Simulation And Mechanism Study Of GaN Diodes

Compared with traditional silicon materials,gallium nitride(GaN)as a representative of wi'de bandgap semiconductor materials has the characteristics of large bandgap,high saturated electron drift speed,small dielectric constant,high breakdown electric field,etc.The Schottky diode(SBD)formed by AlGaN/GaN heterojunction is very suitable for application in high temperature,high frequency,high power and other working environments,thus it has a very good application prospect in the field of radio frequency and power electronics.Conventional high-performance AIGaN/GaN Schottky diodes are usually realized by etching the barrier layer,but this will cause damage to the surface of the barrier layer,thus affecting the reliability of the device.Recess-Free AlGaN/GaN Schottky diodes using non-etching process can avoid these problems and have excellent device performance.However,there are few researches on the Recess-Free structure at present,so this paper mainly studies the AlGaN/GaN Schottky diode with Recess-Free structure in terms of mechanism and simulation.Firstly,the structural parameters of Recess-Free AlGaN/GaN SBD are simulated and optimized by using Silvaco-ATLAS simulation software.The simulated structural parameters include barrier layer thickness,metal work function and Al composition.Firstly,the influence of the change of barrier thickness on the device characteristics is analyzed,and the optimal barrier thickness of the device is obtained.Secondly,the influence of the change of anode metal work function on the device is analyzed,and a compromise is made on the metal work function.Finally,the change of Al composition to the device characteristics and its internal mechanism are analyzed.The simulation results show that the optimal A1 composition is 25%.Secondly,Recess-Free AlGaN/GaN heterojunction Schottky diodes with Ni/Au and TiN anode metals were prepared,and the forward current transport mechanism was studied.The analysis shows that the ideal factor and barrier height calculated by thermionic emission model are strongly correlated with temperature,which is inconsistent with traditional thermionic emission theory.The strong temperature dependence of Schottky parameters is explained by introducing the barrier height model of Gaussian distribution,and Richardson constant,standard deviation of Gaussian distribution and more accurate average Schottky barrier height are extracted.The high standard deviation indicates that the Schottky barrier heights of different anode metals have the same high non-uniformity,and the analysis concludes that the non-uniformity of barrier heights may come from screw dislocations in AlGaN.Finally,the reverse current transport mechanism of TiN anode is also studied for the fabricated Recess-Free AlGaN/GaN Schottky diode.Research shows that the transport mechanism of dominant reverse current is different at different temperatures.At high temperature,it is proved that TiN reverse leakage is dominated by Poole-Frankel(PF)emission mechanism of nitrogen vacancy related traps,and nitrogen vacancy is explained by nitrogen diffusion principle from nitrogen diffusion in AlGaN barrier layer to anode metal.At medium and low temperatures,the transport mechanism of reverse leakage current conforms to Fowler-Nordheim(FN)tunneling,but it is found that the extracted value of FN tunneling barrier height is much lower than the theoretical value.In view of this phenomenon,the FN tunneling model is modified.The model emphasizes that the uneven distribution of barrier height results in FN tunneling current occurring only in a small part of the area with the lowest barrier height(which can be called FN leakage region).The fitting results of the modified FN tunneling model are in good agreement with the experimental data,which verifies the existence of FN leakage region.