Study On The Characteristics Of Enhanced HEMT Devices With P-type Gate

AlGaN/GaN high electron mobility transistors(HEMTs)have received extensive attention in the field of power switching.Due to the spontaneous polarization and piezoelectric polarization effects in the Al GaN/GaN heterojunction,a high-mobility two-dimensional electron gas(2DEG)is naturally formed at the interface between GaN and Al GaN,leading to a depletion mode(V_th<0)device.With the maturity of conventional depletion-mode HEMT devices,with the purpose of ensuring circuit safety and reducing circuit design,the demand for enhanced devices has become stronger,and enhanced mode devices are more conducive to system reliability.However,as of now,there are usually the following methods to achieve enhancement mode operation,such as grooved gate structure enhancement mode devices,enhancement mode devices with p-GaN gates or p-Al GaN gates,fluoride-based plasma treatment and common Source common gate devices,etc.Among these different methods,HEMT with P-type gate is a promising enhancement mode device.Therefore,this article focuses on the process of preparing p-GaN HEMT devices and testing their reliability.Through the selection of etching conditions and the comparison of gate metal,various characterization mechanisms of p-GaN HEMT devices are studied.The following is an overview of the research content of this article:(1)According to the AlGaN/GaN heterostructure material with a 70nm P-GaN cap layer on a 6-inch Si base purchased by Jingzhan,the material was simulated first through Silvaco device simulation,and the simulation comparison researched conventional depletion-mode HEMT devices.The effects of enhanced HEMT devices and different gate metal types on the electrical characteristics of the devices are designed based on the simulation results.After a series of process improvements and explorations,a p-GaN gate HEMT device with enhanced characteristics was successfully prepared.The threshold voltage of the device can reach 2.2 V,its peak transconductance is 116m S/mm,and the maximum saturated output current is 165m A/mm,the current on-off ratio can reach the order of 10⁹,the off-state breakdown voltage is 370V,and the forward gate breakdown voltage at room temperature is 13.1V.The device has good electrical performance.(2)At the same time,based on the process preparation process and device design point of view,the influence of different etching depths on the preparation of the ohmic contact of the device and the influence on the electrical characteristics of the device during the etching of the p-GaN layer were studied.We found that the etching in the P-GaN HEMT device with a depth of 60nm,the ohmic contact prepared on the P-GaN HEMT device has a large resistance value due to the presence of 10nm P-type GaN between the source and drain,resulting in the saturation current of the device being only 54m A/mm.At the same time,the effect of post-gate annealing on the performance of HEMT devices was studied.The devices were annealed under N₂atmosphere for 1 min at a temperature of 350?.After post-gate annealing,the saturation current of the devices increased and the gate leakage of the devices decreased.But at the same time,the threshold voltage of the device is slightly negative.This is because the post-gate annealing process can repair the damage and surface defects of the device surface caused by etching,and at the same time reduce the Schottky contact formed by the gate metal and p-GaN contact Barrier height.(3)The effect of two different gate metals,Ti/Au and Ni/Au,on the electrical characteristics of P-type gate enhancement mode HEMT devices was studied.We found that,unlike traditional n-type GaN,the gate metal with lower metal work function It has a higher Schottky barrier height than P-type GaN.At the same time,the relationship between the gate metal type that forms Schottky contact with p-GaN and the gate leakage of the device is studied.Through research,it is found that the mechanism of the gate leakage leading to the leakage under different gate voltages is different.When the gate voltage is reverse and low forward voltage,the leakage motor is considered to be surface leakage,and its possible dominant mechanism is the two-dimensional variable-range transition.When the gate voltage is greater than the threshold voltage,the value of the gate leakage varies greatly due to the influence of the type of gate metal.The fundamental reason is that the leakage of the driving electrons is different.As the gate voltage increases,the gate current mechanism of the Ni/Au gate metal is the gate current dominated by Schottky diodes,and the Ti/Au gate metal is caused by the high electric field due to the defects in the P-GaN layer.Exciting conduction,forming leakage current,dominates the gate leakage,so PFE is a possible main leakage mechanism.(4)In order to further study the reliability of the device,we studied the breakdown mechanism of the P-GaN HEMT device through the off-state breakdown test of the device.At the same time,by studying the forward gate breakdown mechanism of the device,we conducted a forward gate breakdown test on the device,studied its gate forward breakdown mechanism,and verified the mechanism through simulation results.The withstand voltage of the diode D1 formed by Ti/Au gate metal is much greater than that of Ni/Au,so we can observe that when breakdown occurs,the Ti/Au gate exhibits a higher breakdown voltage.The prepared Ti/Au gate HEMT device has a higher forward gate breakdown voltage of12.6V at room temperature(25°C).In order to further study the reliability of the device at different temperatures,we conducted a variable temperature forward gate breakdown test on the device,and found that its forward breakdown voltage is positively correlated with temperature.The average gate breakdown voltage at 125°C is 1V higher than the average gate breakdown voltage at 25°C.This forward breakdown of the gate metal can be explained by the avalanche multiplication of the depleted region of the Schottky metal and p-GaN junction.The electrons are accelerated by the high electric field in the depleted area,resulting in impact ionization,and finally an avalanche breakdown occurs.