Brominated single-walled carbon nanotubes, with bromine covalently attached to the nanotube surface, have been synthesized by a mild reaction using n-bromosuccinimide (NBS). The latter preferentially attacks metallic single walled carbon nanotubes (SWNTs) over semiconducting ones, and the attached Br leads to a significant density differential between reacted and pristine nanotubes. The differential reactivity between semiconducting and metallic SWNTs enhances the density contrast between them, which may be more effectively spatially separated via density gradient ultracentrifugation than unchemically modified SWNTs. The results of optical absorbance, photoluminescence emission, and resonant Raman scattering show that bromination-assisted density gradient ultracentrifugation (hereafter labeled as Br-DGU) preferentially separated semiconducting nanotubes within a certain diameter range (0.829–0.966 nm, specifically (7,6), (8,4), (9,4), and (10,3)). We have applied the semiconducting species enriched SWNTs to prepare solution-processed FET devices with random nanotube network active channels. The devices exhibit stable p-type semiconductor behavior in air with very promising characteristics. The on–off current ratio reaches up to 1730 within a narrow gate voltage (−2 to 2 V) and an estimated hole mobility of 13 cm2 V–1 s–1.