Vertically oriented, high quality n-type ZnO nanowire/p-Si heterojunction photodiode is fabricated by inexpensive chemical bath deposition technique. Under 5.30 mW/sq.cm, 374 nm UV irradiation in air, photodiode offered a maximum self-biased photocurrent and photosensitivity of -26.55 μA and 23000. Photodiode exhibited very stable, rapid self-biased binary photocurrent switching with a rise and fall time of ~25.27 ms and ~49.82 ms.