The current density-voltage (J-V) characteristics of an organic layer sandwiched between two electrodes were simulated by using the space-charge-limited current (SCLC) model and the trap-assisted tunneling (TAT) model taking into account the leakage current paths. The experimental J-V curves of the Al/Alq(3)/indium-tin-oxide (ITO) and the Al/mCP/ITO devices fabricated by thermal evaporation were in reasonable agreement with the simulated results calculated by the SCLC and TAT models. The tunneling process in an organic layer was significantly related to the nature traps of disordered organic semiconductors. The leakage current of an organic layer was dominantly attributed to the TAT mechanism. (C) 2015 Elsevier B.V. All rights reserved.