The device with the lowest Ge fraction in the base = 16% is analyzed
first. In Fig. 4.10 results of several simulations are presented in their
consecutive order compared to measured data at V =0 V (red
symbols). They are presented as an example of calibration of forward Gummel
plot characteristics. The first simulation is performed with the default models
and the device is defined according to the specification (green dashed lines).
Thus, important effects, such as BGN and SRH recombination are still not taken
into account. Note the significant disagreement between simulation and
measurement, especially in the high-field region. However, the slope in the
collector current density at low and middle voltages is correct, which shows
that the conduction band discontinuity at base-to-emitter junction is modeled
correctly. In the next simulation the BGN is switched on (black dot-dashed
lines) and comparatively good agreement can be observed for I. In the
high-field region still a different behavior is observed. This can be explained
by the complete depletion of the two spacers in the simulation which is not the
case for the real device. As stated in  the spacers are used to
prevent outdiffusion of boron from the base to the emitter in the following
thermal processing. In the final device the SiGe spacers contain boron and are
parts of real the base layer. In the next simulation already gradual boron
doping profiles varying from 5.10 cm at the spacer-to-emitter and
spacer-to-collector interfaces, respectively, to 5.10 cm at the
base-to spacer interfaces are included. The only remaining step is to include
SRH recombination in order to match the base current. Finally, good agreement
for the complete bias range is achieved (solid red lines).
The next simulations are performed with the remaining four devices, which have higher
Ge content in the base. In Fig. 4.11 a comparison between the simulated and
measured Gummel plots for the devices with = 16%, 22%, and 28% Ge are
shown. All simulations are performed with the same set of models and no
adjusting of model parameters is performed. The only exception is done for the
concentration of traps in the base, because with the high Ge content also more
traps are introduced. Thus, quite good agreement is finally achieved.
Finally, the assumption for the doping is confirmed to some extend by SIMS measurements, which were not initially available. As can be seen in Fig. 4.12 it is true that in the real device outdiffusion of boron has taken place. However, the effect appears to be even more pronounced than initially assumed.