The mechanism for electrical conduction is investigated by the dark temperature-dependent current–voltage character-istics of Si PIN photodiodes with different photosensitive areas. The characteristic tunneling energy E00 can be obtained to be 1.40 meV, 1.53 meV, 1.74 meV, 1.87 meV, and 2.01 meV, respectively, for the photodiodes with L=0.25 mm, 0.5 mm, 1 mm, 1.5 mm, and 2 mm by fitting the ideality factor n versus temperature curves according to the tunneling-enhanced recombination mechanism. The trap-assisted tunneling-enhanced recombination in the i-layer plays an important role in our device, which is consistent with the experimental result that area-dependent leakage current is dominant with the side length larger than 1 mm of the photosensitive area. Our results reveal that the quality of the bulk material plays an important role in the electrical conduction mechanism of the devices with the side length larger than 1 mm of the photosensitive area.