Interfacial oxide, grain size, and hydrogen passivation effects on polysilicon emitter transistors

The influence of several process variables on the characteristics of polysilicon emitter transistors is studied. The diffusion length of the polysilicon layer was increased using hydrogen passivation, resulting in an increase in the transistor current gain. The increase observed was less than that predicted by the T.H. Ning and R.D. Isaac (1980) transport model, especially at high emitter doping levels. Similarly, the polysilicon grain size had a smaller effect on the current gain than predicted by this model. On the other hand, an increase in the interfacial layer thickness caused a large increase in the emitter efficiency and also resulted in retarded arsenic diffusion and increased emitter resistance and base current ideality factor. The results suggest that reflection of minority carriers at the interface plays a dominant role in determining the emitter Gummel number in devices with heavily doped emitters.>