Investigation On High Voltage Operation Structure Design And Performance Improvement Technology Of GaN Devices

Due to its excellent material properties,including high breakdown field strength,high electron mobility and excellent high temperature stability,GaN is an advanced strategic semiconductor material.This thesis is carried out to explore huge potential and broad prospects of GaN in high frequency and high voltage applications.Based on the high breakdown field strength of GaN material,this paper investigates the design of high breakdown voltage structure and the improvement technology of its performance.For GaN devices,their high-voltage characteristics are mainly required in two application scenarios:one is RF（radio frequency）high-voltage power amplification,and the other is power conversion circuit.In terms of RF high-voltage devices,the main point of our research in this thesis is to design the structure of the devices that demonstrates high voltage,and develop the enhancement technology of their corresponding performances.Accordingly,the devices can obtain higher output power.In the aspect of power electronic devices,we focus on two issues.One is to improve the breakdown voltage of diodes by optimizing the design of structure and exploring their performance improvement technology.And the other is to reduce the leakage current so that the conversion efficiency can be improved.In view of the above research objectives and problems,this thesis carries out a series of investigations on GaN-based HEMTs in RF territories and GaN-based diodes with high operating voltage.The major results are as follows:（1）To meet the demand of GaN-based HEMTs for high voltage and high power applications in RF territories,the impact of the GaN buffer layer on the high-voltage and RF power characteristics is studied,where"Fe-doping tail"of Fe-doped buffer is proposed.Firstly,an equivalent model of"Fe-doping tailing"is established in the simulations.Secondly,three samples with different Fe-doped buffer layers are analyzed for comparative analysis.The"Fe-doping tailing"is observed by secondary ion mass spectrometry for these samples.It was found that the thicker the Fe-doped buffer layer,the more severe the"Fe-doping tail"is.Then,the devices are fabricated by using the relevant materials,where we can find that the thinner the Fe-doped buffer layer,the weaker the"Fe-doping tail".And in a certain range,the RF output power is higher when Fe-doped buffer layer is thinner.This is mainly related to the trap that introduced by the"Fe-doping tail".On this basis,the frequency conversion conductivity method is used to investigate the trap introduced by the"Fe doping tail".Double-pulse test is also utilized to clarify the effect of the"Fe-doping tail"on the devices.Finally,through a series of systematic studies,the physical models are proposed so that the performance of GaN-based devices with the Fe-doped buffer layer in RF territories can be improved,which also lays the foundation for the following research.（2）The influence of Fe doping in the buffer layer on GaN-based HEMTs is studied in the first part.To further explore the effect of buffer layer and to optimize the characteristics of devices in RF field,an epitaxial structure with"Buffer free"is proposed,which can improve breakdown voltage,suppress the leakage of buffer layer and avoid the influence of related defects of buffer layer on the RF characteristics of devices.In this epitaxial structure,GaN channel layer is directly grown above the Al N nucleating layer.On the one hand,the trap effect caused by buffer layer is directly eliminated;on the other hand,the Al N nucleating layer acts as the back barrier to improve the confinement of 2DEG.According to the analysis of DC characteristics,the Buffer free devices have more negative threshold voltage,higher saturation current and higher voltage compared with the conventional Fe-doped Buffer layer devices.In addition,Buffer free devices perform well in RF power characteristics,with P_OUTof 23.0 W/mm,linear gain of 21.3 d B and PAE of 49.3%at V_DS of 100 V.Currently,this device is the best among those reported"Buffer free"devices internationally.（3）Facing the application requirements of GaN devices in the field of power electronic,the breakdown voltage of devices should be improved,and the leakage current should be reduced.To improve the electric field distribution and suppress the leakage on the side wall of the Si-based GaN quasi-vertical PIN diodes,the fluorine plasma self-alignment technique（BSTFP）with inclined side wall is proposed.The test results show that the diode using BSTFP technology can achieve lower off-state leakage and higher current switching ratio(I_ON/I_OFF)of about 1×10¹¹.The breakdown voltage is 930 V for BSTFP-treated devices and680 V for untreated devices.The breakdown voltage of 930 V is the highest among the Si-based GaN vertical diode reported in the world currently.The reverse leakage current of the devices treated with BSTFP is 10^-4～10^-3 A/cm² at 600 V,which is lower than that of the devices without BSTFP.It can be seen that the breakdown voltage of the device is effectively improved by using BSTFP technology.（4）Facing the application requirements of GaN devices in the field of power electronic devices for high voltage and low loss,the N ion implantation terminal is proposed to improve the electric field distribution at the anode edge and reduce the leakage current of Si-based GaN quasi vertical SBD（Schottky barrier diode）.Firstly,SBDs with different anode including W/Au and Ni/Au are fabricated,and the device characteristics of the two are compared.The results demonstrate that SBDs with anode of Ni/Au have higher breakdown voltage.On this basis,the N ion implantation edge termination（NET）of the device with anode of Ni/Au is applied to further improve the breakdown voltage.Results show that the incorporation of terminal structures on Ni/Au anode quasi-vertical device can effectively reduce the off-state leakage and improve the switching ratio of the device.The reverse characteristics of the device with a N ion implantation terminal are tested,and the breakdown voltage of 540 V is among the best in the world for the Si-based GaN vertical SBD.The BFOM of the device reaches 0.28GW/cm².The NET structure fabricated by N ion implantation reduces the leakage current of the device,and modulates the electric field at the edge of the anode,thus improving the breakdown voltage of the device.