Fabrication and Characterization of GaN-on-Si based RF and Power Devices

Abstract

Owing to the unique capabilities of achieving high current density, high breakdown voltage, high cut-off frequency and high operating temperature, AlGaN/GaN high electron mobility transistors (HEMTs) are emerging as promising candidates for RF power amplifier and power switching devices. Nevertheless, despite the great potential of these new technologies, they still suffer from physical and fabrication issues which may prevent devices fabricated on GaN from achieving the performance required. This thesis presents a comprehensive study on the development of GaN-based high frequency, high power transistors.

This work can be divided into two parts, namely D-mode AlGaN/GaN schottky HEMTs on silicon substrate for high power X-band operation and E-mode Si3N4/AlGaN/GaN metal-insulator-semiconductor heterostructure field-effect transistors (MIS-HFETs) for power switching devices.

One of the main obstacle is the trapping effects, may be exacerbated when devices are operated in Radar systems. In this work, we will use a novel fluoride-based plasma treatment technique to reduce trapping phenomenon which originated from the surface, and then apply this treatment technique in conjunction with a field plate structure to a device for GaN-based RF applications. To improve overall device performance, a backend process with individually grounded source via formation has been developed to integrate large periphery devices. Based upon it, GaN HEMT amplifier with single chip of 3.6 mm gate periphery has been successfully developed. It exhibits very high power density of 8.1 W/mm with 29.4 W output power under VDS = 38 V pulse operating condition.

Compared to the conventional depletion-mode AlGaN/GaN (D-mode), Enhancement mode (E-mode) devices are attracting a great interest as they allow simplistic circuity and safe operation. It is difficult to obtain E-mode operation with a low on-resistance and a high breakdown voltage. A gate recess technique will be crucial to realize an enhancement-mode operation and improve the transfer characteristics. To reduce the on resistance and enhance the drain current density, partially recessed MIS-HFETs are investigated. The gate recess was carried out using a low-damage Cl2/BCl3-based RIE where the target etch depth was remains AlGaN barrier layer in order to improve the transfer characteristics. The occurring degradation of the mobility due to plasma etching-induced damage and scattering effect were effectively removed by partial gate recess technique. The technologies we developed have helped to give definitive direction in developing GaN-based high frequency, high power transistors.