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) Pd2Si + Al, or iii) n⁺polysilicon + Al are compared. For the same base doping profile, β(Al) is typically about 25 percent larger than β(Pd2Si), while β(poly) is typically several times larger than β(Pd2Si). 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.