Study On The Novel GaN-based HEMTs Power Electronic Devices For Power Switching Applications

GaN materials possess large band gap,high electron mobility,high thermal conductivity and good chemical stability,thus GaN-based HEMTs have advantages of high breakdown voltage,high switching speed,great high-temperature characteristics and excellent radiation resistance effect for power switching applications.However,the current GaN devices for power switching applications have not fully exerted their potential and advantages,which are mainly reflected in:1)P-GaN gate HEMTs have been gradually commercialized.Nevertheless,the threshold voltage and gate swing are still low.Those blemishes of p-GaN gate HEMTs lead to the complexity of the gate drive design and the difficulty of the devices being applied in the environment with high gate driving signal noise.Thus the application of GaN-based HEMTs in power switches is limited.2)At present,the breakdown characteristics of the GaN material system have been fully exploited.However,GaN-based power switches are mainly concentrated in the low and medium voltage levels of 100/300/650 V and rarely used in high voltage power switches.3)There are few reports of GaN devices used in bidirectional power switches,and the reverse blocking voltage of the device does not exceed-1000 V,which is far from the limit of GaN materials.4)P-GaN gate HEMTs have been widely used in the civil market.However,the single event effect is still a key problem that restricts the application of GaN devices in space environment.In addition,high-temperature characteristics of AlGaN devices on Si have not been studied and reported.In-depth research on novel GaN-based HEMT power electronic devices for power switching applications have been carried out to solve the above four problems.A novel AlN/p-GaN composite gate structure was innovatively proposed to improve the threshold voltage and gate breakdown voltage of GaN devices,and the gate reliability and threshold voltage stability of devices have been deeply studied.10 nm in-situ AlN dielectric layer was grown on p-GaN cap layer,and the role of the AlN dielectric layer is as follows:1)AlN can lift up the energy band of epitaxial structures and increase the threshold voltage of GaN devices.2)The gate voltage division of the AlN insulator layer accounts for the threshold voltage positive shift and the increase of gate breakdown voltage.3)In-situ AlN on p-GaN cap layer can avoid the introduction of traps at the AlN/p-GaN interface and improve the gate reliability.In summary,by growing 10 nm in-situ AlN dielectric layer on p-GaN cap,the threshold voltage of the device was shifted from 1.8 V to 3.9 V,and the forward gate breakdown voltage was increased from 10.0 V to 17.6 V.The maximum applicable gate voltages were tested by using TDDB.By considering 10 years’ lifetime at 63%failure level,the maximum applicable gate voltage for AlN/p-GaN gate HEMT reaches a remarkable high value of 12.1 V,which is much higher than that of 6.1 V for conventional HEMT.In addition,the threshold voltage stability of the devices was analyzed under drain stress,gate stress and on-state stress,and the influence of the introduction of AlN dielectric layer on the threshold voltage of the device was qualitatively analyzed.In this work,AlGaN material was introduced as channel and buffer layer to improve the breakdown voltage of the device and the breakdown mechanism of GaN devices on different substrates was analyzed.Firstly,the buffer layer structure of AlGaN epitaxial material was optimized,and the AlGaN material epitaxial with electron mobility of 1119 cm2V-1s-1 and square resistance of 777 Ω/□ has been obtained.GaN devices on sapphire substrates are not affected by vertical breakdown,thus AlGaN channel HEMTs with breakdown voltages over 3000 V were fabricated on sapphire substrates.Subsequently,AlGaN material was grown on the Si substrate as buffer and channel layers to improve the vertical and lateral breakdown voltages of the devices,AlGaN channel HEMTs with breakdown voltage of 2000 V were obtained on Si.The two kinds of AlGaN channel HEMTs possess the highest breakdown voltage among of the reported devices.In addition,GaN/AlGaN composite channel HEMTs were proposed,and breakdown voltages of this devices are higher than 4000 V by simulation analysis.In addition,electric field distribution of each layer and breakdown mechanism have been obtained by simulation software.At present,the breakdown of GaN material has been deeply explored,and the introduction of AlGaN material is conducive to promoting the application of GaN-based HEMTs in high-voltage power switches applications.Three kinds of high-performance GaN bidirectional blocking devices were proposed for bidirectional power switches applies.By using Schottky-drain structure and AlGaN channel,RBHEMTs with reverse blocking voltage more than-3000 V were obtained on sapphire substrate.Subsequently,low-cost RBHEMTs with reverse blocking voltage of2000 V on Si were achieved by adopting gold-free process,AlGaN buffer/channel layer and Schottky-drain structure.Finally,combining AlxGa1-xN/superlattice/AlxGa1-xN buffer layer and Schottky-drain,E-mode AlN/p-GaN gate RBHEMTs were obtained on Si.The threshold voltage of this device is 3.9 V,turn-on voltage is 0.61 V,forward gate breakdown voltage is 17.6 V,the forward/reverse breakdown voltages are 2520/-2480 V,respectively.The forward/reverse power figure-of-merit are 512/496 MW/cm2,respectively.This promotes GaN devices to be applied in bidirectional power switches.The degradation mechanism of GaN devices in extreme environments is deeply studied.high-temperature characteristics of AlGaN-channel devices on Si substrates were analyzed.The changes of mobility under different temperatures were studied and the detailed trapstate information was extracted at high temperatures.This filled the related high temperature characteristics research.Subsequently,heavy ion irradiation experiments were conducted under low/high voltages,respectively.The degradation mechanism under low voltage and the failure mechanism under high voltage were deeply analyzed.The influence mechanism of the line-shaped crystal defects introduced by heavy iron irradiation and a large number of electron-hole pairs generated around the defects was analyzed.The study on single event effects of GaN devices lays theoretical foundation for the GaN power switch applied in the space environment.