Study On The Breakdown Mechanism Of GaN-Based Power Electronics Devices

Gallium Nitride-based power electronics devices have great advantages in the area of power electronics, and the research on their breakdown voltages is very important. Recently, there exists a long distance between the breakdown voltages and the theoretical values for Ga N-based power electronics devices, indicating that the breakdown voltage can be further improved. In order to improve the breakdown characteristics of Ga N-based power electronics devices, the breakdown mechanism should be studied. In this paper, the author focus on the breakdown mechanism of Ga N-based power electronics devices.In chapter 2, the problems in process and simulations are discussed. Then, the judgments of five basic parameters are discussed, including the maximum output drain current, threshold voltage, gate leakage current, breakdown voltage and specific on-resistance. Finally, three breakdown mechanisms are summarized, namely the avalanche breakdown induced by high electric field, thermal runaway induced by leakage current and thermal, and air breakdown between gate and drain. Based on the discussion of these problems, the research on the breakdown mechanisms of Ga N-based power electronics devices would be smoother.In chapter 3, three aspects about the breakdown characteristics of Schottky-drain HEMT are investigated. Firstly, the forward and reverse blocking voltages of Al Ga N/Ga N HEMT are simultaneously increased by using Schottky-drain structure, and the breakdown improvement mechanism has been studied. Using Schottky-drain structure, the forward and reverse blocking voltages are improved from 72 V and-5 V to 149 V and-49 V, respectively. In order to investigate the breakdown improvement mechanism, analysis on the leakage current is carried out and the reason for the improvement is explained by simulation. Secondly, the combination of Schottky drain and drain field plate an improve the reverse blocking voltage, which is a new thought and proposed in this chapter. Drain field plate can alleviate the electric field peak at the drain electrode, and the reverse blocking voltage is improved from-67 V to-653 V by using drain field plate. Simulation results suggests that the combination of Schottky drain and drain field plate can improve the reverse blocking capability effectively. Finally, the impact of drain field plate on forward blocking voltage is investigated. In order to avoid the negative impact of the drain plate on the forward blocking voltage, the distance between gate edge and drain field plate edge should be larger than a certain value, which value should make sure that the potential cannot be squeezed.In chapter 4, a group of depletion capacitance models are proposed to explain the improvement of high-k passivation layer on Al Ga N/Ga N HEMT breakdown voltage. For HEMT with passivation layer, the wall and top of the gate can form gate/insulator/semiconductor contact(MIS contact) with the Ga N-based materials, which is the real reason for the breakdown improvement. Based on the proposed depletion capacitance model, it is observed that the gate metal height and field plate thickness can affect the breakdown voltage for the first time. A thick gate metal can improve the breakdown voltage, and a thick field plate can alleviate the electric field peak at the field plate edge. In addition, high performance Al Ga N/Ga N HEMT is designed by suing the proposed depletion capacitance model and the huge electric field modulation of high-k passivation layer. For the HEMT with gate-drain spacing of 7 μm, the breakdown voltage is 1310 V, and the power figure of merit is as high as 3.67×109 V2·?-1·cm-2, which is the highest value for all Ga N-based HEMTs.In chapter 5, three high-performance Ga N-based power electronics devices are fabricated, including high-voltage Al Ga N channel HEMT, enhancement-mode In Al N/Ga N MISHEMT and high-voltage circular Al Ga N/Ga N HEMT. For the Al Ga N channel HEMT with gate-drain spacing of 3 μm, the breakdown voltage is improved from 144 V to 320 V. In addition, the trap states in Al Ga N channel HEMT is investigated by frequency-dependent CV method for the first time. It is found that the trap states in Al Ga N channel HEMT are deeper than those in Ga N cahnnel HEMT by about 0.04 e V. By using the combination of gate insulator and F treatment, the threshold voltage and breakdown voltage of In Al N/Ga N HEMT are improved simultaneously. The threshold voltage is shifted from-7.6 V to 1.8 V by F treatment. Negatively F ions can modulate conduction band, reducing the gate leakage and buffer leakage. For the HEMT with gate-drain spacing of 3 μm, the breakdown voltage is improved from 80 V to 183 V by the reduction of the buffer leakage. Experiments indicate that the combination of the gate insulator and proper F treatment can improve the threshold voltage and breakdown voltage simultaneously, and is an effective method to achieve high-voltage enhancement-mode In Al N/Ga N HEMT. The breakdown voltage of the circular Al Ga N/Ga N HEMT with gate-drain spacing of 18.8 μm is 1812 V. The average breakdown electric field between gate and drain is improved from 0.42 MV/cm of conventional bar-type HEMT to 0.96 MV/cm of circular HEMT. The conventional field plate modulates the electric field in two dimensions to improve the breakdown voltage. The fabricated circular HEMT modulate the electric field at the third dimension, improving the breakdown characteristics significantly.In chapter 6, the limitation of the conventional three-terminal breakdown characterization method is investigated, and a modified method is proposed to solve the problem. For conventional breakdown, seven kinds of breakdown curves are summarized, but the conventional characterization method is only applicable to two of them. For the other five kinds of breakdown curves, the gate leakage current is larger than the drain leakage current for a certain drain bias range. In addition, the source current cannot represent the buffer leakage current, and their values and signs are different. The reason for these problems is that the gate-source current is neglected in the conventional breakdown characterization method. Therefore, the conventional method should be modified to characterize the breakdown mechanism precisely. The similar problems also exist in off-state time-dependent breakdown. The buffer leakage current and gate-source current are extracted through a simple method. The conventional characterization method is modified based on these two leakage currents. By using the modified method, the problems in the conventional breakdown characterization method are solved. Experiments and analysis indicate that the modified breakdown characterization method is very important in the research on the breakdown mechanism of Ga N-based power electronics devices.