Leakage current mechanisms associated with selective epitaxy in SiGe heterojunction bipolar transistors

SiGe Heterojunction Bipolar Transistors (HBT's) have been fabricated using selective epitaxy for the Si collector, followed in the same growth step by non-selective epitaxy for the SiGe base and Si emitter cap. E/B leakage currents are compared with cross section TEM images to identify sources of leakage currents associated with the epitaxy. In addition, the influence of the position of the extrinsic base implant with respect to the polysilicon emitter on the leakage currents is studied. The emitter/base leakage currents are modelled using Shockley-Read-Hall recombination, trap-assisted tunnelling and Poole-Frenkel generation. The position of the extrinsic base implant is shown to have a strong influence on the leakage currents. The Poole-Frenkel effect dominates the emitter/base leakage current in transistors in which the collector area is smaller than the polysilicon emitter. This result is explained by penetration of the emitter/base depletion region into the p+ polysilicon extrinsic base at the perimeter of the emitter. These leakage currents are eliminated when the collector area is increased so that the extrinsic base implant penetrates into the single-crystal silicon at the perimeter of the emitter