To apply Si-based photodetectors in the near-infrared band their responsivity needs improvement. A regular and orderly microstructure array was formed on the surface of silicon-based photodetectors by plasma lithography. Besides, an Al2O3 film was grown on the microstructure surface by atomic layer deposition (ALD) to study its anti-reflection and passivation effects. The surface reflectivity and I-V characteristic curves of the device were compared and the light responsivity of the device under 808nm near-infrared light was calculated. It is found that the responsivity of the device is increased from 0.063A/W to 0.83A/W.