Effects of Heavily Carbon-Doped Base Layers on Performance of Submicron AlGaAs/GaAs Heterojunction Bipolar Transistors

Submicron heterojunction bipolar transistors (HBTs) with maximum frequency of oscillation, f MAX, of 91 GHz have been fabricated using a self-aligned technique and a very heavily carbon-doped (1020 cm-3) base layer. Since the quenching of photoluminescence (PL) intensity in heavily-doped GaAs is mainly due to nonradiative recombination in the bulk material, while contribution from surface recombination is negligible, the use of a heavily carbon-doped base layer in AlGaAs/GaAs HBTs minimizes the influence of surface recombination in the extrinsic base region. Thus, for HBTs with a heavily doped base layer, the ”emitter size effect” (degradation of HBT current gain) is greatly reduced when the emitter width is scaled down to submicron (0.6 µm) dimensions.