Current Gain Degradation Model of Displacement Damage for Drift BJTs

Drift bipolar junction transistors (BJTs), characterized by a graded doping profile in the base region, are susceptible to the recombination in both the base and emitter-base depletion regions when they are damaged by atomic displacement, leading to the gain degradation. The previous model of the gain degradation of BJTs (i.e., Messenger-Spratt model) adopts the assumptions of the neutral base and ideal depletion, which are not valid for the drift BJTs. A drift BJT has an extra build-in electric field which breaks the neutral base condition, and the excess recombination rate is reduced and depends on the drift parameters. Furthermore, the nonideal depletion effects can suppress the excess recombination in the junction region, which will also reduce the susceptibility of the displacement damage (DD). This paper presents a physics-based improvement of the previous model and provides a much better fit to the experimental data of the 3CK3B drift BJTs subjected to deuterium-tritium 14-MeV neutron irradiation. The new model also suggests that the ideal factor of excess base current induced by DD approaches to 1.33 for the BJT technologies with narrow base or operated under low bias conditions.