In this thesis, we develop vacuum level system, E_0 method, for numerical 2-D heterojunction device simulation. In the past, our program use intrinsic Fermi level, E_fi, to calculate carrier concentration and potential. However, the E_fi modeling has problem in heterojunction. The E_0 method has been verified by a pN heterojunction. And discuss problems of simulation happened on abrupt. Finally, we use the E_0 method to compare HBT with BJT to discuss the electrical characteristics.