94-GHz fT, 0.4-dB NFmin HBT with Optimized Si Cap and Extrinsic Base Using Blanket SiGeC Epitaxy

Abstract

The profile design methodology of heterojunction bipolar transistors (HBTs) using a blanket SiGeC epitaxial layer is presented from the viewpoint of a Si cap and an extrinsic base design. Although important, it is difficult to optimize the intrinsic and extrinsic base regions of a HBT using blanket epitaxy since these regions are simultaneously grown and their profiles cannot be controlled independently. In accordance with the optimum Si cap thickness, extrinsic base implantation and cobalt silicidation were carried out to obtain a low base resistance and a high maximum oscillation frequency ( fmax). A cobalt silicide process for the extrinsic base was optimized to accommodate the Si cap thickness without forming highly resistive layers caused by a reaction between Ge and Co. This design led us to realize HBTs with a 94 GHz cutoff frequency ( fT) and an 81 GHz fmax, and a 0.4 dB NFmin at 2 GHz, which could be used for consumer electronic products operating within a 2–5 GHz frequency range.