Si/SiGe heterojunction bipolar transistor with graded gap SiGe base made by molecular beam epitaxy

Si/SiGe heterostructure bipolar transistors (HBT) were fabricated and compared to Si homojunction transistors with the same doping levels. Low-temperature Si-MBE was used to form the heterojunction and the homojunction layer sequences. One HBT structure has a graded-gap base layer, where the Ge fraction is 20% at the emitter side and linearly graded to 0% toward the collector side. This gives an average Ge content of 10% in the base and increases the critical thickness. The second HBT structure has a spike-doped S-base, sandwiched to low-doped SiGe layers. A strain selective etching technique was used to contact the thin base layer. Both HBT structures exhibit a higher current gain than the homojunction structure.>