Self-heating induced soft degradation of the Early voltage in SiGe:C HBTs

A soft degradation of the Early voltage measured in a constant V/sub be/ forced mode in SiGe:C HBTs is observed, which occurs at collector current levels far below the critical current required for base pushout. It is experimentally demonstrated that self-heating is the physical mechanism behind the soft degradation, and a new expression is derived for the critical current at which the onset of the soft degradation occurs. A modified expression for the Early voltage is proposed to take the self-heating effect into account.