Unidirectional bulk conduction and the anomalous temperature dependence of drift current under a trap-density gradient

Nonlinear drift conduction under a trap-density gradient is mathematically formulated. Semianalytical and numerical solutions demonstrate bulk-induced unidirectional current flow, i.e., rectification. The present theory is in excellent agreement with various experimental J-V characteristics (J: current density and V: applied voltage). At low V, the J-V characteristics are ohmic and bidirectional. As the injection increases, the J-V characteristics become nonlinear and exhibit unidirectionality under proper conditions. The major requirements for a large unidirectionality are the trap-density gradient G≫1, an intermediate V, and not too large trap-filling factor Θ, which requires the presence of acceptorlike traps. The unidirectional J-V characteristics due to the difference in trap-filled-to-trap-free-limit voltage V_TFL for forward and reverse bias markedly resemble the standard rectification. In addition, the trap-density gradient yields a positive T dependence of resistance for a proper set of parameters, evident J∝V ^1.5 characteristics, and a photovoltaic effect. The present results suggest that bulk conduction under trap-density gradient explains fractions of resistance switching and rectification phenomena. The semianalytical solutions are verified by numerical solutions and comparison with experiments. In particular, semianalytical solutions for shallow-trap case excellently fit the experimental data by three parameters in practice: two scaling factors and G.