GaN HEMTs Based High-Frequency Switching Devices:Development And Applications

GaN-on-Si high electron mobility transistors(HEMTs)have the advantages of high breakdown voltage,high power density,high frequency,high efficiency,hightemperature operation,and low costs,which are promising for a variety of applications such as consumer electronics,data centers,server power supply,base station switching mode power supply,photovoltaic inverters,and electric vehicles.Enhancement-mode(E-mode)p-GaN/AlGaN/GaN HEMTs have garnered considerable interest from both academia and industry due to the highly-stable p-GaN gate.In recent years,hydrogen(H)plasma-enabled E-mode GaN HEMTs via p-GaN passivation have been widely reported and studied,leading to significant improvements in device breakdown voltage,threshold voltage,gate voltage swing,current collapse,and reliability of GaN HEMTs.This dissertation focuses on the development and application of E-mode GaN high-frequency switching devices,and systematically solves the key technical problems from material epitaxy,devices to packaging applications.The dissertation first develops high-quality AlGaN/GaN HEMT epitaxial growth on Si substrates by optimizing nucleation and buffer layer growth and semi-insulating GaN growth via intentional carbon doping using ethylene(C2H4).Then the dissertation proposes an innovative H plasma technology for p-GaN passivation to realize E-mode high-frequency GaN HEMTs based on the p-GaN/AlGaN/GaN platform,where key fabrication processes are systematically investigated.Furthermore,the dissertation carries out an in-depth study and analysis of the surface leakage of the GaN HEMTs and proposes effective solutions to address this issue.Finally,the dissertation develops packaging techniques and verification circuits for GaN HEMTs in high-frequency applications.Through systematic analysis and research on related technical issues,the foundation has been laid for the later production development and engineering application of E-mode GaN HEMTs.The dissertation includes the following five key topics:1.AlGaN/GaN HEMT growth on Si(111)substratesDue to their large size,low cost,and compatibility with traditional Si CMOS manufacturing,Si substrates are the top choice for GaN heteroepitaxial growth.However,the large thermal expansion coefficient mismatch and lattice mismatch between Si and GaN can lead to large defect density and film cracking.This dissertation utilizes metal organic chemical vapor deposition(MOCVD)to grow GaN HEMTs on Si substrates.By improving AIN nucleation layer quality via optimized growth temperature and using graded AlGaN multilayers to reduce wafer bowing and defect propagation,high-quality AlGaN/GaN HEMTs are successfully grown on 6-inch Si substrates.The two-dimensional-electron-gas(2DEG)density of the grown HEMTs is 9.3×1012 cm-2,the 2DEG mobility is 2150 cm2/V·s,and the square resistance(Rsq)is 312 Ω/□.2.Semi-insulating GaN buffer layers for AlGaN/GaN HEMTsSemi-insulating GaN buffer layers are crucial to improving breakdown voltages of GaN HEMTs.Carbon-doped semi-insulating GaN is widely used in GaN HEMTs,but unintentional carbon doping from metal-organic sources can degrade the crystal quality.This dissertation develops intentional carbon doping using C2H4 as the carbon source to realize high-quality semi-insulating GaN layers.Key growth parameters,including growth temperature,pressure,and Ⅴ/Ⅲ ratio,are tuned to obtain the optimal growth conditions.It is found that the Ⅴ/Ⅲ ratio plays a critical role in the carbon doping in GaN.With a thickness of 3.5 μm and a carbon doping of 1×1019 cm-3,the semi-insulating GaN shows a similar crystal quality and much-improved breakdown voltage compared with GaN without carbon doping,and the breakdown voltage is increased from 300 V to 2200 V at a drain current of 10 μA.3.H-plasma technology for E-mode p-GaN/AlGaN/GaNThis dissertation develops H-plasma technology for p-GaN passivation to realize E-mode p-GaN gated GaN HEMTs with a threshold voltage of 2 V,where key fabrication processes are comprehensively investigated.The effect of H-plasma technology on devices with short gate lengths is studied.With proper design of H diffusion margin,devices with different gate lengths exhibit good uniformity and consistency in threshold voltage.Furthermore,this dissertation develops a novel gate metal process to realize wafer-scale sub-micron-gate GaN HEMTs via stepper and double-layer photoresist,which can be used for mass production.In addition,a low ohmic contact resistance(Rc)of 0.36 Ω·mm is achieved by the combination of slow etching technique,Si ion implantation,and Au-free drain/source contacts.4.Surface leakage study of AlGaN/GaN HEMTsThis dissertation investigates several surface passivation methods to suppress the surface leakage of AlGaN/GaN HEMTs.The use of SiNx/SiON composite gate dielectric can not only suppress gate leakage current,but also have a positive effect on threshold voltage stability,which shows this composite dielectric has a lower interface state.After deposition of surface passivation dielectric layer,the E-mode HRCLHEMTs show increased leakage currents,which is ascribed to surface lateral leakage,and the leakage mechanism is revealed to be a two-dimensional variable-range-hopping model(2D-VRH).The off-state breakdown voltage of the device using the Al2O3 and SiNx composite dielectric layer process is increased to 780 V,the current collapse effect is also alleviated,and both PEALD and PECVD processes are suitable for large-scale applications.5.SMT packaging techniques and verification circuits for E-mode GaN HEMTsIn order to satisfy the high-frequency and miniaturization requirements of GaN HEMT applications,this dissertation develops leadless flip-chip surface mount technology(SMT)packaging techniques by combining PCB with Sn ball insertion and reflow soldering.Then,this dissertation realizes the wafer-scale DFN packaging technique for GaN HEMTs via a series of steps,including wafer sawing,die attachment,wire bonding,modeling,etc.To verify the high-frequency characteristic of E-mode pGaN/AlGaN/GaN HEMTs,GaN HEMTs based DC-DC and AC-DC circuits are designed and demonstrated.The GaN HEMTs show 3.8 MHz switching frequency in 220 V AC to 5 V DC circuits.In addition,the thermal effect is one of the major causes of performance degradation under a long operation time.