The effect of an ammonia plasma treatment on silicon nitride (SiNx) gate dielectrics for organic thin film transistors (OTFTs) with soluble organic semiconductors (OSCs) was studied. Our OTFTs were fabricated by using the soluble derivatives of Poly(bi-thiophene) as the p-type polymer semiconductor material and SiNx thin films with nitrogen oxygen (N2O) and ammonia (NH3) plasma treatments as the gate dielectric. To improve the performance and stability of the OTFT devices, we studied the basic mechanism of the plasma treatment on the SiNx gate dielectric and the gold (Au) electrode surface. The SiNx-OTFTs with a NH3 plasma treatment yielded the improved results in terms of a higher field-effect mobility (mu(fe)) of 0.06 cm(2) V-1 S-1 with a lower interface charge trap density of 4.45 x 10(-11) (cm(2) eV)(-1) and a lower contact resistance of 0.384 M Omega-cm. The repulsive force of the soluble OSC solvent was reduced at the edge of the source-drain (S-D) electrode due to the difference between the surface energies of the channel region and Au S-D electrodes. (C) 2011 Elsevier B.V. All rights reserved.