The n-ZnO/p-Si heterojunction with an ultrathin Al₂O₃ buffer layer was prepared by atomic layer deposition. X-ray diffraction revealed that the crystalline quality of (100)-oriented ZnO films was improved with an Al₂O₃ buffer layer. The n-ZnO/p-Si heterojunction with 5 nm inserted Al₂O₃ layer shows the best electrical characteristics, with a dark current of 0.5 μA at a reverse bias of -2 V and increasing the photo-to-dark current ratio effectively by 8 times. These results demonstrated that Al₂O₃ buffer layer with optimized thickness exhibits significant advantages in enhancing the crystal quality of ZnO film and improving the photoelectrical properties of n-ZnO/p-Si photodetectors.