In this work, n-type organic thin film transistors (OTFTs) were fabricated in the bottom-gate bottom-contact configuration, depositing a fullerene-derived semiconductor (PC70BM) by drop-casting technique on SiO2 substrates treated with a self-assembled monolayer, namely the HMDS. The influence of the deposition temperature of the HMDS on the device performance was investigated, using three different temperatures. The relationship between the properties of the resulting semiconductor films and the electrical characteristics of the transistors was evaluated through admittance measurements. The frequency response of the devices vs. the bias was interpreted applying an electrical equivalent circuit to model the properties of the semiconductor and of the transistor conductive channel. The proposed model shows the critical role played by the quality of the insulator-semiconductor interface on the traps density in the semiconductor, and therefore on the increase of the mobility and on the reduction of the threshold voltage of the transistors. © 2017 Elsevier Ltd.