Effect of emitter contact on current gain of silicon bipolar devices

The common-emitter current gain β of silicon n-p-n bipolar transistors with shallow (200 nm) emitters contacted by either i) Al, ii) Pd
2
Si + Al, or iii) n
⁺
polysilicon + Al are compared. For the same base doping profile, β(Al) is typically about 25 percent larger than β(Pd
2
Si), while β(poly) is typically several times larger than β(Pd
2
Si). The dependence of the base current on temperature and on the thickness of the polysilicon layer indicates that the base current is not determined by the silicon-polysilicon interface properties, such as tunneling through an interfacial oxide layer, but by the transport of holes in the n
⁺
polysilicon layer. A simple two-region (n
⁺
silicon region and n
⁺
polysilicon region) model is presented which satisfactorily explains the experimental results in terms of lower hole mobility in the n
⁺
polysilicon than in the monocrystalline n
⁺
silicon.