Characterization of Photodiodes and their Performance Several quantities characterize photodiodes, including quantum efficiency, response speed, and noise. At the long wavelength cutoff, a dies off and consequently no photons are absorbed to generate EHPs. Three main factors determine the response time. The diffusion of carriers from outside the depletion region produces a delay, for which reason the junction should be formed near the surface to reduce the distance over which such carriers must diffuse. The last major photodiode characteristic, noise, seems to garner the most attention in current research.
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.