LDR vs photodiode mechanism / principle - Page 1
Forum section: electronics / projects

author: 741
author_rank: Frequent Contributor
timestamp: September 12, 2021, 01:21:50 pm
content: After some time looking, I've not found a really good article on photoresistors/LDRs.The explanations I see remind me of those for photodiodes.A photodiode has a PN junction, and the inbuilt junction potential can be augmented or reduced depending upon extrnally applied PD (if any). The net PD across the PN junction sweeps electron / hole pairs to the terminals.For an LDR, there is no PN junction.For both types of device (LDR and photodiode) though, it seems we have light energy creating electron / hole pairs. Then, one difference is what then "happens to" the charge carriers.With an LDR, I do not see what prevents re-combination- Is re-combination prevented when (externally produced) voltage makes use of those extra electrons when pushing current through the LDR?

author: moffy
author_rank: Super Contributor
timestamp: September 12, 2021, 01:46:41 pm
content: For a photodiode, photons of sufficient energy are converted into electrons, which are swept in the reverse direction, by the junction, to normal current flow. You normally reverse bias a photodiode and the current is created by the light hitting it.An LDR does not create a current but changes conductivity due to light hitting it. I guess electrons are liberated, but there is no junction to create a current, just modulate conductivity. The electrons do have a finite lifetime, of the order of ms, because when a light stops shining it takes a finite time for the LDR to reset to its dark state/high resistance.With a photodiode or any diode the lifetime can be reduced by gold doping, creates combination centers. This is done to make fast diodes like the 1N4148.

author: David Hess
author_rank: Super Contributor
timestamp: September 12, 2021, 06:02:59 pm
content: As far as applicaitons go, LDRs have good linearity of resistance with applied voltage while a photodiode is a current source in parallel with a diode so they are applied very differently. Unfortunately there is no modern substitute for an LDR.

author: Kleinstein
author_rank: Super Contributor
timestamp: September 12, 2021, 07:07:26 pm
content: With LDRs there are 2 mechanisms to increase the conductivity. The fast way is just the newly created additional carriers. This gives are fast response, but also relatively low sensitivity. The external exlectric field can sweep the electrons and holes out of the sensitive area. So though not a PN junction it may behave a bit like the photodiode at high enough voltag. Onece sweept away the effect is gone. There are special photoconductive detectors (special materials (direct semiconductors) and small distance between the contacts) to work even in the GHz range, but with relatively low sensitivity and not necessary very linear in resistance.With the common CdS LDRs there is another mechanism to increase the sensitivity: There are some relatively long lived trapping centers that catch the created extra carriers of one polarity. I don't know if this are the electrons or holes that are caught, but the principle is the same. As long as the hole is captured in a long lived state the corresponding electron has no easy way to recombine and can freely move and contribute to an increased conductivity. The longer the holes are trapped the more (the longer) the electrons can contribute to conductivity. Though half the carriers are caught and thus immobile, this mechanism can increase the sensitivity quite a bit, as the partner stays mobile and gets a much longer effective life time (e.g. ms instead of ns range). So slow reponse and good sensitivity are kind of linked.

author: 5065AGuru
author_rank: Frequent Contributor
timestamp: September 13, 2021, 04:08:29 am
content: I recently had to work on a 1962 vintage instrument. In one of the circuits it showed the resistor in a circle for a photo resistive cell which I figured must be a cadmium sulfide device.It seemed impossible that it had a fast enough response time and it was tiny! Finally after lots of searching I found that it was a 1N2175 silicon NPN duo diode and that it's preferred symbol was a back to back diode in a circle. It seems that the two terminal NPN duo diode does indeed act like a variable resistance with a higher speed that the cadmium devices.I'm not sure if they make modern versions but it was an interesting device.Cheers,Corby

