Films of the high-performance solution-processed amorphous oxide semiconductor a-ZnIn4Sn4O15, grown from 2-methoxyethanol/ethanolamine solutions, were used to fabricate thin-film transistors (TFTs) in combination with an organic self-assembled nanodielectric as the gate insulator. This structurally dense-packed semiconductor composition with minimal Zn2+ incorporation strongly suppresses transistor off-currents without significant mobility degradation, and affords field-effect electron mobilities of similar to 90 cm(2) V-1 s(-1) (104 cm(2) V-1 s(-1) maximum obtained for patterned ZITO films), with I-on/I-off ratio similar to 10(5), a subthreshhold swing of similar to 0.2 V/dec, and operating voltage < 2 V for patterned devices with W/L = 50. The microstructural and electronic properties of ZITO semiconductor film compositions in the range Zn9-2xInxSnxO9+1.5x (x = 1-4) and ZnIn8-xSnxO13+0.(5x) (x = 1-7) were systematically investigated to elucidate those factors which yield optimum mobility, I-on/I-off, and threshold voltage parameters. It is shown that structural relaxation and densification by In3+ and Sn4+ mixing is effective in reducing carrier trap sites and in creating carrier-generating oxygen vacancies. In contrast to the above results for TFTs fabricated with the organic self-assembled nanodielectric, ZnIn4Sn4O15 TFTs fabricated with SiO2 gate insulators exhibit electron mobilities of only similar to 11 cm(2) V-1 s(-1) with I-on/I-off ratios similar to 10(5), and a subthreshhold swing of similar to 9.5 V/dec.