Enhancement Mode Tri-gate GaN Power Devices and Logic Circuits

Gallium Nitride (GaN) is one of the most promising materials for high frequency power switching due to its exceptional properties such as large saturation velocity, high carrier mobility, and high breakdown field strength. The high switching frequency of GaN-based power converters can lead to a significant reduction of the size of passive components, such as capacitors and inductors, and thus, increasing the power density of the overall system .

Due to the polarization effect induced high density and high mobility 2DEG in AlGaN/GaN heterostructure, GaN HEMT is an intrinsic normally-on (D-mode) transistor. However, normallyoff transistors are required in most power eletronics systam for safe operation and easier driver design. Despite the presence of several commercial GaN HEMT based devices, current GaN device performance is far from the fundamental materials capabilities, as relatively large onresistance ( RON), smaller threshold voltage ( VTH) and insufficient breakdown voltage ( VBR) . Moreover, CMOS logic in GaN is not feasible today due to the poor performance of p-type GaN devices, which hinders the development of GaN monolithic integration power IC.

Recently, tri-gate structures are attracting considerable attention due to their better gate control and enhanced VBR compared to planar devices, without degrading the RON. In addition, tri-gates allow a controllable positive shift of VTH by changing the fin width, due to the partial relaxation of the AlGaN barrier and the enhanced electrostatic control from the tri-gate sidewalls. This would offer the possibility to maintain high VTH and low RON at the same time.

In this thesis, we proposes a few technologies combined with tri-gate structure to overcome these challenges. Firstly, we present normally-off GaN-on-Si MOSFETs based on the combination of tri-gate with a short barrier recess to yield a large positive VTH, while maintaining a low RON and high current density (ID ).The tri-gate structure offered excellent channel control, enhancing the VTH up to +1.4 V at 1µA/mm for the recessed tri-gate, along with a much reduced hysteresis in VTH, and a significantly increased transconductance (gm). Additional conduction channels at the sidewalls of the tri-gate trenches compensated the degradation in RON from the gate recess, resulting in a small RON of 7.32 ± 0.26 ⠊·mm for gate to drain length (LGD of 15 µm, and an increase in the maximum output current (IDmax).

Secondly, we propose a new concept for normally-off AlGaN/GaN-on-Si MOS-HEMTs based on the combination of p-GaN, tri-gate and MOS structures to achieve high VTH and low RON. The p-GaN is used to engineer the band structure and reduce the carrier density (Ns) in the tri-gate structure for a high VTH. The gate control is mainly achieved from field-effect through the tri-gate sidewalls, and does not rely on injection of gate current. The MOS structure enables much larger gate voltages ( VG) and the effective sidewall modulation resul