Polycrystalline silicon-germanium emitters for gain control, with application to SiGe HBTs

This paper investigates germanium incorporation into polysilicon emitters for gain control in SiGe heterojunction bipolar transistors. A theory for the base current of a polySiGe emitter is developed, which combines the effects of the polySiGe grains, the grain boundaries and the interfacial layer at the polySiGe/Si interface into an expression for the effective surface recombination velocity of a polySiGe emitter. Silicon bipolar transistors are fabricated with 0, 10 and 19% Ge in the polySiGe emitter and the variation of base current with Ge content is characterized. The measured base current for a polySiGe emitter increases by a factor of 3.2 for 10% Ge and 4.0 for 19% Ge compared with a control transistor containing no germanium. These values are in good agreement with the theoretical predictions. The competing mechanisms of base current increase by Ge incorporation into the polysilicon and base current decrease due to an interfacial oxide layer are investigated.