Novel Engineering And Mechanism Studying For GaN Based Vertical Transistors

The fully approaching electrification process has promoted the development of power devices for power conversion.HEMT devices with inherent Al Ga N/Ga N heterostructure have been commercialized on a large scale.However,the unsatisfied electric field crowding of surface,caused by unique structure,has become a critical factor which limits the further improvement of the power density and reliability of Ga N-based devices.A series of problems caused by surface electric field can be removed fundamentally by adopting devices with vertical topological structure.At the same time,the decoupling of chip size and voltage level is also realized,and the current handling capability of the device is not limited to the device surface.In this thesis,based on the existing structural design and performance deficiencies in Ga N vertical device,the internal electric field distribution and reverse blocking capability are concerned and further optimized.At the same time,considering the current carrying capacity of Ga N vertical devices is limited,heterojunction opening under forward bias is utilized to improve the forward conduction of Ga N devices.The main research contents are as follows:（1）In this work,a novel trench-gated CAVET featuring a p-Ga N island for breakdown voltage improvement is proposed.The p-Ga N island inserted between the two p-Ga N current-blocking-layer（CBL）constructs a dual-current-aperture vertical transistor（DCAVET）,which enables to engineer the electric-field（E-field）distribution in the buried p–n-junction for voltage sustaining.The device operation mechanism and characteristics were investigated by TCAD simulation.By delicately designing the device structural parameters and optimizing the doping concentra-tion,the premature punchthrough and high E-field induced avalanche breakdown were effectively suppressed at even higher reverse blocking voltage.The proposed DCAVET achieves excellent balance between punchthrough and avalanche breakdown,delivering a high breakdown voltage of 1504 V and low R_on,sp of 0.77 mΩ·cm².The trench gate DCAVET demonstrates a flexible approach to engineer the E-field distribution to achieve kilo-volts Ga N power transistors.（2）A novel junction barrier Schottky diode（JBS）with p-Ni O_x/n-Ga N heterojunction is also proposed in this thesis,which is employed to improve the forward current carrying capacity and reverse blocking capability of the device.The p-type region of conventional Ga N JBS devices is replaced by Ni O_x island with high hole concentration,which is sputtered on the surface of n-Ga N region.The anode metal forms Schottky and ohmic contact with n-Ga N and p-Ni O_x respectively,constructing a NI-JBS with p-Ni Ox hole injection ring（HIR）.The conduction of p-Ni Ox/n-Ga N heterojunction is used to achieve the injection of holes into the n-Ga N region,in order to obtain the bipolar conduction and conductance modulation effect of the device.The design characteristics of the new device are studied by testing the performance of the fabricated devices with different Schottky/HIR ratio.Adopting low Schottky/HIR ratio design in the device,both positive and reverse blocking characteristics are improved.Ga N NI-JBS presents an innovative application of p-n hetero-structure in Ga N devices to break through the limitations of the process bottleneck on structural design.