Research On The Performance Of GaN-based HEMT And Its Optoelectronic Integrated Devices

As the third generation of wide band gap semiconductor material,gallium nitride(GaN)has many advantages,such as wide band gap,strong radiation resistance,high electron mobility,strong breakdown field,high temperature resistance.So it has a broad application prospect in the microwave high-power field.In the field of microelectronic devices at this stage,high electron mobility transistors(HEMTs)based on AlGaN/GaN heterojunctions have gradually become the focus of research.Because of their unique advantages,they can be widely used in aviation,aerospace and communications.However,temperature reliability and thermal issues restrict the further development and application of GaN HEMT,it is of great significance to study the temperature characteristics and design a reasonable thermal management scheme.In addition,GaN-based HEMT-LED optoelectronic integrated devices have huge potential applications in visible light,communication and display.At present,the influence of the proportion of HEMT and LED on optical power density has not been studied,and the proportional design of devices can be guided by physical model simulation.This thesis explores the above-mentioned problems,mainly studying the performance optimization of GaN HEMT devices,its temperature characteristics,the simulation analysis of flip-chip devices based on diamond substrates and optoelectronic integrated devices GaN-based HEMT-LEDs.The main research contents of the thesis are as follows:1.Research on performance optimization of GaN HEMT devicesThe GaN HEMT devices were tested and found to have the problems of large off-state current(10-6?10-5 A)and large negative threshold voltage.To solve this problem,the device was annealed in 850? nitrogen atmosphere,and the performance of the annealed device was significantly improved,especially in the reduction of off state current and the positive shift of threshold voltage.On the one hand,the post metal annealing treatment effectively improved the GaN/Al2O3 interface,which reduced the off-state current.On the other hand,the thickness of the surface oxide layer increased during the annealing process,which may cause surface passivation to reduce the off-state current.The positive shift of the threshold voltage was due to the reduction of the fixed positive charge in the GaN/Al2O3 interface or Al2O3 after the post metal annealing treatment.In addition,XPS results showed that the Ga-Ga bond peak increased after post metal annealing,and the metal Ga acted as the acceper to promote the shift of threshold voltage.2.Research on temperature characteristics of GaN HEMTThe temperature characteristics of GaN HEMT at 200?500 K were simulated by silvaco software.Compared with the device performance at room temperature,the output current and transconductance decreased at high temperature,while the output current and transconductance increased at low temperature,indicating that the device performance was degraded at high temperature and the device performance was improved at low temperature.The performance of GaN HEMT device at low temperature was investigated experimentally,and the performance of device at 100 K,150 K,200 K,250 K and 300 K was tested.With the decrease of temperature,the output current,threshold voltage,transconductance and mobility of the device would change.The output characteristics of the device increased with decreasing temperature.When Vg=0 V,the saturation output current density at 100 K increased by 48.5%compared to that at 300 K.At low temperature,the negative shift of threshold voltage,the increase of 2DEG concentration and mobility all increased the saturated output current value.The threshold voltage shifted negatively with the decrease of temperature,which was mainly related to the decrease of barrier height ?B and the increase of conduction band difference ?EC.The transconductance and the mobility of 2DEG also increased with the decrease of temperature,indicating that the gate control ability was enhanced at low temperature,and the increase of mobility was also the main factor leading to the increase of output current.3.Research on GaN HEMT flip-chip devices based on diamond substratesAiming at the heat dissipation problem of GaN HEMT,this thesis designed a scheme of GaN HEMT flip-chip device based on diamond substrate,which combined GaN HEMT devices with high thermal conductivity diamond for effective thermal management.The heat dissipation path was different before and after flip chip.The thermal resistance calculation showed that the thermal resistance of the device after flip chip was lower than that before flip chip.The results showed that the saturation current Idmax of the flip device was increased by 15%and the on-resistance Ron was decreased by 16.4%,which indicated that the GaN HEMT flip device could achieve more effective heat dissipation.This provides a new way for the combination of diamond and GaN HEMT for heat management.4.Research on GaN based HEMT-LED integrated device simulationThe GaN based HEMT-LED model of optoelectronic integrated device was established by apsys simulation software.The basic performance of the integrated device was simulated and analyzed.The results showed that the current,optical power and luminous intensity of the integrated device were controllable.The size ratio between the HEMT and LED greatly determines the overall performance of the integrated device,this thesis studied the optical and electrical properties of integrated GaN-based HEMT-LED devices with different LED/HEMT-LED ratios using simulations.As the size ratio of LED decreased,the power density increased first and then decreased,and the optimal size ratio of LED was about 0.2.The existence of peak power density was attributed to the increase of current density and the decrease of the quantum efficiency when the size ratio of LED decreased.The relationship between peak power density and LED size ratio at different components and thickness of electronic barrier layer(EBL)was further explored.For integrated devices with different EBL components,the LED size ratio corresponding to the peak power density of Al0.15GaN device was 0.2,while the LED size ratio corresponding to the peak power density of Al0.12 GaN device was 0.24.For the integrated devices with different EBL thickness,the LED size ratio corresponding to the peak power density of 8 nm EBL device was 0.23,the LED size ratio corresponding to the peak power density of 12 nm EBL device was 0.2,and the LED size ratio corresponding to the peak power density of 16 nm EBL device was 0.17.The peak power density of different EBL components and thicknesses corresponds to different LED size ratios,which are caused by the difference in efficiency reduction.The result of the integrated simulation provides guidance for the size ratio design of the device in the future experiments.