Abstract

We have experimentally studied the effect of twonewbase 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 transittime is achieved while only causing a 4 % increase in base sheet resistance.The impacton base transit time is confirmed with fT data on small areadevices.