Graded-bandgap SiGe Bipolar Transistor Fabricated with Germanium Ion Implantation

A graded-bandgap SiGe heterojunction bipolar transistor (HBT) is fabricated using Ge
⁺
implantation. The maximum current gain and the maximum cutoff frequency are 40 and 8 GHz respectively, while those of the Si control device are 100 and 11 GHz. This relatively high cutoff frequency is obtained for the SiGe HBT despite considerable defects in the emitter and base regions, and is attributed to the drift field in the base region resulting from the graded bandgap.