General > General Technical Chat
PD Photocurrent rise/fall time an unexpectedly strong function of wavelength?!
<< < (3/6) > >>
tom66:

--- Quote from: Marco on June 19, 2022, 04:53:08 pm ---
--- Quote from: jonpaul on June 19, 2022, 12:59:50 pm ---Risetime into a capacitive load dv/dt= I/C

--- End quote ---

That's unlikely to be relevant in the context of a 50 Ohm loaded reverse biased photodiode. It's generally not rising to a fixed voltage, it's rising to the equilibrium voltage which is only a small fraction of the bias voltage. With the simple current source + parallel capacitor model, an instantaneous increase of the photocurrent should lead to a predictable risetime regardless of the magnitude of the change.

--- End quote ---

Not only that, but the change in photocurrent isn't that significant between 530nm and 940nm (roughly 2:1) but the rise time varies by 100:1, so it's clearly not as simple as dv/dt.
jonpaul:
repeat experiment into a transconductance amp or 1 Ohm R so only the current is sensed and voltage change is negligable

Jon
jonpaul:
Check the curves of Quantum Efficiency vs wavelength.

QE 1.0 1 photon >>1 electron.

It is rarely even close to 1.0 at the peak of curve. It is a property of the material.

https://en.wikipedia.org/wiki/Quantum_efficiency
Jon
mawyatt:

--- Quote from: evb149 on June 19, 2022, 02:23:29 pm ---
I vaguely remembered hearing something about spectrally dependent absorbtion depth in semiconductors but I couldn't recall for sure if that was noted about camera image sensors or photodiodes or solar cells or what and also what the real world effects of that were for this case.

Very interesting...

--- End quote ---

Foveon (later acquired by Sigma) made use of this wavelength dependent depth with their unique image sensor which created a "3D" pixel where Blue was captured at the top surface, Green in a mid-layer and Red photons on the bottom surface of the pixel. The original sensors were made at National Semiconductor on a Silicon bipolar process line.

Interesting Carver Mead was one of the original investors in Foveon.

Best,
mag_therm:
Comments here are  about the actual quantum efficiency versus wavelength rather than the rise and fall times although they may be related.
Single photodiodes are held reverse biased and the current variation with light level is used.
Camera sensors have the pd reset/pulled negative just prior to the integrating period with shutter open.
Color information is by a tri-color mask over the sensor which otherwise has all cells equal.
The sensor photodiode levels are written to a raw HEX file prior to any processing.

I extracted data for a few pixels to see the levels of the 3 colors.
https://app.box.com/s/473axtfr6pm43m4pi0et5i97blovdlt5

Note the readings for the LAB 50,0,0 grey image. The raw levels will be multiplied by the manufacturers' correction matrix and should result in
that neutral image. (before color temperature correction and user preference etc)
Firstly, the cameras use 2 times more Green wells than Red and Blue.
This is because the s/n ratio of green is best.
Secondly Red has the lowest levels, along with worst s/n.

The sensor quantum efficiencies versus wavelength  are adjusted by manufacturers for best fidelity. Generally the peak is at about 0.65 um
the blue side rolls off by the choice of coating of the front illuminated sensor.
The red side rolls off (I think unavoidably) because of recombination deeper in the cell before the electrons reach the storage.

My ref here for the above comments is from 2011 and might be getting a bit dated:
Holst and Lomheim "cmos/ccd Sensors and Camera Systems"
Navigation
Message Index
Next page
Previous page
There was an error while thanking
Thanking...

Go to full version
Powered by SMFPacks Advanced Attachments Uploader Mod