Photodiode and photodiode array

A photodiode array comprising:

• a silicon substrate comprised of a semiconductor of a first conductivity type and having a first principal surface and a second principal surface opposed to each other,
• wherein a plurality of avalanche photodiodes each composed of a pn junction between a semiconductor region of the first conductivity type having a higher impurity concentration than the silicon substrate and a semiconductor region of a second conductivity type are arranged on the first principal surface side of the silicon substrate,
• wherein on the second principal surface side of the silicon substrate, an accumulation layer of the first conductivity type having a higher impurity concentration than the silicon substrate is formed and an irregular asperity is formed in at least regions opposed to the avalanche photodiodes, and
• wherein the regions opposed to the avalanche photodiodes in the second principal surface of the silicon substrate are optically exposed.