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 13, 2021, 05:05:01 am
content: Yes, to make it clear: a photoresistor is a monode -- no junction, just contacts on a blob of semiconductor. Doping would provide free carriers and thus dark current, so the semiconductor must be intrinsic (high purity, low defects).Quote from: Kleinstein on September 12, 2021, 07:07:26 pmAh, this must be related to the effect I measured back in college...The experiment was just measuring a simple impulse response, illuminating a CdS photocell with a camera flash and watching the current flow through a shunt resistor. Evidently, I had chosen the resistor too large, resulting in a large signal level (though not clipping, as I recall). I got a result different from the rest of class, which I had no explanation for. But it was a clear effect, having a sqrt(t) dependency. Or at least, a better fit than for the linear curve.Ah, here's the relevant plot:The lumps are quantization noise from the shitty scope used to acquire it (the right side corresponds to a tail of some 10s mV); it's pretty fair to average through them.If the material had a simple time constant behind its conductivity (as you'd more or less expect from recombination), R should increase as a single power of time; but it actually goes at the half power. At least, so it seems, here. If that was actually the finding that others made (hm, I don't recall if I looked at anyone else's data), it could be that extra charges get trapped when the dwell time is longer, versus being cleared fairly quickly from the junction under bias; hence the time dependency that other students apparently didn't have.sqrt(t) suggests either a diffusion mechanism, or an ensemble effect where -- if it's trapping centers, it might be there are a wide variety of them present, some short, some long, and they happen to be distributed in this manner.Hmm, photoflash isn't exactly brief, by itself (~fractional ms usually), and this whole event is only a ms long. That should have a linear, or truncated exponential, light curve though, and wouldn't match the data here. I didn't happen to save the whole waveform (including rising edge and peak; only the decay). Any difference due to terminal voltage, can only amount to a few microseconds of difference, as the voltage drops to normal values fairly quickly...(The prof found my answer of "Idunno" rather unsatisfying...but also didn't have any insights himself. Heh, I wonder if he expected/wanted me to BS it. Wouldn't have known what to research for, at the time. Anyway, if it's trapping centers, how would I even be able to verify that hypothesis without a hell of a lot more work (likely including fab work)?... ah well.)Tim

author: Kleinstein
author_rank: Super Contributor
timestamp: September 13, 2021, 06:46:14 am
content: Just free carriers to contribute to the conductivity would be swept out of the device after some time. Beside the simple recombination there is the current flow to remove the carriers. This would expecially be an issue with very long lifetime.At the university we had an experiment in a lab coarse to measure photoconductive effect in germanium at different temperatures. At lower temperatures (e.g. 100 K) there was a clear sepration into a fast and slow contribution. The slow one got to the seconds range. The fast one was still in the usual 10 µs (AFAIR) range. If not careful one could really miss the slow part.For the usual CdS LDRs the recovery from light is also known to be not just a single time constant, but with some rather long contributions. To really get the reistance high it can take minutes in the dark.

author: 741
author_rank: Frequent Contributor
timestamp: September 13, 2021, 08:35:03 am
content: Found a reference to the "duo-diode" here.https://www.rfcafe.com/references/electronics-world/light-sensitive-photodiodes-electronics-world-july-1969.htmOr page 45 of thishttps://worldradiohistory.com/Archive-Electronics-World/60s/1969/Electronics-World-1969-07.pdfI was interested to read this (somewhat off-topic)QuoteAre these (mode vs. quoted resistances) derivable from datasheet values?Of course we know the ideal termination for a current source is a short circuit - but where does 800 Ohms come from? Something to do with the compliance of the current source presumably?

author: 5065AGuru
author_rank: Frequent Contributor
timestamp: September 14, 2021, 05:12:17 am
content: 741,The data sheets I can find don't quote or chart any values.I do have a 1N2175 on the way to me, maybe I can try to chart some values.In the instrument there is a series resistor from the supply in series with the device.The voltage drop across the device when illuminated is used to synchronize a pulse.Cheers,Corby