Depth‐resolved secondary‐ion mass spectrometry (SIMS) and high‐resolution transmission electron microscopy were used to study the composition and structure of the arsenic‐doped polycrystalline silicon (polysilicon)–single‐crystal silicon interface. This interface remains intact during an 880 °C anneal, with no alignment of the polysilicon occurring. When the interfacial oxide varies from a thin, discontinuous film, to a continuous layer, SIMS analysis is able to quantify differences in the interfacial oxygen content. This interfacial oxygen, expressed as oxygen atoms/cm2, is found to correlate to polysilicon emitter bipolar transistor device current gain.