A photodiode (PD) device that monolithically integrates a PD element with a waveguide element is disclosed. The PD device includes a conducting layer with a first region and a second region next to the first region, where the PD element exists in the first region, while, the waveguide element exists in the second region and optically couples with the PD element. The waveguide element includes a core layer and a cladding layer on the conducting layer, which forms an optical confinement structure. The PD element includes an absorption layer on the conducting layer and a p-type cladding layer on the absorption layer, which form another optical confinement structure. The absorption layer has a length at least 12 μm measured from the interface against the core layer.