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Microbeam measurements and TCAD simulations are used to examine the effects of ion angle of incidence on the charge collected from events occurring in a Silicon Germanium (SiGe) Heterojunction Bipolar Transistor (HBT). The results identify the geometrically driven charge-collection mechanisms that dominate the low LET broad beam SEU response. The deep trench isolation that surrounds the transistor significantly modulates the charge transport and, therefore, the charge collected by the collector. A new way of estimating critical charge, , for upset in SiGe HBT circuits is proposed based on TCAD simulation results and measured broadbeam data.