The Effects of Carbon Doping on the Performance and Electrical Reliability of SiGe HBTs

The effects of carbon doping in the SiGe base on the electrical reliability of SiGe HBTs are investigated. This study uses three separate SiGe HBTs, each with an identical process flow with the exception of the carbon dose introduced during the SiGe epitaxial growth. These devices are characterized for open-base collector-emitter breakdown voltage
$BV_{CEO}$
and open-emitter collector-base breakdown voltage
$BV_{CBO}$
, which are relevant predictors of device voltage handling. Electrical stress was then performed to investigate damage trends with aging. It is shown that the increase in
$BV_{CEO}$
with increasing carbon content is a result of a decrease of peak DC current gain
$\beta$
, and not of any carbon-dependent change in the damage mechanisms. It is also shown that
$BV_{CBO}$
and multiplication factor
$M-1$
do not change with carbon, indicating similar impact-ionization for all profiles. It was also shown that, although the magnitude of base current near operating bias is highest after stress for the highest-carbon device, the relative shift is lower due in part to the existence of pre-stress traps from in-situ carbon.