We report the development of a novel field emission (FE) device based on Al-doped ZnO nanorods (NRs), which can be used as an alternative tool for exhale biosensing applications. Highly crystalline hexagonal array of ZnO NRs doped with up to 5wt% Al content were obtained using hydrothermal growth method. Field emission process with acceptable enhancement factor (761 < beta < 951) were performed for 1 to 4wt% Al-doped ZnO nanorod arrays. By increasing the A1 content above 5 wt%, the morphology of nanorods started to transform into the nanoplates, demonstrating weak FE performance. When a high voltage was applied to 4 wt% Al-content ZnO NRs for a few times (4-6 x), an abnormal microstructure formation with improved FE performance was observed. Significant field emission current with enormous enhancement factor (beta = 5545) was demonstrated from the high Al-content reformed ZnO microstructure assembly. The formation of these microstructures was found to be due to the diffusion and electromigration of Au atoms through the grain boundaries into the NR assembly.