We present the results of a simulation study on the behaviour of 3C-SiC bipolar transistors fabricated using a 6H-SiC heterojunction emitter. We show that despite the potential barrier originating in the discontinuity of the conduction bands, that npn devices offer a higher common emitter gain in comparison to pnp devices. The base voltage corresponding to the maximum gain of the device is controlled by the tunnelling of carriers across the collector-base junction and is different for both npn and pnp devices.