Novel base doping profile for improved speed and power

We have experimentally studied the effect of two new base doping profiles on the base transit time of a GaAs npn heterojunction bipolar transistor. The doping in a region close to the collector is reduced either by a doping grade or a stepwise reduction. Quasi-electric fields resulting from these doping gradients increase the minority carrier velocity and the beta of large area transistors. By focusing these doping changes adjacent to the collector, the amount of low-doped base material and the resulting increase in base sheet resistance can be minimized. For both a step change in doping or graded doping change a 10% decrease in base transit time is achieved while only causing a 4 % increase in base sheet resistance. The impact on base transit time is confirmed with f/sub T/ data on small area devices.