We show theoretically that under graded distributions of carrier traps nonlinear drift conduction exhibits diode-like characteristics, whereas at low voltage it exhibits ohmic characteristics that are symmetric with respect to the bias polarity. In shallow-trap dominated cases, the JV relationship of these diode-like characteristics is J ∝ V m (J: current, V: voltage, 1 ≤ m ≤ 2). These characteristics agree well with experiments. The theory reproduces exactly experimental JV characteristics using three free parameters in practice, only one more than that used in standard space charge-limited conduction theories. The results indicate that bulk-limited conduction can exhibit rectifying JV characteristics without relying on surface barriers. In most cases, the coexistence of deep traps with shallow-traps is necessary for the appearance of rectification.
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