A contribution to the study of the effects of base widening in bipolar transistors

The effects of base widening on the performance of high frequency bipolar transistors operating at high current densities are examined. Principle interpretations given in the literature to explain saturation mechanisms and the associated widening effects are reviewed, and a unified analysis is proposed for the behavior of the collector for every polarization condition. Expressions are derived for the induced base depth and the critical current density. Results obtained with these expressions are compared with those obtained by numerical simulation of the collector-base junction. A model is developed for a transistor subjected to the effects of base widening. Relations are derived which permit calculation of the output characteristics, current gain, and transistor frequency as a function of the polarization conditions. Results obtained for high voltage and high frequency transistors are compared with experimental results.