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PIN Photodiode: needs biasing?

Discussion in 'Electronic Design' started by Robert Scott, Mar 12, 2007.

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  1. Robert Scott

    Robert Scott Guest

    I have been doing some experimenting with a transimpedance amp (LF353) and a PIN
    photodiode. So far I have been using only a simple 1M feedback resistor with
    zero bias on the diode. Then I noticed in the datasheet for the PIN photodiode
    (NEC PH302) it shows a graph of photodiode current as a function of reverse
    bias. According to that graph, the photodiode current should be about double
    with 2 volts of reverse bias on the diode as compared with zero bias. When I
    tried it, the gain of the system remained the same (I have a squarewave light
    signal generated by a LED). It did improve the transient response quite a bit,
    especially when I rasied the bias to 7 volts (due to the reduced diode
    capacitance, I guess) but I saw no evidence of increased photodiode current. Is
    this bogus? I have not seen this stated anywhere else besides the NEC PH302
    datasheet, but Graeme in his classic book on Photodiode Amplifiers does say that
    PIN diodes are usually used with bias. Why is that? Why PINs and not all

    Robert Scott
    Ypsilanti, Michigan
  2. Hello Robert,

    The photo current is independent of the reverse voltage at such low
    I guess they measured the photo current with an ampere-meter with 100kOhm
    input resistance.

    See figure 3.3 of this application-note.

    Best regards,
  3. redbelly

    redbelly Guest


    What I have noticed with unbiased silicon photodiodes in general is
    that they are pretty linear up to about 0.2 or 0.25 V output, no
    matter what current + load resistor combination is used. By the time
    the output gets up to 0.3 or 0.4 V, the response (output vs. incident
    photo power) is pretty well flattening out. You don't mention what
    output current or voltage you are getting, but hopefully this info
    helps you determine if you should need biasing for linearity.

    That being said, it has been 12+ years since I worked with a PIN
    photodiode, and I'm not sure if they behave vastly different, in terms
    of linearity, vs. a regular PN type. If speed is an issue, then as
    you noticed biasing will help you with that.


  4. Joerg

    Joerg Guest

    Provided you built a good TIA where the voltage at the photodiode node
    does not move the reverse bias is primarily intended to reduce the diode
    capacitance and thus improve speed. It also allows you to reduce the
    compensation cap Cfb for the TIA, making it a bit more hot. But that
    really only matters in the 100MHz+ speed ranges, not so much for an LF
    series amp.

    I finished a photodiode design about a month ago and this one went to
    100MHz. I moved the bias around a bit but even there it didn't matter. I
    just kept it in case the client wants to go higher one day.
  5. John Larkin

    John Larkin Guest

    I think current can be lower at zero bias because some photon-smacked
    hole-electron pairs recombine and are lost. With a reverse-bias field,
    they are swept up and collected quickly.

    For the first few volts of reverse bias, junction capacitance drops
    rapidly, so the *circuit* gets faster. That effect flattens out
    quickly, but higher bias sweeps carriers out faster and continues to
    improve *diode* speed.

    At very high voltages (50? 100?) you can get avalanche multiplication
    that greatly increases gain (and noise.) Some diodes are designed to
    do this well.

    Something like that.

  6. Joerg

    Joerg Guest

    Just be aware that some of the communications diodes cannot tolerate
    more than 5V. At least per abs max in the datasheet.
  7. Guest

    The datasheet I found doesn't start at zero. Do you have a link?
  8. Guest

    The datasheet I found doesn't start at zero. Do you have a link?
  9. Robert Scott

    Robert Scott Guest

    It is a crummy optically-scanned image document. Do you have a better one,
    perhaps one that is not just an optical scan?

    Robert Scott
    Ypsilanti, Michigan
  10. Guest

    That is the same one I found. Since the graph doesn't go to zero, how
    did you conclude the shift for 2v back bias?
  11. Hello miso,

    They obviously measured with an ampere-meter with 100kOhm input resistance.
    This is nonsense of course in a photo-diode application when you measure
    over a
    wide range of light(photo current). As soon as there is a voltage drop of a
    very few 100mV on the load resistor, the measured current isn't correct
    the photo diode gets internally forward biased by this value. Either a
    negative bias
    voltage will help to prevent that or a short circuit current measurement
    (transimpedance amplifier).

    Best regards,
  12. redbelly

    redbelly Guest

    Robert is talking about the photocurrent (bottom of p. 3 in the .pdf
    file), whereas you seem to be talking about the graph of leak current
    (top of p. 3).

  13. Guest

    Does this mean the efficiency of the detector increases with bias? Or
    are they lumping leakage in with photocurrent?
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