Research On Novel Structures And Key Technologies Of GaN Based Power Electron Devices

Power electronic devices with high withstand voltage,high current,high thermal conductivity and high switching speed have attracted many attentions to meet the increasing power density,operating frequency,high temperature adaptability,switching speed and voltage application range requirements such as power electronics and power management.Owing to the advantages of high forbidden band width,high thermal conductivity,and high critical breakdown electric field strength gallium nitride?GaN?material has been widely researched.Power deivices based on GaN have gradually become the mainstream of power devices and have broad and bright application prospects.The main object of this dissertation are GaN-based power electronic devices to solve the current problems of GaN-based power devices such as low breakdwon voltage and low switching speed.The main research content of this dissertation is divided into the following six parts:1.The advantages of GaN materials and GaN-based power devices have been described and analyzed from the aspects of materials and device characteristics.Through the comparison of research progress in domestic and foreign,the research results of GaN-based power devices and the existing problems are explained.Combined with the polarization effect of the AlGaN/GaN heterojunction structure,the formation mechanism of the two-dimensional electron gas in GaN-based power devices and the calculation method of its concentration have been explained,and the threshold voltage model,mobility model and IV model have been established based on it.A device-circuit hybrid simulation system has been established,and the relationship between the switching speed of the device and the device structure and parameters have been discussed.By the way the degradation of the switching speed of the device at high frequency and high temperature have been characterized.2.A novel GaN vertical HEMT device?SJ-VHEMT?with vertical super junction has been proposed to improve the breakdown voltage and R_on-BV trade-off of vertical GaN HEMTs.By introducing a pnp super junction structure in the buffer,the charge balance between the p-type doped region and the n-type doped region makes the electric field distribution of the device more uniform,thereby effectively increasing the breakdown voltage.After the optimization of n-type region width,n-type region doping concentration,p-type region doping concentration and buffer layer thickness,the breakdown voltage of the device reached 2604V when the buffer layer thickness is 10?m.And the average breakdown strength is as high as 2.6MV/cm.3.A novel GaN-based power electronic device?HB-HEMT?with a hybrid AlGaN buffer has been proposed to solve problem of electric field crowding during the breakdown process and promote the Baliga's figure-of-merit.The influence of conventional GaN buffer layer and AlGaN buffer layer on the threshold voltage and on-resistance of GaN-based power devices has been discussed and analyzed.By using different Al compositions of AlGaN buffer and channel layer thickness,the threshold voltage of the device is effectively increased.The difference of electric field distribution and breakdown properties of the device with different Al compositions has been analyzed and discussed.Based on the above research a novel GaN-based power device structure?HB-HEMT?with a hybrid AlGaN buffer layer is proposed.After optimization of key parameters,HB-HEMT has high breakdown voltage?1450V?,low specific on-state resistance?0.47m?·cm²?,and high Baliga's figure-of-merit?4.47GW/cm²?at the same time.And the average breakdown electric field intensity reaches as high as 2.42MV/cm.By the way,the simulation results of CV and switching characteristics also demonstrate that the HB-HEMT device also has a low capacitance value and a high switching speed.The rise time t_r and fall time t_f of the device at 1MHz frequency are 0.49ns and 0.67ns respectively,and the turn-on speed and turn-off speed are as high as 653V/ns and 477V/ns respectively.4.A novel GaN-based power electronic device?H-HEMT?with AlGaN buffer layer and a high-k/low-k hybird dielectric layer has been porosed to solve the problem of electric field crowing during the breakdown process and promote the Baliga's figure-of-merit.Through the difference of dielectric constant and Gauss theorem,the electric field modulation mechanism of the new structure has been explained.After optimizing the key parameters of the device,the breakdown voltage,specific on-state resistance and Baliga's figure-of-merit have reached 1490V,0.4m?·cm² and 5.3GW/cm²,respectively,and the average breakdown electric field strength is as high as 2.48MV/cm.In addition,the simulation results of CV and switching characteristics show that the device also has low capacitance value and high switching speed.The rise time t_r and fall time t_f of the device?1MHz?are 0.51ns and 3.66ns respectively,and the turn-on speed and turn-off speed are as high as 627V/ns and 87V/ns,respectively.5.A novel GaN-based power electronic device?HBB-HEMT?with hybrid AlGaN back barrier layer has been proposed to solve the problem of electric field crowding during the breakdown process and promote the Baliga's figure-of-merit.Through the analysis of the difference in the electric field modulation of the device with single AlGaN back barrier,the effect of the hybrid AlGaN back barrier layer on the electric field modulation and breakdown voltage of the device has been explained.Subsequently,the key parameters of the novel device has been optimized,and the novel device has high breakdown voltage?1920V?,low on-resistance?0.45m?·cm²?,high Baliga's figure-of-merit?8.19GW/cm²?and high average breakdown electric field strength?2.74MV/cm?at the same time.6.The fabrication technique of high breakdown voltage GaN-based power electronic devices has been broke through,and GaN HEMTs with over 1000V breakdown voltage have been fabricated to accumulate experiences for the proposed novel GaN devices.The main process flow of the device fabrication used in this work has been described,and then the key processes and key technologies have been optimized.High breakdown voltage GaN-based power devices based on Si substrate and sapphire substrate have been fabricated and characterized in this paper.The breakdown voltage measurement results show that the maximum breakdown voltage of the GaN power electronic device based on Si substrate reaches 1000V with gate to drain distance of 15?m,for which the specific on-resistance,figure-of-merit and average breakdown electric field strength are identified as 1.79m?·cm²,0.57GW/cm²,and 0.67MV/cm,respectively.The GaN power electronic device based on sapphire substrate has a maximum breakdown voltage of 1156V with gate to drain distance of 15?m,for which the specific on-resistance,figure-of-merit and average breakdown electric field strength are identified as 2.38m?·cm²,0.56GW/cm²,and 0.77MV/cm,respectively.In addition,a multi-gate-finger high power GaN-based power devices based on Si substrate have been fabricated and characterized using the optimized fabrication process.The switching properties measurement results?drain voltage set as 20V?show that the rise time and fall time of the device are 3.8ns and 2ns,respectively.