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In diode why there is creation of depletion region?advanced thanks.

  1. Jul 9, 2011 #1
    in diode why there is creation of depletion region?
    advanced thanks.
     
  2. jcsd
  3. Jul 9, 2011 #2

    es1

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  4. Jul 10, 2011 #3
    Re: diode

    but we know that diode does not conduct in reverse bias but LED works in AC also.how is it possible,as when diode will be reverse bias in one cycle of AC ,then also LED bright contantly?
     
  5. Jul 10, 2011 #4
    Re: diode

    Every diode can work in reverse bias. In AC, LED works as a somewhat rectifier. Depending on how much voltage you actually have, It will let current flow only in one direction. As a consequence you will only use half wave, ergo half power.

    google half wave rectifier for more understanding.
     
  6. Jul 10, 2011 #5

    uart

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    Re: diode

    Well actually the LED need not be "bright constantly" for us to perceive it as constant. Due to persistence of the eye, we can not easily detect the flicker of the LED going on-off-on-off etc if the frequency is high enough. Actually LED's are often pulsed rapidly like this, yet the observer cannot detect it and thus the LED appears to be continuously light.
     
  7. Jul 10, 2011 #6
    Re: diode

    Exactly what uart said. Think of how could light bulb work? Current goes up to max then goes down to 0 then goes down to -max then to 0 again. By looking at this, light bulb would turn it self of twice a cycle, because of 0 current.

    But filament in light bulbs gets very hot, so it doesn't turn off that easily . In LEDs doesn't get hot that much, but still period of AC oscillations is 0,02 secs. So basically you cannot see any change.

    If you are really bugged by this, I can get out some facts and numbers.
     
  8. Jul 10, 2011 #7
    Re: diode

    While you are getting these facts compare the heat generated by LEDs with the heat generated by a tungsten filament.

    You might be suprised.

    Most manufacturers have been caught out by the myth wrongly propagated that LEDs 'are more efficient therefore generate less heat' and there have been a number of building fires caused as a result, some serious.
     
  9. Jul 10, 2011 #8
    Re: diode

    I understand that heat is involved in LEDs. In fact I did an experiment at home where I attached a piece of wire to one of the electrodes. I then heated manually the attached wire with a lighter. The LED started glowing brighter. I figured this was because of LEDs negative resistance.

    But since this isn't the whole truth about the heat as you say, I will do a little research to see those facts and numbers.

    Thanks
     
  10. Jul 10, 2011 #9
    Re: diode

    Hmmm,

    I am getting confused now. From the articles that I read, It says that temperature is a big problem with LEDs.

    LEDTemp.jpg

    [source http://www.photonics.com/Article.aspx?AID=29860]

    I know from the fact that most semiconductors, including those ones that diodes are made of, have negative temperature coefficient. This means that the conductivity will go up if the temperature goes up. This effect I saw when I did home made experiment.

    Now this conductivity comes from electron-hole pairs being created.

    But then again, lattice vibrations go up too with temperature. Is there any limit where things start go down hill? Where lattice vibrations overwhelm the e-h being generated?

    Is this whats going on?
     
    Last edited: Jul 10, 2011
  11. Jul 10, 2011 #10
    Re: diode

    It was a suprise to me as well but I read an article in a recent Electrical Trades Journal about LED lights as follows.

    Tungsten lights run much hotter and are much less efficient producers of light than LEDs, if you count efficiency the obvious way light power out/total power in.

    However LEDs output spectrum is very narrow - many are monochromatic. All the rest of the power is wasted as heat.

    In the case of tungsten the spectrum is very broad and the light output is only a small percentage of the total radiative output - most is infra red. So the tengsten filament actually produces a smaller % heat.

    I think the figures are something like LED is about 30% efficient tungsten is less than 5% efficient.

    So a 12 watt LED produces about 4 watts light and a 100 watt tungsten produces 4-5 watts which is why manufacturers say they are equivalent.
    But an LED therefore also produces 8 watts of heat which soon mounts up in an enclosed space and causes problems, even fires. Taking everyone by suprise since the original claim was how cool an LED lamp runs.

    This has caused the original ranges to be withdrawn by Philips etc and building codes changed.

    go well
     
  12. Jul 10, 2011 #11
    Re: diode

    Hmmm isn't (radiated) heat a form of light(electromagnetic radiation) but that we can't see?

    Do you mean that, more heat is radiated with tungsten filaments because of that broad spectrum? I mean, that is their mode of operation in the first place, heat til it glows.

    Most of heat is radiated through infrared i think in tungsten filaments.

    But in LEDs, they radiate monochromatic light and that leads to narrow spectrum leading to the fact that most of the energy that the diode is being fed is "trapped" in it, causing it to overheat?

    I use heat here very loosely, because I really don't like "heat" term, I like to use energy.


    Did I got this right?
     
  13. Jul 10, 2011 #12
    Re: diode

    The power in = V2 /R in both cases.

    In the LED lamp some is radiated as light, the rest appears a heat energy in warming the material of the lamp. There is little IR radiation (unless it is an IR LED )

    In the Tungsten lamp a small % is radiated as visible light, a much larger % is radiated as IR radiation and the rest heats the material of the lamp.

    None of the energy radiated serves to raise the temperature of either lamp.

    I know tungsten lamps get hotter than LEDs but that is because they use much more energy overall.
     
  14. Jul 10, 2011 #13
    Re: diode

    Yes, I think I understand. LEDs use less energy, but most of that energy goes to heating the material. Which is not a wanted effect. That is why they cool them.

    Nice to learn something useful today! Thanks
     
  15. Jul 10, 2011 #14
    Re: diode

    Ok i redid the experiment, and I couldn't see any visible changes to LED when heated manually.

    But I did this.

    I put 2 LEDs into parallel. Then I heated one. That one that I heated started glowing brighter and (I think, bad eyesight) the other started going dimmer.

    Whats the case here? Did I lower the voltage drop on it?
     
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