Optimum low-level injection efficiency of silicon transistors with shallow arsenic emitters

Abstract
The influence of As surface concentration CSEon the emitter efficiency βγand the temperature dependence of βγare reported. The theoretical model that is used to explain the variation of βγwith CSEis based upon the difference in the effective energy bandgaps in the emitter and base regions\DeltaE_{g}. Experimental measurements of\DeltaE_{g}versus CSEare presented. Measurements of βγversus CSEshow that the effective emitter doping densityQ_{E}/x_{eb}reaches a maximum value atC_{SE} \cong 1.5 \times 10^{20}atoms/cm3, corresponding to the threshold above which\DeltaE_{g} > 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, βγincreases monotonically with CSEbecauseQ_{B} = Q_{B}(C_{SE})decreases. In addition, for devices fabricated in this study,\Delta\beta_{\gamma}/\DeltaC_{SE}atC_{SE}=2 \times 10^{20}atoms/ cm3for As-diffused emitters (doped oxide) was ≈ 5 times greater than for ion-implanted-diffused As emitters, showing the superiority of implantation in controlling gain. Finally, transistors that were made withC_{SE} \siml 1.4 \times 10^{20}atoms/cm3(\DeltaE_{g} = 0) showed βgamma(85°C)/ βγ(-15°C) ≤ 1.05.

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