To address the problem of traditional surface illuminated detectors being of low responsivity, this work proposes a large-size interdigitated "finger-type" germanium-on-silicon (Ge-on-Si) photodetector (PD) based on the surface illumination approach. For 1550 nm light with a surface incident power of -20 dBm at room temperature, the best responsivity of the PD achieved is ∼0.64 A/W at 0.5 V. At the same time, the optimal bandwidth reaches 1.537 MHz with 3.5 V applied voltage. In order to suppress the dark current induced noise, a Ge-on-Si avalanche photodiode (APD) with the interdigitated structure is designed. The avalanche voltage is designed ∼13.3 V at room temperature, and the dark current density in linear region is at mA/cm2 order. We believe this type of device can be applied in weak light detection condition.