  Due to the increasing dimension, complexity and cost of the future
astronomical surveys, new technologies enabling more compact and simpler
systems are required. The development of curved detectors allows to enhance the
performances of the optical system used (telescope or astronomical instrument),
while keeping the system more compact. We describe here a set of five curved
CMOS detectors developed within a collaboration between CEA-LETI and CNRS-LAM.
These fully-functional detectors 20 Mpix (CMOSIS CMV20000) have been curved to
different radii of curvature and spherical shapes (both convex and concave)
over a size of 24x32 mm^2. Before being able to use them for astronomical
observations, we assess the impact of the curving process on their
performances. We perform a full electro-optical characterization of the curved
detectors, by measuring the gain, the full well capacity, the dynamic-range and
the noise properties, such as dark current, readout noise,
pixel-relative-non-uniformity. We repeat the same process for the flat version
of the same CMOS sensor, as a reference for comparison. We find no significant
difference among most of the characterization values of the curved and flat
samples. We obtain values of readout noise of 10e$^-$ for the curved samples
compared to the 11e$^-$ of the flat sample, which provides slightly larger
dynamic ranges for the curved detectors. Additionally we measure consistently
smaller values of dark current compared to the flat CMOS sensor. The curving
process for the prototypes shown in this paper does not significantly impact
the performances of the detectors. These results represent the first step
towards their astronomical implementation.
