Organic thin film transistors (OTFT) have gained considerable interest due to their potential applications, and the corresponding device models have been developed in recent years. They are mainly based on the classical MOS transistor model, modified by introducing empirical fitting parameters. Now standard drift-diffusion (DD) equations implemented in MINIMOS-NT are being extended for organic transistors, but because of the low conductivity of organic semiconductors, the carrier transport mechanism and some quantities in DD equations need to be properly defined. A new mobility model based on both variable range hopping theory (VRH) and percolation theory has been formulated, and the results are found to be in good quantitative agreement with experimental data. Likewise, the effects of traps and doping on the charge transport have also been investigated using the same theories. A temperature and electric field dependent mobility model has been developed by extending the Apsley model. Based on the same theory, an extended Einstein relationship considering both temperature and electric field effect is being derived. A field dependent effective transport energy model has been developed. This model coincides with Arkipov's model and explains the relation between an electron's mobility and the applied electric field. Further work will focus on numerical calculations to implement these models into drift-diffusion equations.