On the sidewall effects in submicrometer bipolar transistors

The influence of the emitter sidewall on the dc base and collector current in submicrometer bipolar devices is investigated. Analytical calculations on a simplified two-dimensional transistor model as well as numerical calculations on a more realistic structure show that the two-dimensional nature of the carrier injection into the base and emitter regions near the sidewall can strongly affect the base and collector current. From this analysis it follows that when the emitter width is in the range of the characteristics diffusion length in the emitter or smaller, the base current through the bottom part of the emitter-base junction is strongly diminished when compared with the one-dimensional solution. For shallow emitters the base current through the sidewall strongly depends on the surface-recombination velocity and on the distance between the emitter contact edge and the sidewall junction. The collector current flows mainly through the bottom part of the emitter-base junction. Device design implications are briefly discussed.