Research On GaN Quasi-Vertical Schottky Barrier Diodes And Planar Complementary Logic Inverters

With the requirements of energy saving and emission reduction,as the core of a system for electricrical energy control and conversion,power electronics devices have met huge challenges.Compared with conventional Si-based power electronics devices,GaN-based ones have lower on-resistance,higher energe conversation efficiency,more excellent switching performance,higher thermal stability and higher resistance to electromagnetic interference,which make them able to meet the increasing needs for small size power electronics devices in high frequency,high temperature,high power,high energy efficiency,harsh environment applications in power industry.GaN quasi-vertical Schottky barrier diodes（SBDs）have attracted wide attention from both research labs and industries,due to their advantages of good thermal stability,high device reliability,and superior dynamic characteristics,together with the high performance-to-cost ratio owing to the availability in low-cost and large-size foreign substrate.Although the development of GaN quasi-vertical SBDs in universities and research institutes has made considerable progress during recent years,there is still a big gap in the performance of the developed GaN devices from the limit of the theoretical prediction of GaN materials.The relatively high turn-on voltage and large reverse leakage current limit the further commercialization of GaN quasi-vertical SBDs.In addition,GaN-based integrated circuits can realize the monolithic integration of peripheral circuits（such as control,sensing,protection,driving,etc.）and GaN power electronics,which could reduce the parasitic inductance in the circuit and give full play to the operating frequency and energy efficiency of GaN power electronics devices.However,the development of GaN-based complementary logic integrated circuits is still in its infancy,and the main bottleneck lies in the lack of highperformance GaN-based p-FETs and a suitable strategy to achieve the monolithic integration of p-FETs and n-FETs.Based on the abovementioned background,this thesis focuses on the investigations of the key technologies and relative physics mechanisms for the GaN quasi-vertical SBDs with lower turn-on voltage,lower reverse leakage current,and higher breakdown voltage;and for the GaN-based complementary logic integrated circuits.The main research achievements are listed as follows:1.A GaN quasi-vertical SBD with lower turn-on voltage was realized using Molybdenum with a low work function as the Schottky anode metal.The effects of post-anode-thermal annealing on the characteristics and Schottky interface of Mo/Au Schottky contacts on nGaN were comprehensively investigated.It was found that after annealing at 500℃ in N₂ atmosphere,Mo and n-GaN interact at the interface,and the compound products at the interface can greatly improve the homogeneousness of the barrier,and thereby improve the device performance.Based on the above findings,a GaN quasi-vertical SBD with nearly ideal Schottky current characteristics（ideality factor n=1.01）was developed,exhibiting an ultra-low turn-on voltage of 0.34 V,a high on/off ratio of 1010,a low differential specific onresistance of 0.51mΩ·cm².2.A technique of selective anode Fluorine-treatment was proposed to reduce the reverse leakage current in GaN quasi-vertical SBDs.The anode region of the device was selectively treated by F luorine-based plasma,and fixed negative charges are introduced under the anode to deplete the carriers in the drift layer.The fabricated device with a drift layer thickness of 1 μm showed a turn-on voltage of 0.86 V,a specific on-resistance of 0.49 mΩ·cm²,and a breakdown voltage of 145 V.Compared with the conventional SBD,the reverse leakage current is reduced by 4 orders.Compared with other reported GaN quasi-vertical SBDS in the world,the achieved average breakdown electric field of 1.45 MV/cm is among the highest values,and the overal performance of the device has reached the top level.3.A GaN quasi-vertical SBD with sidewall cathode and fluorine ion implantation field rings was proposed.Compared with the conventional SBD,the breakdown voltage of the fabricated device with a drift layer thickness of 4.5 μm was boosted from 378 V to 483 V,and the turn-on voltage did not change（Von=0.54 V）.The specific on-resistance of the device was reduced from 1.34 mΩ·cm² to 1.23 mΩ·cm² by the sidewall cathode structure.The Baliga’s figure of merit of the as-fabricated SBD is 0.19 GW/cm²,and the overal performance is at the top level compared with other reports in the world.4.The reverse current transport mechanisms in GaN-on-Si quasi-vertical SBDs was investigated over a wide temperature range（298-573 K）.It has been found that at low reverse bias,thermionic emission（TE）is the dominated mechanism.At the voltage range from-1 to-20 V,variable range hopping（VRH）is main current mechanism at low temperatures（298-373 K）,while at high temperatures（498-573 K）,electrons gain enough energy and emit into the trapped states,and then Frenkel-Poole emission becomes dominant.At a relatively high voltage range,the increased electric field promotes the electron hopping along the threading dislocation in the bulk GaN layers,and VRH becomes the main current mechanism at whole measured temperature range（298 K-573 K）.The results show that the VRH leakage related to the dislocations and the leakage caused by the electrons entering a trapped state at high temperatures through FP emission are the main sources of reverse leakage current for GaN quasi-vertical SBD.5.A simple method based on the subthreshold slope was proposed to investigate the interface trap characteristics in a p-channel GaN MOSFET.The energy distribution of the interface trap density has been extracted from the analysis of the transfer characteristics in the subthreshold region of operation.The interface trap densities and respective energy distribution were also calculated from the ac conductance measurements at corresponding voltage biases.Both characterization methods show similar results of trap densities and energy levels,which validates the proposed method.6.A GaN complementary field effect transistor（FET）inverter monolithically integrated with power gate-injection HEMTs was realized on a Si substrate.The as-fabricated GaN complementary inverter shows excellent noise margin with low-level noise margin（NML）of 1.47 V and high-level noise margin（NMH）of 0.98 V at a supply voltage of 3 V at room temperature.At the temperature of 150℃,the GaN complementary logic inverter still exhibits good electrical performance.The power GaN HEMT has a Ron,sp of 3.9 mΩ·cm² and breakdown voltage of 900 V.These results show the great potential of the developed GaN complementary FET technology in the applications of GaN power modules.