PLANARIZATION OF EMITTER BASE STRUCTURE OF HETEROJUNCTION BIPOLAR-TRANSISTORS BY DOPING SELECTIVE BASE CONTACT AND NONALLOYED EMITTER CONTACT

We 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. We use a nonalloyed emitter contact facilitated by delta-doping grown at the surface of the sample, so that our "cap layer" is only 75 angstrom thick. The base is contacted by depositing Au-Zn or Au-Be on the surface and alloying at 420-degrees-C for 10 s, resulting in an ohmic contact with the base and rectifying contact with the emitter. We present large-emitter area (50-mu-m diameter) HBT's with homogeneous-doped bases (gain of 170) and delta-doped bases (10(14) cm-2, gain of 20). Upon reducing the emitter size of the latter to 3 x 8-mu-m the gain actually increased to 30, demonstrating excellent surface passivation.