Doping Effects and Compositional Grading in Al(x)Ga(1-x)As/GaAs Heterojunction Bipolar Transistors

摘要

First-order analytical calculations were made for the energy-band diagrams for n-Al(x)Ga(1-x)As/pGaAs heterojunctions for x = 0.15, 0.3, and 0.5 employing different compositional gradings and doping densities specifically for heterojunction-bipolar-transistor (HBT) applications. In the calculations most recently determined, conduction-band discontinuity delta EC of 65 percent of the bandgap difference delta Eg between the Al(x)Ga(1-x)As and 0.5, respectively, were used. The results show that the position of the heterojunction spike barrier, and the depth and width of the notch in the conduction-band edge for a compositionally abrupt heterointerface depend on the respective doping densities on the p and n sides of the heterojunction. Also, for an abrupt heterointerface the difference in barrier heights for electron and hole injections varies between delta Eg and delta EV (the valence-band discontinuity), depending on the doping densities and the applied bias, and is not necessarily the generally accepted value of delta EV. Analytical expressions and curves were obtained to estimate the minimum compositional grading L for eliminating the spike.