Effect of emitter contact on current gain of silicon bipolar devices

The current gain of silicon bipolar transistors with shallow (200 nm) emitters contacted either by (i) Al, (ii) Pd2Si+Al, or (iii) n⁺polysilicon+Al are compared. For the same base doping profile, β(Al) is typically about 25% larger than β(Pd2Si+Al), while β(n⁺polysilicon+Al) is typically 3 to 7 times larger than β(Pd2Si+Al). A simple two-region (n⁺Si and n⁺polysilicon) model is presented which satisfactorily explains not only our results but also the reported results in the literature on higher current gains in transistors with emitters formed by diffusion from arsenic-implanted polysilicon.