Novel Materials and Structures for Wide and Ultra-Wide Bandgap Semiconductor Switches

Abstract

Semiconductor power switches are necessary for the deployment of next-generation electrical systems, including renewable energy generators, electric vehicle drivetrains, and high-power communications systems. Current silicon-based technologies are limited by insufficient blocking voltages due to bandgap limitations and processing-induced defects, undesirably high on-state resistances due to gate charge trapping at poorly understood dielectric/semiconductor interfaces, and limited reliability due to electrical and thermal failure under aggressive operating conditions. As such, new materials and device architectures are required to achieve previously unattained power, efficiency, and reliability. This dissertation identifies and investigates material candidates and demonstrates their incorporation into new device architectures for power switches. Wide bandgap (WBG) semiconductors such as GaN, and ultra-wide bandgap (UWBG) semiconductors such as Ga2O3 and diamond are employed to address the previously stated limitations. Gate charge trapping in these systems is addressed through use of high-k dielectrics not previously employed for WBG and UWBG switches. ZrO2 and HfO2 dielectrics are presented as candidates for dielectric and interface charge tuning on GaN and Ga2O3, thereby allowing the possibility of threshold voltage manipulation and normally-off behavior in WBG and UWBG switches. Fabrication technologies for WBG and UWBG switches are also reported. Normally-on and -off AlGaN/GaN MOS-HEMTs with threshold voltages between -3 to +4 V are demonstrated through a combination of ZrO2 dielectric selection and AlGaN recess etching. Design and processing for normally-off vertical GaN MOSFETs are also developed, with emphasis on critical challenges in fabricating these devices. Additionally, the fabrication and stability of hydrogen-terminated diamond switches with Al2O3 surface transfer dopants are reported. Finally, new materials and processes for improved electrical and thermal stability in power switches are demonstrated. TiN is presented as a reliable gate electrode for AlGaN/GaN HEMTs, imparting superior resistance to reverse gate bias electrical stress and temperatures up to 800 °C that otherwise destroyed conventional Ni/Au-gated HEMTs. A novel process for plasma-free selective area etching of nanocrystalline diamond heat spreading films is also presented, which promises to avoid plasma damage to the underlying semiconductor and enables etching of diamond films along features inaccessible to a typical plasma-based process.