Operation of prototype photodetector using a-Se based target and diamond cold cathode is investigated. In our previous study, successful operation of the prototype photodetector as a unit pixel in future imaging device was reported. Clear photoresponse was obtained even at 10(-5) Toff, several orders of magnitude higher than the pressure used for conventional imaging tube devices. Electron emission characteristics of diamond are widely reported, but the precise fabrication process of a-Se based target film is covered with patents. In this study, we have fabricated following three types of target films: (A) a-Se film without incorporation, (B) a-Se film with incorporation of arsenic (As), and (C) a-Se film with incorporation of As and tellurium (Te). Successful operation of the photodetector driven by diamond cathode with target C was observed. The detector showed clear response to red, green, and blue light-emitting diodes in addition to white halogen light. 0 2006 American Vacuum Society.