Abstract: Abstact: Heavy impurity doping brings about the so-called Band Gap Narrowing (BGN) effects, which will lead to the shift of the height of the energy barrier through the abrupt HBT interfaces that effects the transport of carriers a lot. In this article, the phenomenon of transport is discussed according to the J-R model and current transport mechanism. By comparison with the experimental data, it can be concluded that BGN is very necessary for an accurate description on the current transport mechanism.