PN heterojunctions comprised of n-type nitrogen-doped ultrananocrystalline diamond/hydrogenated amorphous carbon composite (UNCD/a-C:H) films deposited on p-type Si substrates were fabricated in nitrogen and hydrogen mixed gas atmospheres by coaxial arc plasma deposition. The heterojunction devices showed typical rectification properties similar to those observed for conventional abrupt pn heterojunctions. The possible conduction mechanisms that govern current transport in these devices were analyzed from dark current-voltage characteristics measured in temperature range of 300 down to 80 K. Electrical parameters of the diode such as ideality factor and barrier height were found to be strongly temperature dependent. At moderate forward bias voltages, the current follows a power-law dependence, which is generally attributed to a space-charge-limited-current conduction mechanism for single-carrier (electron) injection behavior. This junction behavior might be attributed to existence of large number of grain boundaries in the UNCD/a-C:H film that provides active centers for carrier recombination-tunneling processes at the junction interface.