops to the long wavelength end. An additional drop can also come from decreasing effectiveness of the anti reflective coating as the refractive index also changes at the long wavelength end. By itself the reduced QE does not effect the speed. It is the extra distance in the bulk of the material that takes additional time. To generate a current from the electron hole pairs generated deep in the bulk, it need to build up a concentration of minority carriers to drive them to the junction. This acts a bit like additional capacitance.
Even though quite fast as a photodetector most photodiodes are slow diodes showing really slow reverse recovery. They may be an option as an RF PIN diode (act as a variable resistor / swtich for high frequency AC) for the lower frequency end, down to some 20 kHz. Chances are photodiode are among the slowest Si diodes one can get, as they use quite pure material. With a high qualtiy solar cell (kind of close to a photodiode) I have measured a reverse recovery time of some 50 µs.
jonpaul:
Good cat: UDT: (now OSI)
App notes, slection chart, specs on a wide varierty,
https://peshkin.mech.northwestern.edu/datasheets/Optoelectronics/Photodiode_UDT_catalog.pdf
https://www.osioptoelectronics.com/default.aspx
Jon
StillTrying:
--- Quote from: evb149 on June 19, 2022, 11:48:51 am ---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.
--- End quote ---
I've read that it can be up to 1000 times slower. :o
When I was flashing LEDs at a photodiode redish colours including IR had slower edges and curved tops, I don't know whether that was the LEDs or the PD.
The fastest I got was with 200mA though super bright blue or green.
https://www.eevblog.com/forum/chat/20w-halogen-bulb-viewed-by-a-photodiode/msg2411274/#msg2411274
To reduce the intensity of the blue or green I often shone the LED at the side of the SFH 213 PD, I was expecting the fast edges to be ruined, but that didn't seem to make any difference to the speed - I could still get <20ns rise and fall and a flat top.
Kleinstein:
Some LEDs in the red / IR range can also be slow with rise / fall times in the 1 µs range. One can test the speed of the LED by looking at the reverse recovery of the LED. The delay / time constant of the LED is similar to the revese recovery time.
TomKatt:
This discussion is well beyond my comprehension, but it reminds me of CuriousMarc's repair video of the photo chopper circuit in his vintage HP plotter...
Love that channel!
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