Leakage current mechanisms in SiGeHBTs fabricated using selective and nonselective epitaxy

SiGe heterojunction bipolar transistors (HBTs) have been fabricated using s elective epitaxy for the Si collector, followed in the same growth step by nonselective epitaxy for the p(+) SiGe base and n-Si emitter cap. DC electr ical characteristics are compared with cross-section TEM images to identify the mechanisms and origins of leakage currents associated with epitaxy in two different types of transistor. In the first type, the polysilicon emitt er is smaller than the collector active area, so that the extrinsic base im plant penetrates into the single-crystal Si and SiGe around the perimeter o f the emitter and the polycrystalline Si and SiGe extrinsic base. In these transistors, the Gummel plots are near-ideal and there is no evidence of em itter/collector leakage. In the second type, the collector active area is s maller than the polysilicon emitter, so the extrinsic base implant only pen etrates into the polysilicon extrinsic base. In these transistors, the leak age currents observed depend on the base doping level. In transistors. with a low doped base, emitter/collector and emitter/base leakage is observed, whereas in transistors with a high doped base only emitter/base leakage is observed. The emitter/collector leakage is explained by punch through of th e base caused by thinning of the SiGe base at the emitter perimeter. The em itter/base leakage is shown to be due to a Poole-Frenkel mechanism and is e xplained by penetration of the emitter/base depletion region into the. p+ p olysilicon extrinsic base at the emitter periphery. Variable collector/base reverse leakage currents are observed and a variety of mechanisms are obse rved, including Shockley-Read-Hall recombination, trap assisted tunneling, Poole-Frenkel and band to band tunneling. These results are explained by th e presence of polysilicon grains on the sidewalls of the field oxide at the collector perimeter.