Heterojunction diodes fabricated by plasma enhanced chemical vapour deposition of n-type amorphous silicon carbide on p-type crystalline silicon are analysed by measuring their current-voltage characteristics. Two carrier transport mechanisms are believed to be at the origin of the forward current. At low bias voltage, the current is due to recombination in the amorphous side of the space charge region, while at higher voltages, the current becomes space charge limited. At reverse bias, the current can be explained by tunnelling models. The space charge limited currents in these heterojunctions have been used to determine the density of states in the n-type a-Si1 - xCx:H gap. The results show the increase in localized states when approaching the conduction band edge.