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This paper proposes analytical models of modulation transfer function (MTF), quantum efficiency (QE), and crosstalk for charge-coupled device (CCD) and CMOS image sensors. A unified MTF model for a CCD sensor built on an epitaxial layer deposited on a highly doped substrate was developed by Stevens. The Stevens model uses sinusoidal illumination to calculate the sensor MTF degradation due to charge diffusion and sampling aperture as a function of spatial frequency. The drawback of this approach is the difficulty to evaluate analytically the electrical crosstalk distribution, which can be a good tool for predicting the detector performances, particularly for smaller pixels. In this paper, we use point-source illumination to evaluate the pixel response function (PRF). This approach is applied to the case of CMOS sensors and buried channel CCD sensors. The MTF model includes the impact of pixel size and charge diffusion. The QE model and crosstalk distribution are directly derived from the PRF expression. The models can take into account an electric field induced by a doping gradient.