Reduction of f/sub t/ by nonuniform base bandgap narrowing

Analytical and simulation results are presented to illustrate qualitatively the effect of doping on base transit time. Nonuniform base bandgap narrowing (BGN) in silicon bipolar transistors can give rise to an electric field that is comparable to and against the built-in field. The base transit time tau is subsequently increased, leading to a deterioration of the cutoff frequency f/sub 1/. It is shown that the BGN effectively reduces the impurity profile grading factor K and subsequently the transit-time coefficient eta . Physically, the minority carriers can be thought of as moving in a new profile characterized by a reduced eta but in the absence of BGN. Unlike earlier investigations which also consider effective BGN dopings but ignore the field effects, this treatment includes their impact on the minority-carrier base transit time. For a steep exponential profile with strong BGN, an increase of eta by a factor 3.57 at 300 K is calculated. Device simulations predict a smaller f/sub t/ reduction factor of 1.5 for more general profiles.<
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