Third Party Manufacturers / Re: Samsung NX1 - FF level quality in an APS-C?? Imaging Resource samples...« on: December 22, 2014, 03:41:58 AM »
FWC is dependent upon surface area...the surface area of the photodiode. With an FSI design, some of the pixel area is dedicated to wiring and transistors, which thus necessitates that the photodiode (the actual light-sensitive part of the silicon) become smaller. By flipping the die and etching on the back, the photodiodes can become larger, since the wiring is all on the other side. That increases FWC, which means for the same size sensor, with even the same size "pixel pitch", your actually gathering more light.
Thanks for the explanation. For some reason, I'd thought the surface merely translated photos into electrons, but that the electrons were stored below the surface.
The electrons are actually stored around a specific potential layer within the photodiode. Exactly how deep that layer is, and how many electrons it can hold, depends on the specific design of the photodiode. Generally, the amount of charge is related to surface area, but research is has derived ways of changing that layer, deepening it, increasing the potential, etc. I don't know if any of those technologies have been employed yet, and if they have, it's probably only in sensors with ultra tiny pixels (1-1.2 micron), and we might not see them in APS-C or FF sensors for quite some time.
Generally speaking, though, it's the photodiode that has the charge releasing and holding potential, and that potential is realized when a photon (or few, depending on Q.E.) strikes the photodiode, releasing it's energy. The energy either becomes heat, increases the photodiodes potential for releasing another electron, or actually releases an electron. The larger the the area of the photodiode (or more specifically, the layer within the photodiode that is actually responsible for holding the charge), the greater the maximum charge the photodiode can hold. The increased area of BSI photodiodes increases the pixel's FWC (full well capacity), which in turn has the capability of increasing dynamic range.