Identify pn vs. pin photodiode? - Page 1
Forum section: electronics / projects

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: August 03, 2023, 07:06:36 pm
content: If I have some random unmarked photodiodes of varying area, is there any way to tell the difference between pn and pin diodes? I can measure capacitance under various bias conditions and maybe speed, but given the different areas what would I look for?Thanks!

author: moffy
author_rank: Super Contributor
timestamp: August 04, 2023, 12:00:48 am
content: Looking at some of the datasheets:https://www.infineon.com/dgdl/Infineon-BAR66-DataSheet-v01_00-EN.pdf?fileId=8ac78c8c8386267f0183a27edd391ea2etc. they do appear to be low reverse capacitance, but I guess that might vary with size.The most straightforward method might be to plot the forward voltage at say three currents 1ma, 5ma, 10ma and see if the plot of voltage is linear or logarithmic with respect to current.

author: TimFox
author_rank: Super Contributor
timestamp: August 04, 2023, 12:05:43 am
content: The graph ofIfvsVfin that PIN-photodiode data sheet is a semi-log plot that shows a logarithmic voltage as a function of current (current exponential in voltage), as in a PN junction.The resistance and capacitance values quoted are important for calculating the noise of a photodiode-amplifier combination.

author: moffy
author_rank: Super Contributor
timestamp: August 04, 2023, 12:09:27 am
content: Quote from: TimFox on August 04, 2023, 12:05:43 amYou are right, my bad, its the dv/di not V/I.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 04, 2023, 03:53:48 am
content: I would expect PIN has higher breakdown voltage, probably steeper CJ(Vr)?There's probably something about optical pulse response and carrier drift that can be measured, though that might be fairly specialized, may be masked by device characteristics (like if it has high internal resistance), and I'm no expert on optics so I couldn't tell you what to look for.Oh, reverse recovery will tell you something about drift, of course.Tim

author: Someone
author_rank: Super Contributor
timestamp: August 04, 2023, 04:48:59 am
content: If you cant measure the difference does the distinction really matter?

author: coppercone2
author_rank: Super Contributor
timestamp: August 04, 2023, 05:12:47 am
content: said no one ever

author: jbb
author_rank: Super Contributor
timestamp: August 04, 2023, 06:34:22 am
content: Err, photodiode-noob guess here… how about testing the diode reverse recovery (when transitioning from forward conduction to reverse blocking) when diode is in a dark box? As I understand it, PIN diodes should have quite a bit more.

author: Kleinstein
author_rank: Super Contributor
timestamp: August 04, 2023, 09:47:48 am
content: Quote from: jbb on August 04, 2023, 06:34:22 amNot necessary. The reverse recovery time is mainly the minority carrier lifetime and this can also be quite long for a PN diode from relatively pure material.One should see the different in how the capacity changes with reverse voltage. The PIN diode should have more change in the capacitance and likely also saturation after some 5 V or so.

author: TimFox
author_rank: Super Contributor
timestamp: August 04, 2023, 02:40:25 pm
content: Before my retirement, we used large-area photodiodes (roughly 10 mm2), which we ran in "photovoltaic" mode (zero bias) into a transimpedance amplifier.A competing vendor of diodes suggested we change to PIN diodes, but we found that our existing photodiode part was already a PIN diode.We only paid attention to the capacitance and the resistance (slope) through zero, which affected our noise calculations.Another mode of operation, which was not good for this application, applies a reverse bias to the diode, connected into a similar transimpedance amplifier.The trade-off between these two bias modes is that reverse bias gives a "dark current" (with shot noise), which is strongly temperature dependent (faster than doubling per 10 K), but the diode's capacitance decreases and the shunt resistance increases, both of which reduce noise, however the shot noise increases the noise.We were more concerned here with the temperature stability, but other applications work better with reverse bias.

author: tggzzz
author_rank: Super Contributor
timestamp: August 04, 2023, 04:05:29 pm
content: Quote from: TimFox on August 04, 2023, 02:40:25 pmThe other advantage of a reverse-biassed PIN diode is that thephotocurrentis proportional to the optical power over a very wide range. For an AC signal, the dark current is irrelevant, except that it decreases the signal-to-noise ratio.

author: TimFox
author_rank: Super Contributor
timestamp: August 04, 2023, 04:35:59 pm
content: In photovoltaic mode (zero bias), if the diode goes into a dead short (virtual ground of my transimpedance amplifier), the current is still proportional to the optical power over a wide range.In solar-power photovoltaic operation, the diode goes into a load and the load voltage will eventually change the diode bias and reduce its sensitivity.It's interesting that for the zero-bias operation, although the I-V curve is quite different on each side of zero, the slope of the curve through the origin is a well-defined value, and the Johnson noise of that resistance is important, although the shot noise is negligible at zero.The capacitance at zero bias, however, can be huge. The input noise voltage of the amplifier "flowing through" that capacitance can be a substantial current noise (analogy, not the actual analysis).

