Choice of epitaxial structure to promote performance and uniformity of GaAs/AlGaAs heterojunction bipolar transistors grown by MOVPE

Numerical simulations with a one-dimensional physical device model are used to investigate the importance of the design of the emitter and base regions of n-p-n GaAs/AlGaAs HBT structures. A structure of optimum gain and speed is determined together with limits for the variations of the aluminum gradings and doping placement. The simulations show that the edge of the base doping on the emitter side has to be controlled within a few nanometers to avoid serious degradations of the current gain and the cutoff frequency. Variation in the position of the base doping relative to the aluminum gradings is a plausible source of the spread in current gain observed in measurements of devices made by metalorganic vapor-phase epitaxy (MOVPE). It is predicted that longer emitter aluminum gradings will reduce the sensitivity to these variations together with the peak values of gain and speed. It is suggested that the grading length be chosen as a compromise between performance and uniformity depending on the accuracy of the manufacturing method and the demands of the application