Optimum low-level injection efficiency of silicon transistors with shallow arsenic emitters
The influence of As surface concentration CSE on the emitter efficiency βγ and the temperature dependence of βγ are reported. The theoretical model that is used to explain the variation of βγ with CSE is based upon the difference in the effective energy bandgaps in the emitter and base regions ΔEg. Experimental measurements of ΔEg versus CSE are presented. Measurements of βγ versus CSE show that the effective emitter doping density QE/xeb reaches a maximum value at CSE≅1.5×1020 atoms/cm3, corresponding to the threshold above which ΔEg>0. For the case of a constant active base doping/cm2QB, this also corresponds to an optimum in the emitter efficiency βγ. However, it is shown that in typical sequential diffusion processing of transistors, βγ increases monotonically with CSE because QB=QB(CSE) decreases
