The input bias current response of LM124 operational amplifiers to proton irradiation is shown to correlate with the response of identical part types exposed to neutrons and, subsequently, irradiated with X-rays. Moreover, the bias current responses to proton and combined neutron/X-ray exposures are more sublinear than the sum of neutron and X-ray radiation responses measured independently. These data indicate that the ionization defects introduced by proton irradiation moderate the effects of proton-induced displacement damage in the critical bipolar transistors within the LM124. An analytical model is presented that characterizes the various mechanisms of proton radiation effects in bipolar transistors, including the combined effects of oxide charge and bulk traps on carrier recombination. Prototype transistor responses simulated using the model show good correlation with the sublinear characteristics observed in the experimental data. Furthermore, the model is used to assess whether a bipolar junction transistor (BJT), designed with specific structural characteristics and operating in a proton-rich environment, will suffer the most degradation from damage caused by bulk displacement, ionization damage, or a combination of both.