An embodiment of an image sensor comprising photosensitive cells, each photosensitive cell comprising at least one charge storage means formed at least partly in a substrate of a semiconductor material. The substrate comprises, for at least one first photosensitive cell, a portion of a first silicon and germanium alloy having a first germanium concentration, possibly zero, and for at least one second photosensitive cell, a portion of a second silicon and germanium alloy having a second germanium concentration, non-zero, greater than the first germanium concentration.