A front-side type image sensor includes a substrate successively comprising a P− type doped semiconducting support substrate, an electrically insulating layer and a semiconducting active layer, and a matrix array of photodiodes in the active layer of the substrate, wherein the substrate comprises, between the support substrate and the electrically insulating layer, a P+ type doped semiconducting epitaxial layer A method of forming such a structure includes epitaxially growing a P+ type doped semiconducting layer on a P− type doped semiconducting support substrate, providing an electrically insulating layer and an active layer over the P+ type doped semiconducting layer, and forming photodiodes in the active layer.