[en] The effect of (NH₄)₂S_x treatment on the surface properties of SiO₂ is studied. (NH₄)₂S_x treatment leads to the formation of S–Si bonds on the SiO₂ surface that serves to reduce the number of donor-like trap states, inducing the shift of the Fermi level toward the conduction band minimum. A finding in this case is the noticeably reduced value of the SiO₂ capacitance as the sulfurated layer is formed at the SiO₂ surface. The effect of SiO₂ layers with (NH₄)₂S_x treatment on the carrier transport behaviors for the pentacene/SiO₂-based organic thin-film transistor (OTFT) is also studied. The pentacene/as-cleaned SiO₂-based OTFT shows depletion-mode behavior, whereas the pentacene/(NH₄)₂S_x-treated SiO₂-based OTFT exhibits enhancement-mode behavior. Experimental identification confirms that the depletion-/enhancement-mode conversion is due to the dominance competition between donor-like trap states in SiO₂ near the pentacene/SiO₂ interface and acceptor-like trap states in the pentacene channel. A sulfurated layer between pentacene and SiO₂ is expected to give significant contributions to carrier transport for pentacene/SiO₂-based OTFTs. (paper)