according to one exemplary embodiment , a heterojunction bipolar transistor comprises a base having a concentration of germanium , where the concentration of germanium decreases between a first depth and a second depth in the base . according to this exemplary embodiment , the base of the heterojunction bipolar transistor further comprises a concentration of a diffusion suppressant of a base dopant , where the concentration of the diffusion suppressant decreases between a third depth and a fourth depth so as to counteract a change in band gap in the base between the first depth and the second depth . for example , the diffusion suppressant can be carbon and the base dopant can be boron . for example , the concentration of diffusion suppressant may decrease between the third depth and fourth depth so as to counteract the change in band gap at approximately the second depth .