ANALYSIS AND MODELING OF THE BASE CURRENTS OF SI SI1-XGEX HETEROJUNCTION BIPOLAR-TRANSISTORS FABRICATED IN HIGH AND LOW-OXYGEN CONTENT MATERIAL/

A comparison is made of the de electrical characteristics of Si/Si1-xG ex heterojunction bipolar transistors (HBTs) fabricated on high oxygen content material grown using molecular beam epitaxy and low oxygen co ntent material grown using chemical vapor deposition. The base current s of Si/Si0.85Ge0.15 HBTs with a high oxygen content are significantly higher than those of comparable devices with a low oxygen content and the base currents of both are higher than those of silicon homojuncti on devices. In addition the base current in the low oxygen content dev ices increases significantly when the germanium concentration is incre ased from 15% to 23%. The roles of the lifetime in the base and the ou t-diffusion of the boron from the Si1-xGex base are investigated using a two-dimensional drift-diffusion device simulator. It is shown that the increased base currents in the HBTs are caused by recombination in the neutral base, and that the lifetime in the Si1-xGex is an importa nt parameter in determining the base current, Modelling of the measure d base currents indicates that the Lifetime in the high oxygen content Si0.85Ge0.15 is reduced by a factor of approximately 15 compared with silicon, but in the low oxygen content Si0.77Ge0.23 by a smaller fact or of approximately 4. Boron out-diffusion from the base is present in both the high and low oxygen content HBTs, but it appears to be signi ficantly less in the former. A high oxygen content in Si1-xGex HBTs th erefore has the disadvantage of decreasing the lifetime, but the advan tage of decreasing the boron out-diffusion from the base. (C) 1995 Ame rican Institute of Physics.