Thanks to its three-dimensional integration and the use of amorphous as well as crystalline silicon, the TFA technology is suitable for advanced image sensor applications. This paper describes the fundamentals of the properties: sensitivity, dark current, temporal and fixed-pattern noise of these TFA image sensors. It compares the different sensitivity definitions, especially current sensitivity and the charge conversion factor. Further, the dark current sources are pointed out, and their temperature behavior is described. By noise calculations, different pixel input stages are compared with regard to low light level detection.