I. Introduction
When bipolar junction transistors (BJTs) are exposed to radiation environment, they are easy to suffer from ionization and displacement damage. In this case, the base current of the BJTs increases, while the collector current remains approximately constant [1], leading to decrease the current gain, especially at low bias levels [2]–[6]. Ionization defects include the interface traps and oxide-trapped charge. Interface traps in both the emitter-base depletion region and the neutral base region can increase surface recombination velocity, resulting in an increase in base current [7]–[10], which is independent of the base polarity in bipolar transistor. However, the positive oxide-trapped charge can enhance base current in NPN transistor, while suppress base current in PNP transistors [5], [10], [11].