Wide bandgap GaN and related group III-nitride semiconductors possess many fundamental material properties that could lead to power switching devices outperforming the ones made of mainstream Si technology, enabling next-generation power conversion systems with higher efficiency and compact size. The most desirable device structure, i.e., insulated gate normally-off FET, however, is still challenged by reliability and stability issues originated from traps at the interface between the gate dielectric and III-nitride semiconductors, and traps in the buffer layer. This thesis focuses on developing advanced processing techniques to create high-quality interface and improve the gate dielectric quality for GaN metal-insulator-semiconductor high-electron-mobility transistors (MISHEMTs). To su...[ Read more ]

Wide bandgap GaN and related group III-nitride semiconductors possess many fundamental material properties that could lead to power switching devices outperforming the ones made of mainstream Si technology, enabling next-generation power conversion systems with higher efficiency and compact size. The most desirable device structure, i.e., insulated gate normally-off FET, however, is still challenged by reliability and stability issues originated from traps at the interface between the gate dielectric and III-nitride semiconductors, and traps in the buffer layer. This thesis focuses on developing advanced processing techniques to create high-quality interface and improve the gate dielectric quality for GaN metal-insulator-semiconductor high-electron-mobility transistors (MISHEMTs). To suppress the buffer trapping effect, an enhanced back barrier technique was realized with fluorine ion implantation.

To achieve low trap density at the interface between gate oxide and III-nitride semiconductors, AlN could play a special role as it was naturally compatible with III-nitride semiconductors and could be used to prevent oxygen from deteriorating the III-nitride surface. However, other than MOCVD and MBE growth techniques that require high thermal budget, low-temperature growth of AlN has been difficult. In this work, single-crystal-like AlN thin film has been successfully grown at 300℃ using plasma-enhanced atomic layer deposition (PE-ALD) technique. The AlN film exhibits a monocrystalline structure and an atomic sharp AlN/GaN heterointerface. In addition, AlN/GaN heterostructure based metal-insulator-semiconductor (MIS) structures using in situ ALD-grown Al₂O₃ as the gate dielectric were fabricated to investigate its electrical properties. The AlN/GaN heterostructure can deliver well-defined 2DEG channels owing to the strong polarization of AlN film. Meanwhile, effective gate control and enhanced channel mobility at the heterointerface was also obtained by the low interface trap density. The resulting high-quality AlN film shows great potential in enhancing device performance when integrated into GaN devices.

Although the interface trap density can be significantly reduced by using an AlN-based nitridation interfacial-layer, thermally-assisted electron emission from deep interface trap states could lead to thermal instability in threshold voltage (V_TH). To address this issue, a thin barrier layer is proposed to bring the deep interface traps below the Fermi level at pinch-off so that they become inactive. In this work, normally-off MISHEMTs featuring a partially recessed (Al)GaN barrier are realized by a fluorine-plasma implantation/etch technique. The partially recessed barrier leads to improved thermal stability, while the fluorine implantation can convert the device from normally-on to normally-off without completely removing the barrier and sacrificing the high mobility heterojunction channel. The proposed MISHEMT exhibits a threshold voltage (V_TH) of +0.6 V at a drain current of 10 μA/mm, a maximum drive current of 730 mA/mm, an ON-resistance of 7.07 Ω∙mm, and an OFF-state breakdown voltage of 703 V at an OFF-state drain leakage current of 1 μA/mm. From room temperature to 200℃, the device exhibits a small negative shift of V_TH (~0.5 V) at temperatures increasing from 25℃ to 200℃.

Lastly, GaN MISHEMTs with fluorine-implanted enhanced back barrier (EBB) were implemented. The EBB structures are achieved by standard F⁻ ion implantation below the 2DEG channel in gate region. The implanted F⁻ ions can produce a higher potential barrier at the back of the 2DEG channel, resulting in better carrier confinement in the channel and effective blocking of the source-to-drain injection current in the OFF-state. Furthermore, improved dynamic performance during high-voltage switching operations was also obtained. In particular, the presence of low-density fluorine ions in the channel region introduces impurity scattering that can effectively suppress the generation of hot electrons, leading to smaller dynamic on-resistance degradation. The MISHEMTs with EBB layers with improved high-voltage performance with lower OFF-state leakage and suppressed dynamic R_ON degradation are suitable for power switching operation.