Electrical Characteristics of Low Thermal Budget Polysilicon Emitters for Si/Si1-xGex Heterojunction Bipolar Transistors

摘要

Electrical results are presented on low thermal butdget polysilicon emitters for Si/Si^1-xiGe^x HBTs. Rapid thermal anneals of 30s in the temperature range 775-900°C are investigated, and arsenic and phosphorus emitter dopanits are compared. The base current is shown to be very sensitive to the temperature of the anneal that is used to diffuse the emitter dopant from the polysilicon into the underlying single-crystal silicon. Unusually high base currents are observed for the lowest anneal temperatures, and a sudden drop in base current, is observed at higher anneal temperatures, with the drop occurring at a temperature of approximately 875°C for phosphorus-doped emitters, and 900°C for arsenic. SIMS doping profiles are presented which show that the drop in base current correlates with an increase in the dopant concentration at the polysilicon/silicon interface. This behaviour is explained by the segregation of dopanit at the polysilicon/silicon interface, which passivates interface states and creates a low-high-low potential barrier. The combination of these two effects leads directly to a decrease in the recombination velocity at the polysilicon/silicon interface, and hence a decrease in the base current. The trade-off between the dopant type, anneal temperature, and undoped Si1-xGex spacer thickness is investigated, and it is shown that the use of phosphorus instead of arsenic as the emitter dopant allows the anneal temperature to be decreased by approximately 50°C.