Research On Breakdown Mechanism And New Structures Of GaN Power Devices

As one of the third generation semiconductors gallium nitride(GaN)has great material properties,especially in the field of power electronics,gallium nitride power devices have attracted much research attention and many results have been reported over the world.Considering the current situation that the voltage blocking ability of gallium nitride power device is far from the theoretical limit and the shortcomings of the existing technologies,this paper has carried out a deep study which consists of the theoretical analysis of the voltage blocking mechanism of GaN power devices and the exploration of innovation device structures.Firstly,the existing techniques to improve the breakdown voltage of GaN HFET devices are summarized,such as field plate technology,substrate removal technology and ion implantation technology and so on.Then the drawbacks of the above technologies have been discussed and analyzed.Several general breakdown mechanisms of GaN HFET devices are briefly described and analyzed,including buffer layer leakage current,gate leakage current and avalanche breakdown caused by strong electric field,and some corresponding improved methods are proposed.Then,based on the breakdown mechanism and the voltage blocking mechanism of the prior art,a new structure of GaN BCD-HFET with back composite dielectric layer is proposed.The back composite dielectric layer is composed of a high-K dielectric material layer and a low-K dielectric material layer,and the composite dielectric layer is located on the back of the buffer layer of the device.Since the back composite dielectric layer can introduce additional electric field spikes in the channel of the GaN HFET device,the uniformity of the electric field distribution in the channel is greatly enhanced and the breakdown voltage of the device is increased significantly.In addition,the BCD-HFET structure is compatible with the substrate removal process and it can effectively improve the limiting effect of the substrate material on the device's voltage withstanding capability.The simulation results show that the breakdown voltage of the new structure is 658 V,which is 86% higher compared with the breakdown voltage(354V)of the conventional GaN device with substrate removal.In addition,the simulation results also show that the use of back composite material with good thermal conductivity can effectively improve the DC output characteristics of the device.Finally,a novel GaN CPL-HFET structure with negative ion implantation passivation layer is proposed.The negative ion implantation region is formed by implanting negative ions into the passivation layer between the gate and drain of the device using ion implantation technique.Due to the depletion of the two-dimensional electron gas in the channel,when the drain is under higher reverse voltage bias the depletion region in the channel is expanded and the electric field distribution in the channel is modulated.What's more,the peak of electric field of the gate edge is reduced,and the electric field under the negative ion implantation region is improved.Consequently the average electric field intensity in the device is greatly improved,and the breakdown voltage of the device increases obviously.Study results show that compared with the breakdown voltage of conventional structure(496V)and field plate structure(665V),the optimized CPL-HFET device breakdown voltage approached to 1125 V and the breakdown voltage increased by 127% and 70% respectively.In addition,the proposed device also maintains good frequency characteristic and good on-resistance characteristic.