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Scaling has been the principal driving force behind the successful technology innovations of the past half-century. This paper investigates the impacts of scaling on SiGe heterojunction bipolar transistors (HBTs), which have recently emerged as a strong contender for RF and mixed-signal applications. The impacts of scaling on key performance metrics such as speed and noise are explored, and both theory and data show that scaling, both vertical and lateral, has mostly beneficial effects on these metrics. However, it is shown that the scaled devices are increasingly vulnerable to device reliability issues due to increased electric field and operation current density. Bipolar transistor scaling rules are reviewed and compared with accumulated reported data for verification. A review of scaling limits suggests that bipolar scaling has not reached the physical fundamental limit yet, promising a continued improvement of bipolar performance in the foreseeable future.