Research On Enhancement-mode GaN HEMT Based On Superlattice Structure And Its High Electric-field Reliability

Gallium nitride-based high electron mobility transistors（HEMT）have the advantages of high breakdown voltage,high power density,and low switching loss that are difficult to be achieved by traditional silicon-based devices.Thus,GaN HEMT is widely used in the field of power electronics.Due to the polarization effect of Al GaN/GaN,most of the conventional GaN HEMT are natural Depletion-mode devices,which limits their application in the field of ultra-high-speed digital integrated circuits,and increases the complexity of circuit design and production costs.Therefore,the research of Enhancement-mode GaN HEMT is of great significance in practical applications.Although Enhancement-mode GaN HEMT devices have significantly improved in terms of performance,efficiency,energy consumption,size,etc.,there are still problems such as complex manufacturing processes and poor device reliability,which restrict the further popularization and application of GaN power devices.In response to the above problems,this work carried out systematic research on Enhancement-mode GaN HEMT from epitaxial growth,device preparation to reliability testing.The detailed research work and results are as follows:First,the MOCVD pulse method is used to grow the GaN/AlN superlattice barrier layer to achieve a high-quality barrier layer with high aluminum composition.The research results show that the GaN/AlN superlattice structure has a regulatory effect on the stress and defects in the barrier layer,and can avoid the influence of alloy scattering on the carrier mobility.Therefore,the material quality and electrical properties of the superlattice HEMT epitaxial film are better than those of the alloy HEMT epitaxial film under the same Al composition.When the Al composition in the superlattice barrier layer is increased to 50%,the material still maintains good crystalline quality and surface morphology,and the total dislocation density is only6.8×10⁸ cm^-2.Thanks to its high aluminum composition,the sheet resistance of the heterojunction in the superlattice barrier layer is as low as 270Ω/sq,and the output current of the prepared HEMT device is 113%higher than that of the traditional device,which provides a material basis for the preparation of subsequent Enhancement-mode devices.Then,based on the unique enhanced doping characteristics of the superlattice structure,a novel Mg-doped superlattice Enhancement-mode GaN HEMT device was designed and fabricated on the grown GaN/AlN superlattice epitaxial film.The theoretical and experimental results show that the superlattice structure also has an enhanced doping effect on solid thermal diffusion,which can effectively improve the doping efficiency of Mg and promote the formation of p-GaN/AlN in the barrier layer under the gate,so that the superlattice device with25%Al composition can achieve normally-off characteristics.However,further study found that for the GaN/AlN superlattice barrier layer with high aluminum composition,the acceptor doping is more difficult,and the enhanced doping effect of the superlattice structure is limited.Therefore,it is proposed to use ICP to lightly etch the area under the gate,which can effectively promote Mg diffusion by increasing the acceptor contact area and artificially introducing defects,so as to achieve Enhancement-mode devices with a threshold voltage of 0.51 V,an output current density of 300 m A/mm@V_G=3 V and with a threshold voltage of 0.72 V,an output current density of 347 m A/mm@V_G=3 V on the high-Al-content superlattice barrier layer,effectively avoiding technical problems of conventional p-gate devices such as high requirements for etching accuracy and difficulty in doping.Finally,the high electric-field reliability research of the superlattice Mg-doped Enhancement-mode device prepared in this work is carried out,and the corresponding reliability improvement scheme is proposed based on the analysis result.It is analyzed that under constant on-state stress,the inherent traps of the material and the defects introduced in the device preparation process will trap high-energy hot electrons and deplete the conductive channel,resulting in the degradation of device performance.However,due to the large conduction band difference of GaN/AlN,which improves the confinement of 2DEG,the reliability of the device under on-state stress is improved,and the maximum output current degradation of the device is only 4.7%.Under constant off-state stress,the stress regulation effect of the GaN/AlN superlattice structure can improve the ability of the high-Al-content barrier layer to withstand the inverse piezoelectric effect.But the Mg ions under the gate will brings the problem of unstable threshold voltage under the strong electric field between the gate and the drain.In summary,this work proposes an Enhancement-mode GaN HEMT based on a GaN/AlN superlattice structure.While improving the material quality and carrier transport characteristics,it achieves normally-off characteristics through a simple and efficient Mg doping method.This thesis provides new ideas for the preparation of Enhancement-mode devices,and has certain guiding and reference significance for the further research and development of GaN-based materials and devices.