OPTIMUM BASE DOPING PROFILE FOR MINIMUM BASE TRANSIT-TIME

We investigated base doping profile which gives the minimum base transit time. We selected the peak base concentration so that the base resistance is the same at any base width for all profiles: box, Gaussian, and exponential. The exponential profile always gives the minimum base transit time at any base width. Thus this profile is the best if there is no other base-concentration-limiting factor, which is the case for hetero-bipolar transistors. However, emitter-base tunneling current limits the base concentration above in homojunction transistors. The base concentration of the box profile is minimum at the same base resistance. We can achieve the shallowest base width with the box profile, and the base transit time at this base width is smaller than that of other profiles at each profile's allowable base width. Consequently, for homojunction transistors, the optimum base doping profile changes from exponential to Gaussian and from Gaussian to box as the base width decreases. We also investigated the base transit time of epitaxial base transistors with a 50-nm base width and found that the base transit time increases by a factor of 1.5 when the peak base concentration is located at a depth of 20 nm.