A photodiode (A) comprises a substrate, a light receiving layer having a band
gap
wavelength and including a pn-junction and at least an absorption layer having
a band gap wavelength g. One of the absorption layers is sandwiched between
the substrate and the light receiving layer, the band gap wavelength g of
the absorption layer is shorter than the receiving signal wavelength 2
but longer than noise wavelength 1(1g2).
Otherwise a photodiode (B) has two absorption layers epitaxially made on the substrate.
One absorption layer is formed on the top surface of the substrate. The other absorption
layer is formed on the bottom surface of the substrate. The absorption layers annihilate
the noise 1. The PD has no sensitivity to 1.
