Why do digital cameras need a long exposure time in low light conditions? Sometimes the exposures are 15-30 seconds or longer. Doesn't the electronic sensor achieve a steady state under 1 second or so? I can understand long exposures in film, where the light effects are cumulative. But in an electronic sensor? Do they do digital signal processing behind the scenes to build an image?
The light conversion element in most image sensors as actually a reverse biased diode carefully crafted to retain charge until reset. A photon enters the bulk of the photodiode and interacts with the Silicon lattice creating a electron/Hole pair. The electric field within the diode from being reversed biased sweeps the minority carrier away while the majority carrier is kept within the photodiode. As more photons are intercepted more carriers are collected. With a lower rate of generation (i.e. with lower rate of photon arrival) one must accumulate for a longer period to have the same signal level.
In other words, photodiodes are integrating sensors.
Digital camera sensors do achieve a steady state, and how quickly depends on the system noise, both electrical and thermal. The steady state is a saturated sensor, or all white in photo terms. This happens if the shutter is open or closed. The sensor has to be read or refreshed just before each photo. For CMOS sensors noise can overwhelm the signal in a second - at least for earlier and low cost sensors. CCD's have been developed for photo and astronomy work for a lot longer and are today very low noise in comparison. I can remember a CCD camera designed at UC Santa Cruz for the Lick Observatory using a Kodak CCD in about 1988.
The CCD chips will also achieve a steady state or "full electron wells". Cooling the sensor lowers the noise significantly and allows 20 or 30 minute exposures or even more than 1 hour in some cases. A lot of CCDs have charge wells that are full at 48K or 65K electrons and are limited to data in the 12 to 16 bit range even if you can count electrons. Astro-photographers will add multiple images together to get greater dynamic range.
Cooling to -55C is not uncommon for astronomy and spectroscopy. They use Peltier coolers that have become inexpensive due to use in PCs to cool the CPU chips. Ice water is circulated through the cooler's radiator or a finned air radiator and stacked Peltier devices (and often a small fan) are needed to get more than a 20 or 30 deg difference between the CCD chip and the ambient air. Check the web sites of Astronomy Magazine and Sky and Telescope to see sample equipment and photos.
A Nikon D90 has a setting for 20 seconds and if noise reduction is turned off, it will show an amazing amount of stars and detail of the night sky. The same chip could be used for 20 minute exposures if cooled.