Effect of base profiles on the forward transit time of the bipolar transistors in BiCMOS circuits

The effect of the base doping profile on the base and the emitter transit time of bipolar transistors (BJTs) in BiCMOS circuits is presented. Comparing the uniform base profile with the exponential base profile in low injection, the uniform base profile gives a lower base transit time for a given base resistance and peak base concentration, while the exponential base profile gives a lower base transit time for a given base resistance and base width at a large base width. At high injection, the uniform base profile always gives the minimum base transit time. The uniform base doping profile is the optimal profile for BiCMOS logic at which the bipolar transistors are operated in high injection