{"id":1920,"date":"2026-05-07T06:48:25","date_gmt":"2026-05-07T06:48:25","guid":{"rendered":"https:\/\/photo-detector.com\/?p=1920"},"modified":"2026-05-07T06:48:30","modified_gmt":"2026-05-07T06:48:30","slug":"photovoltaischer-vs-photoleitender-modus","status":"publish","type":"post","link":"https:\/\/photo-detector.com\/de\/photovoltaic-vs-photoconductive-mode\/","title":{"rendered":"Photovoltaik- vs. Photoleitungsmodus: Welcher ist zu w\u00e4hlen?"},"content":{"rendered":"\n

You\u2019re halfway through your front-end design, staring at the schematic, and the question hits again: should I run this photodiode in photovoltaic mode<\/strong> or photoconductive mode<\/strong>? One gives you beautiful low noise. The other gives you speed. Pick the wrong one and you\u2019ll spend weeks chasing ghosts in the signal.<\/p>\n\n\n\n

After spending way too many nights in the lab with Si PIN photodiodes, I\u2019ve learned the hard way that there isn\u2019t one \u201cbest\u201d answer. There\u2019s only the right answer for your<\/em> application. Let\u2019s talk about how to actually make that call.<\/p>\n\n\n\n

What Photovoltaic Mode Actually Is (and Why Zero Bias Matters)<\/h2>\n\n\n\n

Photovoltaic mode<\/strong> means running the photodiode at zero bias \u2014 no external voltage across it. The diode generates current purely from incident light, like a tiny solar cell.<\/p>\n\n\n\n

The biggest advantage? Extremely low dark current. We\u2019re often talking single-digit picoamps or even lower with good Si PIN photodiodes. That translates directly into lower noise, especially at low light levels where every electron counts.<\/p>\n\n\n\n

I remember one medical spectroscopy project where we were trying to measure absorbance changes of 0.001 AU. In photovoltaic mode<\/strong> the noise floor stayed low enough that we didn\u2019t need heroic averaging. When we tried switching to reverse bias later, the increased dark current wrecked our signal-to-noise ratio within minutes.<\/p>\n\n\n\n

The formula for photocurrent stays the same in both modes:<\/p>\n\n\n\n

Iph = R \u00d7 P<\/strong><\/p>\n\n\n\n

Where R is responsivity (A\/W) and P is optical power. But the noise current is where things get interesting. In photovoltaic mode your dominant noise sources are usually Johnson noise from the feedback resistor and the op-amp\u2019s current noise. The photodiode\u2019s own shot noise from dark current is almost negligible.<\/p>\n\n\n\n

Photoconductive Mode \u2014 Speed at a Cost<\/h2>\n\n\n\n

Flip the situation around and apply reverse bias. Now you\u2019re in photoconductive mode<\/strong>.<\/p>\n\n\n\n

The reverse voltage does two useful things:<\/p>\n\n\n\n

    \n
  1. It widens the depletion region, reducing junction capacitance (Cj).<\/li>\n\n\n\n
  2. It increases the electric field, speeding up carrier collection.<\/li>\n<\/ol>\n\n\n\n

    Lower Cj means higher bandwidth. The relationship is roughly:<\/p>\n\n\n\n

    Cj \u221d 1 \/ \u221a(Vbi + Vr)<\/strong><\/p>\n\n\n\n

    Where Vbi is the built-in voltage (~0.7V for silicon) and Vr is your reverse bias.<\/p>\n\n\n\n

    I\u2019ve measured bandwidth improvements of 3\u20135\u00d7 when moving from 0V to \u201310V on standard 1mm\u00b2 Si PIN photodiodes. That\u2019s huge when you\u2019re designing a 100MHz TIA.<\/p>\n\n\n\n

    But here\u2019s the trade-off nobody puts in the marketing slides: dark current jumps dramatically. What was 5pA at zero bias can easily become 500pA or several nanoamps at \u201310V, depending on the device and temperature. And remember, shot noise goes with the square root of current. That noise is now in your signal chain whether you like it or not.<\/p>\n\n\n\n

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