Thin film transistors (TFTs) with Si incorporation in zinc tin oxide (ZTO) channel layer were fabricated using a sol–gel process, and the effect of Si incorporation in ZTO systems was investigated with respect to optical, structural, and electrical properties. The Si effectively controlled the generation of the oxygen vacancies examined by x-ray photoelectron spectroscopy, which affected the electrical properties of the silicon zinc tin oxide (SZTO) TFTs. As the Si concentration increased in the ZTO systems, the threshold voltage shifted in the positive direction, the on–off current ratio increased due to the effective reduction of the off current, and the subthreshold swing decreased. At a Si mole ratio 0.02, the SZTO TFTs exhibit favorable electrical properties of Vth = 3.0 V, μFE = 1.90 cm2 V−1 s−1, S.S = 0.38 V/decade, and Ion/off = 1.66 × 107. Thus, the SZTO is a promising material for backplanes of displays.