The principal difference between the BJT and HBT is in the use of different semiconductor materials for the emitter and base regions, making a heterojunction. This limits the injection of holes from the base into the emitter region, since the potential barrier in the valence band is higher than in the conduction band. Unlike BJT technology, this allows a high doping density to be used in the base. The high doping density reduces the base resistance while maintaining gain. The efficiency of the heterojunction is measured by the Kroemer factor.