Top-contact self-assembled monolayer field-effect transistors (SAMFETs) were fabricated through both spin-coating and solution assembly of a semiconducting phosphonic acid-based molecule (11-(5-butyl-[2,2′;5′, 2″;5″,2‴;5‴,2]quinquethiophen-5-yl)undecylphosphonic acid) (BQT-PA). The field-effect mobilities of both spin-cast and solution assembled SAMFETs were 1.1-8.0 × 10 ^-6 cm ² V ^-1 s ^-1 for a wide range of channel lengths (between 12 and 80 μm). The molecular monolayers were characterized by atomic force microscopy (AFM), attenuated total reflectance-Fourier transform infrared spectroscopy (ATR-FTIR), and near edge X-ray absorption fine structure (NEXAFS) spectroscopy. It was found that the BQT-PA monolayer films exhibit dense surface coverage, bidentate binding, and tilt angles of ∼32°and ∼44°for the thiophene rings and alkyl chain, respectively. These results indicate that rapid throughput of fabricating SAMFETs is possible even by spin-coating.