Crystallographic orientation effects on the performance of InP-based heterojunction bipolar transistors

We report the crystallographic orientation effects in InP-based heterojunction bipolar transistors (HBTs). The DC and RF characteristics of DHBTs fabricated on the same wafer were found to be dependent on the emitter orientation. Self-aligned InP/InGaAs DHBTs with [0 1 -1] emitter direction exhibit higher current gains, more stable and also better RF performance, while maintaining similar breakdown voltages, as compared to [0 1 1] oriented devices. Most of the differences are attributed to the resulting emitter-base sidewall profiles obtained after mesa etching. Without ruling out piezoelectric and stress effects, generally observed in III-V based HBTs, a contribution to the orientation effect, especially on the DC characteristics seem to be related to the more effective extrinsic base passivation for the [0 1 -1] orientation. For a given bias point, the maximum oscillation frequency (fmax) is also slightly higher in [0 1 -1] oriented devices, due to a smaller base resistance resulting from a smaller base-emitter spacing, while the cut-off frequency (fT) remains comparable in both orientations.