The roles of net positive oxide trapped charge and surface recombination velocity in producing excess base current in bipolar junction transistors (BJTs) are identified. Although the interaction of these two quantities is physically complex, simple approaches for estimating their magnitude from measured BJT characteristics are presented. The oxide charge is estimated using a transition voltage in the plot of excess base current versus emitter bias. Two approaches for quantifying the effects of surface recombination velocity are described. The first measures surface recombination directly using a gated diode, while the second estimates its effects using an intercept current that is easily obtained from the BJT itself. The results are compared to two-dimensional simulations and measurements made on test structures. The techniques are simple to implement and provide insight into the mechanisms and magnitudes of the radiation-induced damage in BJTs. A physically-based comparison between hot-carrier and ionizing radiation stress in BJTs is presented as well. Although both types of stress lead to qualitatively similar changes in the current gain of the device, the physical mechanisms responsible for the degradation are quite different. Implications for correlating and comparing hot-carrier-induced and ionizing-radiation-induced damage are discussed. Finally, the worst-case increase in base current is shown to be dose-rate independent. This fact allows the worst-case response of bipolar devices to be determined using convenient laboratory dose rates.
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