>I thought photodiodes were operated reverse biased and
>a photon caused current to flow across the junction.
Avalanche and PIN photodiodes are reverse biased. Photons release held-back electrons to permit current to flow. They are very fast but noisy. Avalance diodes are analagous to photomultiplier tubes, where there is a huge amount of amplification built in (electons cascade and release more electrons). PIN diodes are used for fiber optic communication. Most low light applications use photodiodes in what is called the photovoltaic mode. Photons collide with the silicon, causing electrons to separate and they in turn migrate favorably to one of the junctions (I can't remember which one). Because there is no bias there is no leakage, nor shot noise from escaping electrons, and they're very low noise devices. PV mode is the way the common, voltage producing photocell in a calculator works. Unlike a CCD element, the PV cell isn't made to have its own substantial capacitance, so an external capacitor has to be used if you want to store charge, but in principle it's the same process.
Aart M. Olsen aart@uiuc.edu 217-333-7467 College of Veterinary Medicine Univ of Illinois at Urbana-Champaign