Owing to the unique capabilities of achieving high breakdown voltage, high current density, high cut-off frequencies, and high operating temperatures, AlGaN/GaN high electron mobility transistors (HEMTs) are emerging as promising candidates for radio-frequency (RF)/microwave power amplifiers and high-temperature electronics. Compared to the conventional depletion-mode (D-mode) AlGaN/GaN HEMTs, enhancement-mode (E-mode) devices present two major advantages: (1) the reduced circuit complexity by eliminating the negative voltage supply; (2) the implementation of direct-coupled FET logic (DCFL) for digital circuits by integrating E/D-mode AlGaN/GaN HEMTs together. In this work, we will use a novel fluoride-based plasma treatment technique to fabricate high-performance E-mode AlGaN/GaN HEMTs, and then apply this treatment technique to new device structure and integration technology for GaN-based mixed-signal circuit applications.

This work can be divided into three parts, namely planar-integration of E/D-mode AlGaN/GaN HEMTs, Si₃N₄/AlGaN/GaN metal-insulator-semiconductor heterostructure field-effect transistors (MIS-HFETs), and E-mode dual-gate (DG) AlGaN/GaN HEMTs. At first, to achieve high density and high uniformity GaN-based digital circuits, a planar fabrication technology has been developed to integrate E/D-mode AlGaN/GaN HEMTs on the same chip. A DCFL inverter and a 17-stage ring oscillator are demonstrated using this technology, in which the whole process is conducted on a planar surface. After 153-hour thermal stress measurements at 350°C, the fabricated devices maintain the same DC and RF characteristics, suggesting excellent thermal reliability of this planar process. Both discrete E/D-mode HEMTs and integrated DCFL circuits exhibit proper functions within the temperature range from room temperature (RT) to 350°C, demonstrating a promising potential for GaN-based high-temperature digital ICs. Secondly, to enhance the gate voltage swing and suppress the gate leakage current at high temperatures, E-mode Si3N4/AlGaN/GaN MIS-HFETs are adopted based on CF₄ plasma treatment and a two-step Si ₃N₄ deposition technique. In the new MIS structure, the forward gate bias can be applied up to 7 V, the highest value reported in AlGaN/GaN HEMTs up to now. In addition, E-mode AlGaN/GaN MIS-HFETs show no current collapse under pulse operation as a result of the Si₃N₄ passivation effects in the access region. The DCFL ring oscillator, which consists of E/D-mode AlGaN/GaN MIS-HFETs, reveals a stable operation from RT to 415°C, indicating the excellent high-temperature working capabilities. At last, an E-mode DG AlGaN/GaN HEMT, composed of an E-mode and a D-mode gate electrode, is designed and fabricated. Compared to the E-mode single-gate AlGaN/GaN HEMTs, a 9-dB gain improvement has been achieved at 2.1 GHz in the DG devices. This achievement can be attributed to the higher output impedance and smaller feedback capacitance in DG architecture.