The synergistic effects of the total ionizing dose (TID) and analog transient radiation effects in electronics (ATREE) in vertical NPN (VNPN) bipolar transistor were explored experimentally. The technology computer-aided design (TCAD) simulation was also accomplished to analyze the impact of TID-induced positive charge and Si/SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> interface traps on the primary and secondary photocurrents of the transistor under the transient ionizing radiation environments. The results indicate that the TID-ATREE synergistic effects in the VNPN bipolar transistor include faster decrease of secondary photocurrents and shorter transient durations across various dose rates. TID-induced Si/SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> interface traps enhance the carrier recombination, decreasing the number of electrons emitted from the emitter to the base region near the oxide layer and the number of carriers collected by the collector contact, which is the dominant cause of the synergistic effects, however, TID-induced positive charge in the oxide layer has little influence.