Research On The Interface Characteristics Of AlGaN/GaN MISHEMT Devices

Gallium nitride(GaN)-based high electron mobility transistor(HEMT)has received broad attention because of its outstanding properties in high electron mobility,high current density and high breakdown voltage etc.However,there are many problems,which restricts the large-scale application of AlGaN/GaN HEMT.The usage of the metal-insulator-semiconductor(MIS)structure can effectively reduce the gate leakage and improve the breakdown voltage of the device.In addition,the passivation can improve the interface quality,suppress the current collapse and improve the threshold hysteresis.Therefore,AlGaN/GaN MISHEMT has become a very importanct research topic.In this thesis,our research works mainly focused on the interface improvement of AlGaN/GaN MISHEMT,including device simulation and the improvement of the technologie.We mainly reserched the influences on output and tranfer performances caused the interface states.We also used multiple insulator and different pro-deposition surface process to improve the devices' performance.The main contents of this thesis are as follows:1.GaN device's performance,especially influenced by defferent interface states were simulated.The interface states are studied from the changing of the channel carrier concentration caused by the interface states.The results of the simulation provide a good foundation for selecting the epitaxial structure and for realizing high performance GaN devices.2.LPCVD-Si3N4 material was used extensively to be an efficient gate dielectric for AlGaN/GaN HEMT,and then the electrical properties of the device were analyzed,The results show that the threshold hysteresis of the device is large.In order to improve this phenomenon,a two-layer dielectric method is used,the first layer uses PECVD-grown SiON,and the second layer is high-temperature LPCVD-grown Si3N4.This composite material can act as a gate dielectric layer and as a surface passivation layer,suppressing performance degradation of the device.The transfer test under variable temperature and different gate voltages shows that the threshold voltage of the device using the composite insulating materials is more stable.3.The effect of different interface treatments on device performance was studied by means of N2 and N2O plasma treatment.The results showed that plasma processing interface can increase the threshold stability of the device.For example,the maximum threshold hysteresis of the device is 200 mV by N2O plasma pre-treatment,and the breakdown voltage of the device reaches 805 V.And through the multi-frequency C-V curve test results,the relationship between interface trap density and energy level is extracted.