Research On AlGaN/GaN Normally-off High Electron Mobility Transistor Devices With P-GaN Cap Layer And Insulator Gate

GaN material and AlGaN/GaN heterostructure are very suitable for the field of power electronics due to its superior characteristics such as high concentration two-dimensional electron gas,high breakdown voltage,and high breakdown electric field.However,transistors based on the AlGaN/GaN heterojunction structure have high concentration two-dimensional electron gas at the heterojunction interface,which makes the transistors D-mode operation.Due to this problem,this kind of transistors cannot meet the requirements of the power electronics field for high device safety factor and low conversion power consumption.Therefore,in recent years,normally-off GaN-based high electron mobility transistors(HEMT)devices are being extensively studied.Among the many solutions for normally-off GaN-based HEMT devices,p-GaN structure devices exhibit the advantages of uniform threshold voltage,high withstand voltage,and strong reliability,which are very suitable for power applications.By combing simulation and experiment,this paper designed and fabricated normally-off GaN-based HEMT devices with a p-GaN cap layer and a self-alignment insulator gate.The main contents are:(1)In order to solve the problem of large source-drain series resistance by conventional fabrication process and decreased electron mobility due to Mg diffusion from p-GaN layer to the channel,we proposed a self-alignment gate with a p-GaN/i-GaN composite cap layer structure.The diffusion of Mg was suppressed by introducing an i-GaN barrier layer,and the gate metal was fabricated to cover the entire p-GaN layer to control the entire channel by a self-aligned process.The device was simulated and analyzed by Silvaco simulation software,and it was found that by introducing the i-GaN barrier layer,the threshold voltage of the device would be lowered,but the device could still work in the enhancement mode.Therefore,the compromise to select the appropriate i-GaN layer thickness is the key to obtain excellence device performance.After comparing and analyzing devices with different gate lengths,it is found that the gate electrode can only control the channel in the area directly below it,and the self-aligned process can effectively improve the gate control to the channel.The AlGaN/GaN HEMT was experimentally fabricated with p-GaN/i-GaN composite cap structure by applying the gate-first and low temperature ohmic process.The optimized process is applied with ICP etching time of 28 s,and post annealing at 500^oC for20 min.The threshold voltage of the device is obtained as 1.3 V,which realizes the normally-off operation of the device.The device has a saturated output current density of 40 mA/mm,a maximum mobility of 711 cm²/V·s at V_g=2 V,and good temperature-dependent characteristics.In addition,the device has a lower reverse leakage current of 10^-11A,indicating that the introduction of the p-GaN/i-GaN cap layer can suppress the off-state leakage of the device.(2)In order to solve the problems of low threshold voltage and large forward gate leakage current in the above devices,we proposed a combination of insulator gate structure and self-aligned technology to improve device performance.Through Silvaco simulation,the threshold voltage is closely related to the thickness and dielectric constant of the gate dielectric.The increase in the thickness of the gate dielectric and the decrease in the dielectric constant can both increase the threshold voltage of the device.In addition,the change in the thickness of the dielectric layer and the dielectric constant will not affect the saturation output current density of the device.Based on the simulation results,we have fabricated Si N_x/p-GaN/i-GaN composite gate devices based on self-alignment process.The measured results show that by introducing the Si N_x dielectric layer,the threshold voltage of the device is increased.The threshold voltage of the device reaches 2.2 V,which is 0.9 V higher than that of the device without a dielectric layer.At the same time,the gate leakage current between the two devices is compared.Compared with the non-dielectric gate device,the insulator gate device exhibits a lower gate leakage(when the gate voltage is 10 V,the forward gate leakage current is at 10^-8A).At the same time,the maximum mobility of the device channel is about 625 cm²/V·s,which exhibits the same electron transport performance as the above-mentioned devices.The device breakdown test shows that the device reaches a hard breakdown at a reverse voltage of 1870 V.