CHARGE SEPARATION FOR BIPOLAR-TRANSISTORS

The role of net positive oxide trapped charge and surface recombinatio n velocity on excess base current in BJTs is identified. Although the interaction of these two radiation-induced defects is physically compl ex, simple approaches for estimating these quantities from measured BJ T characteristics are presented. The oxide charge is estimated using a transition voltage in the plot of excess base current vs. emitter bia s. Two approaches for quantifying the effects of surface recombination velocity are described; the first measures surface recombination dire ctly using a gated diode. The second estimates its effects using an in tercept current that is easily obtained from the BJT itself. The resul ts are compared to two-dimensional simulations and measurements made o n test structures. The techniques are simple to implement and provide insight into the mechanisms and magnitudes of the radiation-induced da mage in BJTs.