Using 70nm thick spin-coated film of newly synthesized octadecyl substituted copper tetrabenzotriazaporphyrin (10CuTBTAP) as an active layer on a highly doped silicon (110) gate electrode substrates, output characteristics and transfer characteristics of bottom-gate bottom-contact organic thin film transistors have been measured at room temperature. A compact model for thin film transistors has been employed as a part of circuit design tool to extract device parameters such as the charge carrier mobility μ, the threshold voltage VT and the contact resistances. Parallel measurements and analysis were performed on similarly constructed devices with a copper phthalocyanine analogue (10CuPc). The results reveal that the 10CuPc layer is relatively more susceptible to trapping degradation near the gate region than a 10CuTBTAP layer, which is significant in order to achieve stability in these transistors. The application of the simple square law in the classical MOS model provides a quicker but approximate interpretation of the transistor performance without providing information on the gate voltage dependence of mobility and the effects of the contact regions. In this comparative study, the analysis of the contact regions is found to be very important for determining the difference in the performance of two transistors.