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photodetectors etc.

Discussion in 'Mathematics and Physics' started by PG1995, Jan 4, 2014.

  1. PG1995

    PG1995 Active Member

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  2. dougy83

    dougy83 Well-Known Member

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    Pulling the following out of the air:
    Photoconductors change conductivity in response to incident light; e.g. photodiodes/phototransistors/CdS cell/etc.
    Photoresistors change resistivity in response to incident light; e.g. CdS cells
    Photovoltaic produce voltage in response to incident light; e.g. photodiodes/"solar cells"

    Photodetectors will include anything that can be used to sense light, which will include all of the above.
     
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  3. 4pyros

    4pyros Well-Known Member Most Helpful Member

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    BTW, you forgot LEDS as Photovoltaic.
     
  4. dave

    Dave New Member

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  5. KeepItSimpleStupid

    KeepItSimpleStupid Well-Known Member Most Helpful Member

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    Yea, but... If you take the top off of a TO-3 transistor and shine light on it, guess what happens?

    Actually we shined a laser on it.
     
  6. jpanhalt

    jpanhalt Well-Known Member Most Helpful Member

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  7. PG1995

    PG1995 Active Member

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    Thank you, everyone.

    Don't you think there is a lot of overlap between photoconductors and photoresistors?

    Don't you think this classification is confusing as far as light detectors part is concerned?

    Thanks a lot.

    Regards
    PG
     
  8. steveB

    steveB Well-Known Member Most Helpful Member

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    A glass body germanium diode is light sensitive.

    PG, I'm not sure it is a strict classification. In my mind, it is all variations on a theme, so to speak. When I hear the term photovoltaic, my mind thinks of solar cells and power application. When I hear photodetector I think of sensing and communications applications. It's really about applying the physics to particular applications. When you do that, the behavior can inspire a different terminology.

    There are also photodetectors based on other approaches. For example, thermopiles are used in laser physics. http://www.newport.com/Thermopile-Laser-Power-Sensor-Technology-Guide/1009203/1033/content.aspx
     
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  9. dougy83

    dougy83 Well-Known Member

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    Then you forgot to mention all clear-bodied PN junctions as photodetectors.
    I guess. A photoresistor is also a photoconductor, but not necessarily vice versa.
    It seems pretty clear. All items in the light detectors list can be used to detect the presence of light. The other list includes items that emit light.

    Have you seen http://www.electronics-tutorials.ws/io/io_4.html ?
     
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  10. PG1995

    PG1995 Active Member

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    Hi

    It's understandable that when wavelength increases, the responsivity of a photodiode decreases. But I'm confused that the responsivity also decreases as the wavelength decreases; at lower wavelength the photons are going to have more energy. Why would responsivity decrease with decreasing wavelength? Someone has told me that as energy of photons increases, their absorption probability decreases. Please help me. I'm using this reference.

    Regards
    PG
     

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    Last edited: Jan 19, 2014
  11. steveB

    steveB Well-Known Member Most Helpful Member

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    The long wavelength side has a sharp drop off because of the semiconductor's energy bandgap. At some point the photons just don't have enough energy to make the transitions that would generate a hole-electron pair. On the other side, you might have less absorption in the semiconductor (allows the light to pass through), opacity of window/lenses, reflections, scattering loss or any of several other effects that could reduce conversion efficiency. For example silicon makes a good photodetector over a wide range that includes some UV, all visible and IR below 1000 nm, but above 1000 nm the band edge kills it and it is useless (unfortunately) for 1300 nm and 1550 nm communications systems. Germanium goes to higher wavelength and some of the III-V and II-VI semiconductors can extend even further.
     
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  12. PG1995

    PG1995 Active Member

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    Thank you, Steve.

    Could you please also help me with this query? Thanks.

    Regards
    PG
     

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  13. steveB

    steveB Well-Known Member Most Helpful Member

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    Diodes have a build in potential which implies a large electric field in the intrinsic layer (assuming you have a PIN diode). This field sweeps the holes one way and the electrons the other way. Both charge motions contribute to current in one direction, and in the wire, the flow must be electron movement.

    In general it is not unusual to have a current source that allows current flow with no voltage. This is a model only anyways. In reality there will be a small potential difference and a finite nonzero resistance in the wires.
     
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  14. PG1995

    PG1995 Active Member

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    Thank you.

    Please don't mind my asking but did you really reply to this query about photoelectric effect? Thanks.

    I think as the resistance of LDR decreases, the relay will be turned off, and as the resistance increases the relay will be turned on. Do I have it correct? I'm using this reference. Thanks.

    Regards
    PG
     

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  15. steveB

    steveB Well-Known Member Most Helpful Member

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    Hahaha, no I didn't response correctly. Sorry, i thought it was photodiode you are talking about. I'll come back and answer the correct question soon. I'm still watching the football championship and my team is losing.
     
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  16. steveB

    steveB Well-Known Member Most Helpful Member

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    OK, for the photoelectric effect. You'll want to study the full effect to understand the whole conduction curve, but answering your specific question is easier.

    If the potential is zero, then you will realize that those electrons would just like to get back onto the conductor, at any point, and usually at the nearest point. Since the potential is the same over the entire conductor (anode, cathode and wires), the electrons are happy to get back on at any point. However, the electrons are emitted with kinetic energy and get launched into the anode cavity. So, they will prefer to go back on the anode, more often than on the cathode.
     
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  17. steveB

    steveB Well-Known Member Most Helpful Member

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    Yes, you have it correct. The relay will be energized when the resistance increases beyond a particular threshold level.
     
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  18. MrAl

    MrAl Well-Known Member Most Helpful Member

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    Hi,

    All of the LDR's i've ever used have a decreasing resistance with increased incident light. So the resistance gets lower with increasing light intensity.

    You can take a regular 1N4001 diode with a glass case and shine a light on it and see it develop a voltage because it's basically the same as a photovoltaic cell. Back in the 80's we had to switch from using glass encased to dark epoxy encased because of this sensitivity to light.

    The noun "photodetector" takes on many different meanings because it's basically just stating that it is a device that detects light without further mention about what else it can do. Some photodetectors not only sense light but they do not output anything unless a specific light pattern protocol is detected, and then they may only output a reduced version of what they received. This is typical of the detectors used in products that use IR remote controls. They actually have built in demodulators.
     
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  19. PG1995

    PG1995 Active Member

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    Hi

    Could you please let me know that if a PIN photodiode used for light detection in reverse bias uses higher voltage than a P-N photodiode? An avalanche photodiode uses a very high voltage. Thanks.

    Regards
    PG
     
  20. steveB

    steveB Well-Known Member Most Helpful Member

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    It probably does typically, but it's really dependent on the particular device details (materials, doping levels and structure) and the breakdown voltages that follow from that. PIN diodes are often used in high speed receivers, and then a higher voltage helps sweep the carriers out of the intrinsic layer as fast as possible, due to the high electric field. You can run these diodes even with zero bias voltage, and they will work, but with slower response. So the upper limit on reverse bias voltage for the particular device is one factor to consider, and then the application is the next factor to consider.
     
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  21. PG1995

    PG1995 Active Member

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    Hi

    I think it's saying that once avalanche process has been initiated by a single photon then it can keep itself going on. How it this avalanche breakdown controlled? Thanks.

    Regards
    PG
     

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