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)?
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.
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
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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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