The effects of base dopant outdiffusion and undoped Si/sub 1-x/Ge/sub x/ junction spacer layers in Si/Si/sub 1-x/Ge/sub x//Si heterojunction bipolar transistors

The effects of base dopant outdiffusion and nominally undoped Si/sub 1-x/Ge/sub x/ spacer layers at the junction interfaces of Si/Si/sub 1-x/Ge/sub x//Si n-p-n heterojunction bipolar transistors (HBTs) have been studied. It has been found that small amounts of boron outdiffusion from heavily doped bases of nonabrupt interfaces cause parasitic barriers in the conduction band, which drastically reduce the collector current enhancement in the HBTs. Undoped interface spacers can remove the parasitic barriers, resulting in a strongly improved collector current enhancement.>