Ionizing radiation experiments on gated bipolar transistors used as radiation test structures show a broadening in the peak base current profile after irradiation. The primary mechanism for this effect is identified as the change in the charge state of radiation-induced interface traps in the oxide over the base. From theoretical analysis using Shockley - Reed - Hall (SRH) statistics, a mathematical formulation of effective Fermi level, E-f,E-eff is derived to describe the charge state of interface traps in a gated bipolar transistor under forward bipolar operation as the surface of the transistor is changed from accumulation to inversion by biasing on the gate terminal. (c) 2007 Elsevier Ltd. All rights reserved.