The cause of slow response, which has been observed in amorphous selenium (a-Se) photodetector with nitrogen-doped chemical vapor deposition diamond, is discussed. The effect of high resistance and capacitance of the electric circuit in the photodetector was investigated by using cathodes with different thicknesses. Furthermore, the emission current from the diamond cathode was measured independently to examine its response to transient anode voltage. The results suggest that the surface potential of a-Se shows transient and unstable behavior, and causes an unstable extraction voltage for the diamond cathode. Hence, the slow response is not due to the diamond cold cathode, but unstable surface potential of a-Se. In order to obtain a faster response of the photodetector, a triode structured photodetector should be developed. (C) 2003 American Vacuum Society.