Photoelectric conversion device, method for manufacturing the same and image pickup system

A photoelectric conversion device comprising:

• a semiconductor substrate;
• a photoelectric conversion element that includes a first semiconductor region of a first conductive type and a second semiconductor region of a second conductive type, arranged on the semiconductor substrate so as to form a PN junction between the first and second first semiconductor regions;
• and
• a transfer MOS transistor for transferring a charge accumulated in the second semiconductor region,
• wherein the first semiconductor region has an impurity distribution such that a plurality of impurity concentration peaks are formed along a direction of depth, in which
• a first impurity concentration peak located most deeply among all of the peaks with respect to a light receiving surface of the photoelectric conversion element is highest,
• a second impurity concentration peak located in a vicinity of the second semiconductor region is lower than the first impurity concentration peak, and
• a third impurity concentration peak located between the first and second impurity concentration peaks is lower than the first and second impurity concentration peaks, and
• wherein a part of the second semiconductor region is disposed under a part of a gate electrode of the transfer MOS transistor.