To investigate the effect of composition of SiNx on the properties of organic thin-film transistors (OTFTs), we fabricated bottom gate top contact OTFTs devices with different composition SiNx gate insulator. Pentacene based OTFTs with SiNx insulator, prepared using an interface modi. cation process of UV-ozone treatment, exhibited effective mobility of 0.63 cm2/Vs and on/off current ratio of 10(5). Overall improvement in field-effect mobility, threshold voltage was observed as silicon content in SiNx increases. The results demonstrate that the viability of using SiNx for OTFTs and of UV-ozone treatment could be used to improve the properties of organic thin-film transistors. The dependence of the contact angle on the SiNx film composition is evident for the untreated samples, the contact angle increases as the silicon content in the untreated nitride film increases. In contrast, the rise in contact angle across all samples after surface treatment signifies effective surface modification to promote hydrophobicity of the nitride surface. The hydrophobic surface is needed for the organic semiconductor. (C) 2009 Elsevier B.V. All rights reserved.