Investigation Of Normally-off P-GaN Gate HEMT On Silicon

GaN-based high-electron-mobility transistors(HEMT)have several advantages such as high conversion efficiency,high switching frequency,miniaturization,high operating temperature and high current density,which indicate a strong potential for power electronic system application.However,conventional HEMT is normally-on devices which increases the complexity of circuit design and power consumption and is not practical for direct use in the circuit.Thus,it is important to design a normally-on device that is in the off state at zero gate bias for promoting HEMT applications.p-type gate HEMT is one of the feasible solutions to realize enhanced devices,However,due to the process and device characteristics,problems such as yield,breakdown voltage,threshold voltage,gate swing and reliability still remain.This work mainly focuses on p-GaN gate HEMT.Firstly,the process and structure of the new device are explored,then the passivation layer and leakage mechanism of the device are studied,and finally the structure design of p-gate HEMT device is studied.The specific research contents are as follows,1.Normally-off p-GaN gate HEMTs were prepared by dry thermal oxidation of p-GaN.Oxidation rate variation with temperature can be classified into three sections.GaN is hardly oxidized at 800? or below;At a temperature around 850?,it starts to react with oxygen at a slower rate of 41 nm/h;At an evaluated temperature above 1000? oxidation gets severer at rate of 1146 nm/h.It is also found that surface becomes deteriorated at all temperatures.In the process of fabricating normally-off devices by oxidizing active region p-GaN,oxygen will preferentially difffuse along the surface dislocations and oxidize the underlying AlGaN barrier layer or even GaN channel layer,resulting in the destruction of local lattice and the damage of 2DEG.Thin layer is implemented before oxidation to block surface dislocations.It has greatly improved surface morphology after oxidation but failed to prevent oxygen preferential diffusion.2.The p-GaN gate HEMT is obtained by oxygen plasma technology.Based on the principle of donor-acceptor self-compensation,oxygen is employed to make p-GaN insulating for the first time and relieve the 2DEG in the active area.The threshold voltage of p-type gate device is 1.02 V,the maximum drain current is 301 mA/mm,the switching ratio is 108,and the device has good temperature and dynamic stability.We have systematically explored process window of O plasma treatment and annealing temperature.It is found that O plasma process time can excess 5 min,the power reaches more than 30 W,and the annealing temperature is about 350?.The oxygen distribution and surface oxide layer were then analyzed by secondary-ion mass spectrometry(SIMS)and X-ray photoelectron spectroscopy(XPS).3.Novel applications of oxygen plasma in p-type gate devices have also been investigated.Low power O plasma is used only to oxide the upper p-GaN below the gate of several nanometers.The threshold voltage of the device increases from 1.27 V to 1.81 V.The gate leakage of the device remains,and the threshold voltage has good temperature stability.In addition,the possibility of p-GaN activation by oxygen plasma treatment is verified by the reaction between hydrogen and oxygen.4.Passivation layer of devices by plasma technology is optimized.Through the plasma treatment in PECVD,it is proved that the leakage is related to the surface state of p-GaN,and different treatment conditions have different effects on the leakage.Finally,by introducing the surface etching process,increasing leakage is effectively suppressed,while electrical characteristics of the device has restored.Variable temperature electric transport test,atomic force microscope and X-ray photoelectron spectroscopy were conducted to analyze the leakage mechanism.Results show that the leakage arises from surface,which conforms to the two-dimensional range hopping model(2D-VRH).Surface roughness and stoichiometric ratio of p-GaN after different treatment may influence leakage current.5.We propose poly-Si as an alternative material for p-GaN to achieve normally-off device.Firstly,intrinsic poly-Si on GaN is grown by low-pressure chemical vapor deposition and p-type poly-Si can then be realized either by ion implantation or thermal diffusion.The fabricated poly-Si/AlGaN/GaN HEMT yielded a threshold current of ?,ideal factor of 1.48 and reverse voltage exceeding 1138 V,but it is still normally-on due to the lack of p-type concentration at the interface of poly-Si and AlGaN.Further work on process and structure optimization can be executed to achieve enhanced device by making the threshold voltage drift forward positively.