Abstract
Si/SiGe heterojunction bipolar transistor (HBT) structure, grown by ultra-high vacuum chemical vapor deposition (UHVCVD), was investigated with cross-sectional transmission electron microscopy (XTEM). It was determined that a very thin layer exists in the strained SiGe layer grown on Si, which is in parallel to the interface of SiGe and Si, and the position of thin layer changes with the intrinsic SiGe spacer width but not with the Ge content in the strained SiGe base layer. The base collector junction turn-on voltage of the HBT decreases from ∼0.6 V to ∼0.2 V when the spacer width decreases from 100 A to 75 A, and increases from ∼0.2 V to ∼0.4 V when the Ge content in the strained SiGe base layer decreases from 16% to 10%. A possible explanation for this phenomenon is that the Ge atom accumulates to form a very thin Ge-rich layer.