Modeling gallium arsenide heterojunction bipolar transistor ledge variations for insight into device reliability

It is widely known that under normal bias conditions, GaAs heterojunction bipolar transistor (HBT) device degradation proceeds by a gradual buildup of defects in the base and base–emitter junction depletion regions. The buildup of these defects is associated with a solid-state phenomenon known as recombination enhanced defect reaction, which is the formation and migration of defects associated with nonradiative electron–hole recombination events. These defects are often associated with midgap traps, which serve as additional recombination centers for electron–hole pairs. The resulting increased recombination current is an additional base leakage current, which reduces current gain. By extension, a high electron–hole recombination density in a region with an initially high defect density––such as an unpassivated or poorly passivated base surface––will lead to quick device degradation. This paper reports the modeling of the effects of various different extrinsic base passivation ledge parameters––material composition, thickness, width, and spacing from ledge to base contact––to determine the microscopic effects these parameters have on electron–hole recombination density. Through this we can qualitatively predict the effects these parameters will have on HBT reliability.