Abstract:
In this thesis analytical expressions of effective base width and base transit
time for exponentially doped base of a BJT considering Kirk effect are developed.
The present treatment also includes bandgap narrowing and finite
velocity saturation effect in the calculation of base transit time. In modern
bipolar junction transistors, the collector is more lightly doped than
the base. Therefore, high values of current and collector-emitter voltage
saturate the carrier velocity. With carrier-velocity saturation the positions
of the space-charge region boundaries become function of collector current.
We have studied the influence of velocity saturation in the base-collector depletion
layer, and compared the base transit time with velocity saturation
to that without velocity saturation. From the comparison we have observed
that the base transit time is increased by velocity saturation. Considering
bandgap narrowing we found that the base transit time will be underestimated
if we do not consider it. The aiding electric field weakens if bandgap
narrowing takes place, thereby reducing the drift-aided movement of minority
carriers in the base which increases the base transit time. In our
work we have incorporated Kirk effect for calculating effective base width
and base transit time and found that incorporating Kirk effect the effective
base width is increased significantly. Since carriers injected from the emitter
have to spend more time in the effective base region, the base transit
time becomes higher than that without considering base width widening
condition or Kirk effect. So for proper modeling of bipolar junction transistors
finite velocity saturation, bandgap narrowing and Kirk effect must be
considered.
