Research On Dynamic Characteristics And New Structure Of GaN Power Devices

GaN power devices have been gradually applied to power electronic systems due to their excellent switching rate,breakdown voltage and current density,providing more efficient solutions for electronic systems.However,there are still some key technical problems in the application of GaN power devices,such as the uneven channel electric field distribution of HEMT devices with GaN buffer layer,which causes the device to break down in advance.GaN HEMTs have reliability problems such as threshold voltage drift and on-resistance increase under dynamic stress conditions.Therefore,this thesis focuses on GaN HEMT devices,from the aspects of dynamic characteristics of the devices and exploration of new structures.The main research achievements are as follows.Firstly,Keysight B1505A test equipment and variable temperature test system were used to test the dynamic threshold voltage,pulse output and dynamic on-resistance of GaN HEMT devices,to study the factors affecting the dynamic characteristics of devices,and explore the physical mechanism leading to the change of electrical characteristics.It is found that the threshold voltage of P-GaN HEMT device increases with the increase of the gate-drain off-state bias stress,and decreases with the increase of the open-state drain voltage.Meanwhile,the longer the bias stress time is,the greater the drift degree of the threshold voltage of the device is.The current collapse effect of GaN HEMTs is also closely related to pulse voltage stress.The larger the off-state bias stress of the drain and gate is,the more obvious the current collapse effect is and the smaller the output current is.The longer the pulse bias stress time,the more serious the device current collapse phenomenon is.It is also found that the temperature has the greatest influence on the output current of the self-made device.The output current at 150℃is 30.6%lower than that at 25℃.At the same time,it is found that the dynamic on-resistance of the device increases with the increase of the off-state gate stress,the off-state drain stress and the off-state stress time.Secondly,a GaN HEMT with Multilayer Compound Dielectric(MCD-HEMT)is proposed in order to further improve the breakdown voltage and switching speed of the device based on the dynamic characteristics of the device.The new structure significantly increases the breakdown voltage of the device by using multilayer media to modulate the field in the channel direction.It is found that when the gate-drain distance of the MCD-HEMT is 6μm,the DC output characteristics of the device are basically the same as that of the traditional Al GaN buffer device(A-HEMT),and the breakdown voltage is as high as 1738V,which is 912V higher than that of A-HEMT.The average breakdown electric field is 2.9MV/cm.The specific on-off resistance is 0.40 mΩ·cm~2.Finally,by establishing a hybrid simulation system,the switching characteristics of MCD-HEMT devices are studied and their switching performance advantages are verified.Simulation analysis shows that the switching characteristics of MCD-HEMT devices are better than A-HEMTs,with the rise time of 0.51ns and the fall time of 3.2ns.At the same time,the application of MCD-HEMT at high frequencies was further studied by increasing the pulse frequency to 1MHz,and it is found that MCD-HEMT can maintain high switching speed at high frequencies.