Research On Breakdown Characteristics And Breakdown Mechanism Of GaN HEMT Devices

Power semiconductor devices have a very wide range of applications in the power electronics industry and are one of the basic components of electronic products.In recent years,power electronic devices have developed in the direction of high frequency and high power.As the characteristics of materials such as Si and GaAs have gradually reached their theoretical limits,in order to achieve better device characteristics,the third generation of wide band gap semiconductor materials have come into people's sight.GaN materials have characteristics such as high critical breakdown electric field,high electron saturation speed,low intrinsic carrier concentration,low dielectric constant and high thermal conductivity.They have unique advantages in high temperature,high frequency and high power RF applications.The breakdown voltage is very important for power devices.However,the breakdown voltage of GaN-based devices is still lower than the theoretical limit,indicating that the breakdown voltage of GaN-based devices has a lot of room for improvement.In this paper,the breakdown characteristics of AlGaN/GaN HEMT devices under different structures are studied by experiments and simulations,and their breakdown mechanisms are analyzed further.Based on three different structures of gate-drain distance,recess gate and barrier layer thickness,the influence of each structure changes on the breakdown characteristics of the device is studied in this paper.For the AlGaN/GaN HEMT device with Si substrate and C-doped buffer layer,through a combination of experiments and simulations,the DC characteristics and the breakdown characteristics of devices with gate-drain distance of2?m-26.5?m are compared.The results show that the breakdown voltage of the device increases first and then saturates as the gate-drain distance increases.The study indicated that when the gate-drain distance is small,avalanche breakdown is caused by the peak electric field existing on the gate edge near the drain.Increase of the gate-drain distance can disperse the peak of the electric field,so that the breakdown voltage increases with gate-drain distance.When the gate-drain distance increases to more than 6.5 ?m,a peak electric field appears at the edge of drain.Due to the thinner C-doped buffer layer and large leakage current,breakdown occurs in the vertical direction,so the breakdown voltage does not substantially increase with the gate-drain distance increasing.The recess gate has a certain effect on the breakdown characteristics of the device.It is found through experiments that after the gate of the device undergoes low-speed and low-damage ICP etching,not only the threshold voltage shifts positively and the transconductance peak increases,but the breakdown voltage is also higher than conventional devices.The breakdown voltage of the recess gate device(442V) is 10%higher than the breakdown voltage of the conventional device(396V).This is possibly because the width of the depletion region of the recess gate device is expanded,and the gate is closer to the channel region,which causes the peak electric field separated.The thickness of the barrier layer also has a great influence on the characteristics of the device.The experimental results show that the device with a thicker barrier layer has a higher breakdown voltage and a larger saturation current,but the threshold voltage will shift negatively and the transconductance peak will also decrease.Finally,the effects of different etch depths in the source-drain ohmic region on device characteristics were explored through experiments.First,the source and drain ohmic regions of the HEMT device were etched at four different depths: 0nm?9nm?18nm and complete etching,and their ohmic contact resistances were 0.39?·mm,0.36?·mm,0.21?·mm and 0.54?·mm respectively.The experimental results show that as the etching depth increases,the ohmic contact resistance firstly decreases and then increases,and the breakdown characteristics of the device also show the same tendency.By comparing the breakdown voltages of devices with gate-drain distance of 3.5?m and 4.5?m,it is shown that under the condition of 0nm etch depth,avalanche breakdown occurs due to the high electric field on the side of the gate near the drain.For devices with etch depths of 9nm and 18 nm,due to the lattice damage caused by dry etching and the spiky effect caused by RTA,there are a large number of metal spikes at the drain,which causes localized carrier injection due to electric field concentration,resulting in premature device breakdown.Moreover,with the etch depth increasing,the etch damage increases,the metal spikes are deeper and the drain lattice is more fragile,so the breakdown voltage will be lower.For devices with ohmic region completely etched,the lateral contact between the drain electrode and the channel reduces the impact of damage and spiky effect,and increases the breakdown voltage.