The single-event upset (SEU) responses of three D flip-flop circuits, including two unhardened, and one current-sharing hardened (CSH) circuit, are examined using device and circuit simulation. The circuit that implements the conventional D flip-flop logic using standard bipolar NAND gates shows much better SEU performance than the other two. Cross coupling at transistor level in the storage cell of the other two circuits increases their vulnerability to SEU. The observed differences are explained by analyzing the differential output of the emitter coupled pair being hit. These results suggest a potential path for achieving sufficient SEU tolerance in high-speed SiGe heterojunction bipolar transistor (HBT) digital logic for many space applications.