Influence of built-in drift fields on the performance of InP-based HBTs grown by solid-source MBE

The versatility of solid-source molecular beam epitaxy for the growth of InP/InGaAs heterojunction bipolar transistors (HBTs) is provided by its excellent control of doping and composition grading profiles in combination with its efficiency for carbon doping. Various designs using doping grading or composition grading in the base are investigated to provide a built-in quasi-electric field that enhances electron transport. All graded-base devices exhibit higher current gains (beta), as compared to uniform-base structures, but the beta improvements are found to be nonproportional to the generated built-in drift fields. The best performances are obtained with a 9% linear composition grading profile. As compared to conventionally grown uniform-base structures, the linearly graded-base HBTs show higher current gains (up to 42%), which is of particular importance particularly in analog and mixed-signal applications.