Modeling HBT ledge variations for insight into GaAs HBT reliability

We have modeled 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. Obviously, recombination density affects current gain. Additionally, it is well known in the industry that electron-hole recombination at midgap traps associated with crystalline defects drive HBT degradation through reduction of current gain during bias stress. For example, because a passivation ledge eliminates the infinite supply of recombination centers at exposed surfaces, a passivation ledge greatly improves device reliability. Claims that GaInP ledges produce more reliable devices than AlGaAs ledges have also been widely made. Using commercially available HBT simulation software provided by Gateway Modeling, we have investigated the recombination rate in and near the base as a function of various different ledge parameters.