Single-walled carbon nanotube (SWNT) arrays are the key to making integrated circuits smaller than 10 nanometers. Herein, a brand-new approach is proposed to efficiently prepare semiconducting (s-) SWNT arrays by implementing a simple phenylation to modulate the metallic SWNT bandgap through the radical reaction between SWNTs and benzoyl peroxide molecules. Electrical measurement indicates that the percentage of s-SWNTs in the functionalized arrays could be higher than 97.8% after phenylation, promoting its exceptional performance as a field-effect transistor with an on-off ratio of 11 300. Our work paves a new avenue for the design and synthesis of high-purity s-SWNT arrays, which are highly important for future applications in carbon-based nano-electronic devices.