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Modern bipolar transistors use polysilicon emitters and an epitaxial grown silicon germanium (SiGe) base. For device optimization, both the SiGe base and the region of the diffused emitter is of special interest. In this paper, electron holography is applied to visualize and directly measure the two-dimensional distribution of the local potential in a high-performance SiGe heterojunction bipolar transistor. Special emphasis is put on investigating the region of the emitter diffused into the epitaxially grown base layer. In addition, we investigate the self-aligned base-link construction. We compare electron holographic measurements of the whole transistor to secondary ion mass spectrometric (SIMS) data and discuss the results.