A method for fabricating an image sensor is provided to increase generation of optical carriers by forming a thin p-type impurity region so that an n-type impurity region is thickened. A semiconductor substrate(403) is prepared which has a first epitaxial layer of first conductivity type. A gate electrode(407) for a transistor is formed on the semiconductor substrate. An impurity region(408) for a photodiode having second conductivity type is formed in the first epitaxial layer exposed to one side of the gate electrode. A floating diffusion region(410) is formed in the first epitaxial layer exposed to the other side of the gate electrode. The impurity region and a part of the first epitaxial layer in contact with the impurity region are etched to form a trench. A second epitaxial layer for a photodiode doped with impurities of first conductivity type by an in-situ method is formed in the trench so as to be connected to the impurity region and a part of the first epitaxial layer in contact with the impurity region. The process for forming the trench includes the following steps. A silicon nitride layer(411) is formed in which a region for forming the trench is opened. An etch process is performed by using the silicon nitride layer as an etch barrier layer.
