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

The influence of As surface concentration C
SE
on the emitter efficiency β
γ
and the temperature dependence of β
γ
are reported. The theoretical model that is used to explain the variation of β
γ
with C
SE
is based upon the difference in the effective energy bandgaps in the emitter and base regions
\DeltaE_{g}
. Experimental measurements of
\DeltaE_{g}
versus C
SE
are presented. Measurements of β
γ
versus C
SE
show that the effective emitter doping density
Q_{E}/x_{eb}
reaches a maximum value at
C_{SE} \cong 1.5 \times 10^{20}
atoms/cm
³
, corresponding to the threshold above which
\DeltaE_{g} > 0
. For the case of a constant active base doping/cm
²
Q
B
, this also corresponds to an optimum in the emitter efficiency β
γ
. However, it is shown that in typical sequential diffusion processing of transistors, β
γ
increases monotonically with C
SE
because
Q_{B} = Q_{B}(C_{SE})
decreases. In addition, for devices fabricated in this study,
\Delta\beta_{\gamma}/\DeltaC_{SE}
at
C_{SE}=2 \times 10^{20}
atoms/ cm
³
for 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 with
C_{SE} \siml 1.4 \times 10^{20}
atoms/cm
³
(
\DeltaE_{g} = 0
) showed β
gamma
(85°C)/ β
γ
(-15°C) ≤ 1.05.