Origin of large contact resistance in organic field-effect transistors

The large contact resistance (R_c) in organic field-effect transistors (OFET) is one of the main limitation factors which prevent the reliable operation and further reduction in device dimensions. In this paper, we report dependence of the R_c on the gate dielectric materials, which means that the density of charge traps in access region (from contact to channel) of devices plays a primary role for the large R_c rather than energy mismatch between Fermi level of the metal electrode and valence band level of an organic semiconductor. Based on the finding, we fabricated top-gate OFET devices, the structure of which minimizes access region resistance. Very low R_c of below 0.1 KΩ cm was successfully achieved in the top-gate OFETs. A field-effect mobility (μ_FET) of 8.3 cm²/V s and near zero threshold voltage (V_T) were obtained in top-gate devices based on dioctylbenzothienobenzothiophene (C₈-BTBT).