The base current (I_B) of silicon bipolar transistor degrades when it is subjected to total ionizing dose (TID) irradiation, which is due to the generation of oxide trapped charges (N_ot) in the oxide layer and interface traps (N_it) at the silica/silicon interface. In this work, we investigate the statistical characteristic of I_B of bipolar transistors and its possible microscopic origin. Especially, we carry out TID irradiation experiments on a large sample size of gated lateral PNP (GLPNP) transistors. Forty GLPNP transistors are sequentially irradiated to the total doses of 0.6 krad (Si), 2.6 krad (Si), 4.0 krad (Si), 7.4 krad (Si), and 10.8 krad (Si). The statistical characteristics of their I_B, N_ot, and N_it are obtained from the Gummel, gate sweep (GS), and sub-threshold sweep (DS) curves, respectively. It is found that no matter what the dose is, I_B, N_ot, and N_it all follow a lognormal distribution. However, the distribution parameters change as the irradiation dose increases. Remarkably, the statistical median and standard deviation of I_B as a function of dose show a strong correlation with those of N_ot, but essentially differ from those of N_it. This fact uncovers that for our research objects and dose rate, the sample-to-sample variability of I_B mainly stems from the variation of N_ot. These interesting results should have potential applications in exploring the mechanism and evaluating the irradiation reliability of bipolar microcircuits.