Abstract: Bipolar transistors are widely used in electronic circuits and serve as core components in various devices. In the complex radiation environments, electronic circuits are exposed to more than one type of radiation particle. Research has found that the degradation of devices exposed to simultaneous radiation of multiple particles cannot be simply regarded as the sum of degradation results caused by individual particle irradiation. Therefore, it is highly significant to study the synergistic damage effects of simultaneous irradiation by different particles on devices. However, due to the inherent limitations of irradiation tests, most ground-based facilities can only generate a single type of particle. To thoroughly explore the co-damage mechanisms induced by multiple particles between devices, a series of irradiation experiments were carried out on bipolar transistors in this paper, and then the results were discussed and analyzed. Experimental studies were systematically conducted on bipolar transistors under neutron fluence of 8×10¹² cm⁻² and gamma total dose of 100 krad(Si) using the CFBR-Ⅱ fast neutron reactor and cobalt-60 gamma irradiation facility. The investigation encompassed individual neutron/gamma irradiation and sequential combined irradiation scenarios, with measurements focusing on variations in reciprocal current gain (Δ(1/β)), base current (I_B), and their dynamic response characteristics as functions of base-emitter voltage (V_BE). Results demonstrate that gamma irradiation alone induces the weakest degradation in both transistor types, whereas neutron irradiation causes significant damage: at V_BE=0.6 V, neutron-induced damage accounts for 57.24% of the total cumulative damage in NPN bipolar transistors and 83.12% in PNP bipolar transistors. Notably, PNP bipolar transistors exhibit synergistic enhancement effects under both “gamma-first→neutron” and “neutron-first→gamma” irradiation sequences. In contrast, NPN bipolar transistors display irradiation-order dependence, showing synergistic enhancement in “neutron-first→gamma” sequences but synergistic weakening in “gamma-first→neutron” sequences. Furthermore, I_B variations between irradiation conditions converge as V_BE increases, indicating voltage-dependent saturation of defect interactions. Mechanistic analysis reveals type-dependent degradation mechanisms: for PNP bipolar transistors, irradiation primarily increases interface traps at the SiO₂/Si interface, while for NPN bipolar transistors, gamma irradiation predominantly generates oxide trapped charges. The subsequent neutron irradiation and room-temperature annealing partially neutralize these positive charges in NPN bipolar transistors, explaining their negative synergy in “gamma-first” sequences. The maximum damage for both device types occurs under “neutron-first→gamma” irradiation because neutron-induced displacement defects in the oxide layer increase the density of interface states after gamma irradiation, while ionizing radiation damage accelerates carrier alternation capture in the silicon body, further reducing carrier lifetime. Synergistic enhancement is observed in PNP bipolar transistors under “gamma-first→neutron” irradiation due to the combined effects of defect clustering and induced damage, whereas synergistic reduction is noted in NPN bipolar transistors due to annealing of positive oxide charges and increased minority carrier recombination caused by positive oxide charge accumulation. This paper presents the irradiation damage degradation mechanisms of two types of bipolar transistors under the sequential irradiation conditions of gamma rays and neutrons, and provides an important theoretical basis for the radiation hardening measures of bipolar transistors in mixed neutron/gamma environments.