Dynamic analysis of the nature of nonlinearity of the lighting characteristics of the active pixels of a CMOS image detector

The dynamics of the information-signal readout in active photosensors of modern CMOS image detectors is studied by the methods of mathematical simulation. Under the assumption of a specific (quasicylindrical) shape of the pixel photodiode and with the appropriete application of the exact nonstationary solutions of the 2D diffusion equation, an universal curve of relaxation of the photodiode potential at the signal preset stage and at the readout stage is constructed. An algorithm allowing calculation of the pixel lighting characteristics at any ratio of frame duration and the exposure and readout times is proposed. The corresponding numerical calculation yields the shape of the lighting characteristic close to that observed experimentally. The proposed model explains the anomalously high photosensitivity at small signals by incomplete erasing of the frame exposure at the potential preset stage before the exposure.