Besides the central pn-junction and the central electrode, a PD chip has a
peripheral pn-junction and a peripheral electrode which do not appear on
the sides. The ends of the peripheral pn-junction are covered with a
protection layer for preventing self-shortcircuit. A reverse bias is
applied to the peripheral electrode for making a wide depletion layer
beneath the peripheral pn-junction. Extra carriers generated by
peripherally-incidence rays are fully absorbed by the peripheral depletion
layer and annihilated by the reverse bias.
