Ionizing-radiation-induced oxide-trapped charges and interface states cause the current and 1/f noise degradation in bipolar junction transistors. In order to better understand these two degradation mechanisms and develop hardening approaches for a specific process technology, it is necessary to measure the effect of each mechanism separately. In recent years, several techniques have been developed, but no charge-separation approach based on 1/f noise for NPN bipolar junction transistors is available. In this paper, the effects of ionizing-radiation-induced oxide trapped charges and interface states on base current and 1/f noise in NPN bipolar junction transistors are studied in detail. Firstly, a new model of base surface current of NPN bipolar junction transistors is presented with some approximations, based on an available model for the base surface current under certain conditions; this model can identify the physical mechanism responsible for the current degradation. Secondly, combining the theory of carrier number fluctuation and the new model of base surface current another model is developed which can well explain the 1/f noise degradation. This model suggests that the induced oxide-trapped charges would make more carriers, involving the dynamic trapping-detrapping, which leads to the 1/f noise to increase; and the induced oxide-trapped charges and interface states can also bring about an increase in base surface current which can also cause the l/f noise increase. These two models suggest that the current and1/f noise degradations can be attributed to the same physical origin, and these two kinds of degradations are the result of accumulation of oxide-trapped charges and interface states. According to these two models, simple approaches for quantifying the effects of oxide-trapped charges and interface states are proposed. The base surface current can be extracted from the base current using the available method. The oxide-trapped charge density is estimated using the amplitude of 1/f noise (10-100 Hz) and the base surface current. Given the estimated oxide-trapped charge density, the interface state density can be estimated using the base surface current. These methods are simple to implement and can provide insight into the mechanisms and magnitudes of the radiation-induced damage in NPN bipolar junction transistors.