Planarization of emitter-base structure of heterojunction bipolar transistors by doping selective base contact and nonalloyed emitter contact

The authors report an n-p-n heterojunction bipolar transistor (HBT) with a planar (and thus passivated) emitter-base structure fabricated using a simple, low-temperature technique. They use a nonalloyed emitter contact facilitated by δ-doping grown at the surface of the sample, so that the cap layer is only 75 Å thick. The base is contacted by depositing Au-Zn or Au-Be on the surface and alloying at 420°C for 10 s, resulting in an ohmic contact with the base and rectifying contact with the emitter. The authors present large-emitter area (50-μm diameter) HBTs with homogeneous-doped bases (gain of 170) and δ-doped bases (10¹⁴ cm^-2, gain of 20). Upon reducing the emitter size of the latter to 3×8 μm the gain increased to 30, demonstrating excellent surface passivation