  The practical damage of silicon bipolar devices subjected to mixed ionization
and displacement irradiations is usually evaluated by the sum of separated
ionization and displacement damages. However, recent experiments show clear
difference between the practical and summed damages, indicating significant
irradiation synergistic effects (ISEs). Understanding the behaviors and
mechanisms of ISEs is essential to predict the practical damages. In this work,
we first make a brief review on the state of the art, critically emphasizing on
the difficulty encountered in previous models to understand the dose rate
dependence of the ISEs. We then introduce in detail our models explaining this
basic phenomenon, which can be described as follows. Firstly, we show our
experimental works on PNP and NPN transistors. A variable neutron fluence and
$\gamma$-ray dose setup is adopted. Fluence-dependent `tick'-like and sublinear
dose profiles are observed for PNP and NPN transistors, respectively. Secondly,
we describe our theoretical investigations on the positive ISE in NPN
transistors. We propose an atomistic model of transformation and annihilation
of $\rm V_2$ displacement defects in p-type silicon under ionization
irradiation, which is totally different from the traditional picture of Coulomb
interaction of oxide trapped charges in silica on charge carriers in irradiated
silicon. The predicted novel dose and fluence dependences are fully verified by
the experimental data. Thirdly, the mechanism of the observed negative ISE in
PNP transistors is investigated in a similar way as in the NPN transistor case.
The difference is that in n-type silicon, VO displacement defects also undergo
an ionization-induced transformation and annihilation process. Our results show
that, the evolution of displacement defects due to carrier-enhanced defect
diffusion and reaction is the dominating mechanism of the ISEs.
