Optimum base doping profile for minimum base transit time

The author investigates the base doping profile which gives the minimum base transit time. The peak base concentration is selected 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 heterojunction-bipolar transistors. The shallowest base width was achieved 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. The author 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