Design and Fabrication of AlGaN/GaN-on-Si FETs for Ka-band MMICs

Abstract

As the power amplifiers (PAs) become increasingly important for use in communication of wireless and satellite, and military applications, a high operation frequency bands are highly desired for microwave transistors and monolithic microwave integrated circuits (MMICs) due to the tremendous usage of bandwidth. Since the performance of current Si, GaAs, or InP technology does not satisfied next-generation, high-power amplifiers with their inherent material limitation, GaN based materials have been intensively explored for past few decades in the aspects of their excellent properties, such as wide bandgap and high electron saturation velocity. Especially, GaN-on-Si technology is of particular interest to radio frequency (RF) industry because of the potential for low cost and the large volume of wafer production. This work firstly introduced the overall key process technologies of GaN millimeter-wave (mmw) device including ohmic contact, isolation, passivation, and gate process. For passivation process, we have decided the thickness of dielectric layer considering both mitigations of peak electrical field at the gate edge and degradation of small-signal characteristics due to an increase of parasitic capacitances. Also, gate structure was taken account of minimizing parasitic capacitances with short gate field-plate. The gate metal stack for millimeter-wave applications was suggested nickel and gold with molybdenum as diffusion barrier metal to achieve good thermal stability on RF performances. The final goal of this work was improvement of output power of Ka-band GaN HEMT device and MMIC PA using the proper AlGaN/GaN heterostructure on Si substrate with several technologies that dealt with modifying GaN epitaxial structure, applying high-k dielectric on passivation process, and employing recessed metal-insulator-semiconductor (MIS) structure on gate instead of metal-semiconductor (Schottky) based on above overall processes of GaN mmw device. Using AlGaN/GaN heterostructure with undoped GaN buffer demonstrated better small-signal and large-signal characteristics at 18 GHz load-pull measurement which indicated the direction of epitaxial structure for mmw application. The MIS gate structure with high-k dielectric satisfied low gate leakage current and low current collapse at a time resulted in higher output power than the conventional Schottky gate structure. To demonstrate 26.5~27 GHz GaN MMIC PA, we also optimized each fabrication process of passive elements including NiCr thin film resistor (TFR), metal-insulator-metal (MIM) capacitor, and coplanar waveguide (CPW) transmission line. Finally, the fabricated MMIC PA with employing thin SiNx interface layer for dual MIS structure, ion implantation for device isolation, and field-plated Y-gate for low gate resistance and parasitic capacitance achieved higher output power without degradation of gain at higher operation voltage than conventional Schottky gate at higher center frequency. The front process technologies we developed in this work showed the potential of GaN-on-Si technology for mmw application with watt-level output power. The packaging with good thermal management and design method of MMIC for the higher output power and PAE (power-added efficiency) might improve the performance of GaN-on-Si MMIC PA.