A new self-aligned AlGaAs/GaAs HBT based on refractory emitter and base electrodes

A self-alignment technique for AlGaAs/GaAs heterojunction bipolar transistors (HBTs) using refractory metal film, W, as the emitter and base electrodes is presented. A nonalloyed contact formation combined with selective reactive ion etching of W or WSi/sub x/ against GaAs and SiO/sub 2/ produces a self-aligned structure. An emitter contact that is thermally stable is obtained by using a Zn diffusion process to make the extrinsic base contact layer. An f/sub T/ value as high as 82 GHz was obtained. The self-alignment technique combined with the Zn diffusion process will achieve a much higher f/sub T/ if a thinner base HBT structure is used.>