AlGaAs/GaAs heterojunction resonant tunnelling bipolar transistors, with superlattice base, have been fabricated. For the single heterojunction structure an S-shaped NDR behaviour can be obtained when it is forward biased, due to the impact ionization enhancement in the AlGaAs/GaAs heterojunction. While it is reverse biased an N-shaped NDR behaviour comes after an insignificant S-shaped behaviour. It is believed to be the initiation of a resonant tunnelling process across the superlattice region. As for the double heterojunction structure, the I-V characteristic shows a bi-directional S-shaped switching behaviour. The different performance between the two structures can be modelled as the effect of the band offset of the heterojunction.