Background radiation is conceptually the simplest. It is primarily a problem in IR photodiodes such as HgCdTe devices. Since the environment, including the instrument itself, constantly emits infrared radiation with temperature-dependent wavelength, this must largely be controlled in instrument design. LWIR and MWIR detectors are often operated at low temperatures to prevent the IR background from swamping the desired signal. This complicates the study of these diodes considerably, as their characteristics must be understood over a wide temperature range, from sub-40K up to room temperature.
Dark current is the physically more complicated form of noise.
Its sources vary not only with bias but with the temperature, which varies with the required operational wavelength range. As its name implies, this is the current through the diode when it is receiving neither an optical signal nor background radiation. In journal articles researchers frequently characterize dark current with the RoA product, though its definition is conspicuously absent from many texts on the subject. At the low voltages typically used in operation, RoA is a approximately proportional to the dark current. The study and reduction of the dark current is one of the most common topics in current literature on HgCdTe photodiodes, and it is all the more involved in these devices because the stoichiometry varies in addition the operating temperature and bias as mentioned above.