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photodiode: photovoltaic vs photoconductive

Discussion in 'Electronic Basics' started by [email protected], Mar 29, 2007.

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  1. Guest

    Three questions:

    1) On a photodiode data sheet, you see the spectral response (i.e.
    responsitivity in A/W vs the wavelength), however it never clarifies
    if this is for photovoltaic (no bias applied) or photoconductive (with
    a bias) mode. Which mode is this for?

    2) For a photodiode, would its responsitivity (or Quantum Efficiency)
    be different between photovoltaic or photoconductive mode.

    3) Can you have a quantum efficiency more than 100% (e.g. from what I
    understand, not for silicon photodiodes, but potentially for other
    materials)?

    Thank you.
     
  2. John  Larkin

    John Larkin Guest

    I'm guessing thay's be the same. It has to do with material bandgaps.

    Some will give you more current in biased mode, because the bias
    reduces spontaneous recombination of electrons and holes.
    Avalanche silicon photodiodes can have high gain factors, hence high
    qe. Each photoelectron, accelerated by the high-voltage field, knocks
    a bunch more loose in a cascade.

    John
     
  3. Ban

    Ban Guest

    A photodiode functions always in the same mode, it is just connected
    differently.

    +Vcc
    |\|
    .------------|+\ +A*Vin
    | +Vin | >-+--o
    \ V .-|-/ |
    \ - | |/| .-.
    | | ===| |
    | | GND| |
    === | '-'
    GND | |
    '------+
    |
    photovoltaic .-.
    mode | |
    | |
    '-'
    |
    ===
    GND
    The photocurrent is shunted by its own junction. There is a logarithmic
    response in the O/P voltage vs incident light, which can reach a maximum of
    ca. 0.5V. The diode is forward biased and has a high capacitance across its
    junction. Since the forward voltage varies a lot with temperature this
    circuit benefits from temperature stabilization, especially cooling.

    +Vcc
    ___ |
    .--|___|----)----.
    | Rfb |\| |
    +---------|-\ |
    | | >---+-o
    \ - .---|+/
    \ ^ | |/| +Vout
    | +Vbias |
    | | |
    === === ===
    GND GND GND
    (created by AACircuit v1.28 beta 10/06/04 www.tech-chat.de)

    Voila, photoconductive. The diode is turned around. The opamp operates in
    transimpedance mode. If you make Vbias=0V you are still photoconductive. The
    sensitivity is almost independent of the bias voltage, but the leakage
    current (called dark current) adds to the photocurrent. at room temperature
    it is very low. O-bias has the advantage of no dark current, but the
    response to light variations is slower, because the junction capacitance
    gets reduced by higher negative bias.

    If you compare both modes it is clear, that if you want a linear
    relationship you have to use the latter. But the amp can saturate, so many
    designs switch/vary Rfb. It all depends on your application.
     
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