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: August 04, 2023, 04:52:28 pm
content: I have a need for speed, so reverse bias is the way to go. Plenty of light, so sensitivity shouldn't be a problem. What I'm interested in is measuring the rise and fall times for various LEDs. I know I have a couple very fast PIN parts, but I also have a bunch of other candidates with no info. I was surprised at how expensive a really fast PIN device is, thus my interest in figuring out what else I've got.One really odd thing. I was looking at a company that sells PIN diodes and they made a point of their packaging protecting the devices from bright ambient light. Seems weird for a photo device- is there some condition where they can be damaged by ambient light?

author: TimFox
author_rank: Super Contributor
timestamp: August 04, 2023, 05:23:45 pm
content: Is their packaging designed to avoid ambient light from other directions than the optical axis?I don't see how the packaging could limit (in the sense of a fuse or current limiter electrical circuit) the usable light down the axis.However, parasitic paths (perpendicular or off-axis) could be a solvable problem.Perhaps they are using something similar to "photochromic" materials in sunglasses?They react quite slowly to darken in strong light.

author: tggzzz
author_rank: Super Contributor
timestamp: August 04, 2023, 06:12:12 pm
content: Quote from: Conrad Hoffman on August 04, 2023, 04:52:28 pmNumbers not adjectives?My go-to resources would be TAoE and more specifically Phil Hobb's "Building Electro-Optical Systems: Making it all Work"https://electrooptical.net/#bookQuoteURL? It might be a translation infelicity.

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: August 04, 2023, 08:00:57 pm
content: Numbers? Hmmm... I'd like to confirm that an LED rise or fall time is under 30 ns.

author: TimFox
author_rank: Super Contributor
timestamp: August 04, 2023, 08:10:02 pm
content: Another possibility about light rejection: we purchased flat chip photodiodes, multiple long channels on roughly 6 x 20 mm rectangle, mounted on our substrate by the diode vendor.It was important to block light that could enter the chip from the side edge.

author: khs
author_rank: Regular Contributor
timestamp: August 04, 2023, 10:00:15 pm
content: Quote from: Conrad Hoffman on August 04, 2023, 08:00:57 pmHere maybe a candidate: S5971 BW 100 MHz, price about 9 Euro, Farnell #1495572.

author: moffy
author_rank: Super Contributor
timestamp: August 04, 2023, 10:22:58 pm
content: Since you are interested in rise times, perhaps the simplest test would be to create a test board where you reverse bias the diodes you have by say 5v into a transimpedance amplifier and pulse a known fast LED or Laser diode and measure the response. That will let you know both sensitivity and speed.

author: TimFox
author_rank: Super Contributor
timestamp: August 04, 2023, 10:28:55 pm
content: You don't even need a transimpedance amplifier (with its properties) to test the LED: you can terminate the reverse-biased photodiode into a good 50 ohm load.

author: moffy
author_rank: Super Contributor
timestamp: August 04, 2023, 10:48:14 pm
content: Quote from: TimFox on August 04, 2023, 10:28:55 pmI would have thought that sensitivity might be an issue, especially depending on the scope available?

author: TimFox
author_rank: Super Contributor
timestamp: August 04, 2023, 10:49:28 pm
content: Yeah, it depends on the LED current, the distance to the photodiode, and the oscilloscope sensitivity: engineering.50 ohm fast amplifiers are less problematic than feedback transimpedance amplifiers for waveform fidelity.(I have worked with both.)

author: moffy
author_rank: Super Contributor
timestamp: August 04, 2023, 11:00:22 pm
content: Quote from: TimFox on August 04, 2023, 10:49:28 pmYes a fast amplifier after the 50R would work and is simpler than a transimpedance amp.

author: TimFox
author_rank: Super Contributor
timestamp: August 05, 2023, 02:51:06 am
content: Anyway, 1 mA diode current into 50 ohms is 50 mV, clearly visible on any oscilloscope.

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: August 05, 2023, 03:03:24 am
content: Yes, I was planning on just going into a 50 ohm termination. That's what they show in an old United Detector Technology (UDT) catalog for the highest speed. I don't know much about LED speed, but I think if they use a phosphor type conversion they can be quite slow.

author: moffy
author_rank: Super Contributor
timestamp: August 05, 2023, 03:48:11 am
content: Just a quick look came up with:https://au.mouser.com/ProductDetail/ams-OSRAM/SFH-4555?qs=VcOkZLzzauZt30jRKNAFMA%3D%3Dan infrared LED with 12ns rise and fall times for about $1, they seem to have plenty in stock also.

author: Kleinstein
author_rank: Super Contributor
timestamp: August 05, 2023, 12:33:07 pm
content: If speed is relevant one should still bias the photodiode, even if just a 9 V Battery and capacitor. Just 50 Ohm load can often work, especially for not too larde diodes.