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Carrier transport mechanism in n-type nanocrystalline diamond (NCD)/p-type Si heterojunction diodes prepared by microwave plasma-enhanced chemical vapor deposition is studied in a temperature range of room temperature to 473 K. Current-voltage measurements show at most three orders of magnitude of rectification at ±20 V of biasing and room temperature, depending upon the deposition temperature. The current-voltage characteristics are described with the high ideality factor and the low current injection barrier due to the disordered NCD/Si heterojunction interface, mainly associated with grain boundaries in the NCD film. The Arrhenius plots of the currents reveal that the thermal excitation of carriers limits the conduction, and the apparent activation energy decreases drastically upon the bias voltage change from reverse to forward. The current injection mechanism at the interface is explained along the predicted energy-band diagrams, such that the major carriers from the defect states of the NCD are injected into the conduction band of the Si by forward biasing. [ABSTRACT FROM AUTHOR]