Si/SiGe heterojunction bipolar transistor (HBT) layer structures, grown by ultra high vacuum chemical vapor deposition (UHVCVD), were investigated by high resolution X-ray diffraction (HRXRD) and cross-sectional transmission electron microscopy (XTEM). The SiGe base layer thickness in the Si/SiGe HBT structure is determined by HRXRD and confirmed by XTEM. HRXRD indicates that the high quality SiGe base layer and abrupt interfaces between Si and SiGe were obtained. The Ge content in the SiGe base layer, determined by HRXRD, is 16% for sample #a and #b and 10% for sample #c. XTEM shows no defects in both the strained SiGe base layer and the HBT structure. Mesa-type Si/SiGe HBT was fabricated and ideal base current over several orders was obtained.