Difficulties in the fabrication of planar GaAlAs/GaAs heterojunction bipolar transistors

Planar GaAlAs/GaAs heterojunction bipolar transistors have been fabricated by using zinc diffusion to contact the p-type base and Al-Ge-Ni metallization for all ohmic contacts. The transistor structures were prepared by MOCVD (metal-organic chemical vapor deposition), and the zinc diffusion was carried out by the `leaky tube´ procedure through sputter-deposited amorphous silicon masks. The planar devices demonstrated poor transistor action because parasitic junction currents deteriorated the overall device performance, and because excessive impurity redistribution resulted in widened bases and displaced base-emitter and base-collector junctions