Photoresist systems in floating T-gate fabrication for GaN high electron mobility transistors

Gallium Nitride (GaN) high electron mobility transistors (HEMTs) have garnered significant attention in micro/millimeter wave and terahertz technologies due to their unique material properties, including a large bandgap, a strong electron mobility, a high charge density, and resistance to high-temperatures. Implementing a floating T-gate structure through electron beam lithography has been explored to enhance the frequency performance of GaN HEMTs. To address this, bilayer and trilayer resist systems have been developed successively. This comprehensive review details the critical issues of T-gate manufacturing strategies using EBL technology. We explore the benefits and drawbacks of various approaches, considering factors such as cost, resolution, process complexity, and the challenges encountered during the stripping process. In addition, we summarize and discuss the potential negative electron beam resists that have emerged in recent years for T-gate fabrication. Herein, this review represents the first comprehensive compilation of the fabrication techniques for floating T-gates from the perspective of electron beam photoresists. It aims to provide researchers and practitioners with a consolidated resource encompassing the challenges and advancements in T-gate fabrication, highlighting the role of electron beam lithography and photoresist selection.