A comparison between Silicon (Si) which is an indirect band-gap semiconductor and Gallium Arsenide (GaAs) as a direct band-gap semiconductor for vertical-aligned nanowire radial pn junction-based photovoltaic (PV) devices is presented. The study takes place through determining the fill factor, the power conversion efficiency, the optimum device length and the spectrum of the quantum efficiency. The sensitivity of both Si and GaAs nanowire to temperature variations is also investigated. Finally, the array effect for nanowires of each material alone then of arrays of mixed elements' types is simulated. The results are found to be in accordance with the available experimental measurements.© (2011) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.