Base dopant outdiffusion in SiGe heterojunction bipolar transistors

Outdiffusion of the base dopant into the emitter and collector of SiGe heterojunction bipolar transistors (HBTs) is known to seriously degrade device performance. This can be avoided by introducing undoped spacer layers, the thickness of which is usually determined empirically. This paper presents for the first time a quantitative analysis of the base dopant diffusion. The result is a graph from which the necessary minimum undoped spacer thickness can be determined depending on the base doping level and the given fabrication related thermal budget. Several fabricated HBTs with different spacer thicknesses were submitted to RTA anneals and the outdiffusion was determined experimentally. By comparison with the predicted data the diffusion constant of boron in strained SiGe layers can be determined. First results indicate that the boron diffusion in highly doped SiGe is almost an order of magnitude lower than expected.