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Homework Help: : effect of light on IV characteristic of pn junctions

  1. Jan 18, 2008 #1
    [SOLVED] urgent: effect of light on IV characteristic of pn junctions

    ^ ^;; Last resort time, needs to be answered by 4pm today. (18/01/08)

    It's a pretty qualitative question, and if possible I'd like a qualitative answer. The reason for this is that this is only a weekly experiment I've performed and while I now need to write an essay on it, I haven't actually been taught that much of the theory - so as few equations as possible please.

    I've been asked to investigate pn junctions, and one of the experiments involves the effect of light on the IV characteristic. From what I know, I would've expected this to produce electron hole pairs resulting in an increased leakage current in the reverse bias direction, and this is indeed what I saw experimentally. However I also expected that it would increase the number of charge carriers in the forward bias direction, leading to a steepening of the curve corresponding to higher currents at all voltages. The trouble is that instead, there was a small translation to the right, meaning negative currents for forward bias voltages, and overall a lower current at any given voltage.

    What's going on here??
  2. jcsd
  3. Jan 18, 2008 #2
    Well...I'm looking at this picture and trying to channel half-forgotten electronics classes


    I'm thinking basically the result of the light is gonna be more electrons liberated from atoms on both sides, on the P side the electrons will diffuse towards that central depletion area, on the N side the holes will, and both sides will have more of their majority charge carrier left over, like you said. I'm not sure that's gonna affect the IV curve, not more so than the fact that by having more of the minority carriers diffuse to the depletion area, you increase it. The delta-V across that depletion area is the necessary voltage to turn the diode "on", and by pumping in more electron-hole pairs that recombine, you physically increase the size of the depletion area, and the voltage across it. This means that the "turn on" voltage(where the current starts skyrocketing with a small increase in voltage)is higher, and the whole curve is shifted right
  4. Jan 18, 2008 #3
    What kind of diodes did you use? All diodes do not respond in the same way to ambient light but photo diodes do. The data sheets of the respective diodes/transistors usually provide all the information regarding voltage, current and temperature characteristics. Just tally the results you get with the data provided in the data-sheets.
  5. Jan 18, 2008 #4
    I didn't think about that, but a photodiode is usually pin junction(so like a pn but with intrinsic undoped semiconductor in between)He just said pn so I assumed a typical diode

    The very specific amounts by which things are shifted in different circumstances can vary from diode to diode, but he wanted a qualitative explanation, and pn diodes all have similar physics. It's probably not a zener diode, but even a zener functions similarly in forward bias
  6. Jan 18, 2008 #5
    I agree. Although the general forward and reverse characteristics of all PN-junctions are the same, we have to take into account the biasing effects and the doping levels if one is interested in investigating in a particular phenomenon. In case of Zener, the forward characteristics are same as any other diode but it conducts more in the reverse bias once it reaches the breakdown voltage and the doping level of a zener is quite high.
  7. Jan 18, 2008 #6
    The experiment was performed on eight different diodes, but based on what I've been told of pn junctions the effect should be the same, though of different magnitudes. The observations were actually made on a photodiode, since it was the only one which showed a large enough response to be clearly visible on the characteristic.
    Unfortunately I wasn't provided with any data sheets, either, and the trouble is I'm not sure how I would explain it even if it did agree.
  8. Jan 18, 2008 #7
    :uhh:I fear this is getting a bit above my level. (I also don't quite see what a Zener diode has to do with it, just an example of diode-specific effects?) If it's a diode-type specific effect then hopefully I won't be marked down for not knowing why it happens.
  9. Jan 18, 2008 #8
  10. Jan 18, 2008 #9
    ... You increase.. what? By applying a forward bias potential to turn the diode "on", you physically increase the size of the depletion zone...? By annihilating more e/h pairs, maybe you would produce more ions in the depletion zone to increase the opposing voltage across it, but surely the presence of a forward current shows that this is overcome? ...nyeah, that last bit totally lost me... sorry....
    Last edited: Jan 18, 2008
  11. Jan 18, 2008 #10
    H'mm... this is something I've overlooked, good point.
  12. Jan 18, 2008 #11
    Er, if you reverse bias the diode you make the depletion zone larger, which makes a larger delta-V, which is like having a really big resistor there

    Eventually you can make the reverse bias voltage just so large that it forces the charge carriers through unwillingly, which is the breakdown voltage

    For forward bias you make the depletion zone smaller as holes from the P type combine with those electrons you see in the depletion zone, and electrons from the N-type combine with the holes

    The smaller voltage drop across the depletion zone is like a small resistance, and indeed that's basically what a diode does, it's a huge resistance to current going the wrong way, and basically conducts current going the right way

    As for the fact that you obtained your observations with a photodiode, I'm not sure how right that's gonna be since normally that's not how you use them. You stick them in your circuit and they'll either conduct if light falls on them or won't if it's dark(with a little "dark current" which is like the leakage current in a normal pn diode)
  13. Jan 18, 2008 #12
    Well, thanks for the reponses. Deadline was just now, in the end I really just had to make do & hope for the best, I'll ask one of the professors later, just as soon as i can track one down. :grumpy: Who knows, may have all been some experimental glitch.
  14. Jan 18, 2008 #13
    Yah, that's the thing, I tried to explain your results, so maybe I was trying to "make it" work in my head and it doesn't actually make sense

    There are a LOT of handy semiconductor physics sites. It's one of those types of things you need explained to you just right. I remember using this one to help me in my electronics class

    http://britneyspears.ac/physics/pn/pnjunct.htm you can tell how old it is 'cuz Britney was still a sex symbol >_>

    There was another one I remember that had like active illustrations to show you what was happening. Nifty for diodes, VERY helpful for transistors
  15. Jan 18, 2008 #14
    Thanks, I think I did come across that site, but the trouble is it'd be a whole lot of effort to explain a whole lot more than I need really - I couldn't go to that much detail in my essay, and I don't have the time right now (stupid idea having coursework during exam week anyway) to learn it thoroughly for completeness. I'll keep it bookmarked for the solid state physics module though. :)
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