Nano-crystalline ZnO thin films have been deposited by radio frequency (RF) sputtering on Si substrates and used to fabricate metal-semiconductor-metal photodetectors (MSM-PDs). With this heterojunction structure and proper geometry of MSM interdigitated structure design, photocurrent enhancement has been accomplished. Both MSM-PDs using structures of ZnO/n-Si and ZnO/p-Si gave one to 2 orders higher magnitude photocurrent, respectively, than did the Si only structure. The higher than expected photocurrent has been achieved with n-ZnO/p-Si MSM-PDs showing photo to dark current ratio of 6310 and responsivity of 1.26 A/W. Avalanche multiplication in the ZnO layer is the main reason for improved photocurrent collection and spectral response enhancement in the UV range. Current transport was dominated by space charge limited current due to the nanocrystalline nature of the ZnO. (C) 2011 The Electrochemical Society. [DOI: 10.1149/1.3614415] All rights reserved.