An integrated optical sensor is formed by a pinned photodiode. A semiconductor substrate includes a first semiconductor region having a first type of conductivity located between a second semiconductor region having a second type of conductivity opposite to the first type one and a third semiconductor region having the second type of conductivity. The third semiconductor region is thicker, less doped and located deeper in the substrate than the second semiconductor region. The third semiconductor region includes both silicon and germanium. In one implementation, the germanium within the third semiconductor region has at least one concentration gradient. In another implementation, the germanium concentration within the third semiconductor region is substantially constant.