The synergistic effect of total ionizing dose (TID) on single event effect (SEE) in SiGe heterojunction bipolar transistor (HBT) is investigated in a series of experiments. The SiGe HBTs after being exposed to 60Coγ irradiation are struck by pulsed laser to simulate SEE. The SEE transient currents and collected charges of the un-irradiated device are compared with those of the devices which are irradiated at high and low dose rate with various biases. The results show that the SEE damage to un-irradiated device is more serious than that to irradiated SiGe HBT at a low applied voltage of laser test. In addition, theγ irradiations at forward and all-grounded bias have an obvious infl uence on SEE in the SiGe HBT, but the synergistic effect after cutting off theγ irradiation is not significant. The infl uence of positive oxide-trap charges induced by TID on the distortion of electric field in SEE is the major factor of the synergistic effect. Moreover, the recombination of interface traps also plays a role in charge collection.