Boron Spike Effect on Cut-Off Frequency and Early Voltage in SiGe HBTs

This paper reports an experimental study of a boron spike effect on electrical characteristics of SiGe HBTs fabricated by cold wall type ultra-high vacuum (UHV)/CVD technology. Two types of pre-treatment procedures were carried out to investigate the effect of the boron spike formed during heating step before epitaxial growth. One was Type-A (heating temperature=800°C) and the other was Type-B (HF dip and heating temperature=720°C). In Type-B, HF dip to eliminate surface contamination was necessary for reduction of heating temperature. The peak concentration of the boron spike at epi/sub interface were 1E17 cm^¿3 (for Type-A) and 2E16 cm^¿3 (for Type-B), respectively. As a result, Early voltage (VA) was enhanced from 5V in Type-A transistor to 20V in Type-B transistor. Cut-off frequency (fT) of Type-B transistor had smaller VCB dependence than that of Type-A.