Electron Mobility And ?-? Output Characteristics In AlGaN/GaN MOS(MIS)HEMTS

Even without doping,there are two-dimensional electron gases(2DEGs)with high concentration and high mobility in AlGaN/GaN heterostructures because of the strong spontaneous and piezoelectric polarization effects,and these kinds of structures are especially suitable for producing high electron mobility transistors(HEMTs).HEMTs consisting of AlGaN/GaN heterojunctions can work under conditions of high temperature,high frequency and high power due to some advantages of nitride materials,such as high melting point,high electron mobility,high saturation electron rate and high breakdown field strength.These HEMTs have potential and extensive applications in radar early-warning,wireless communication,satellite communication and high frequency microwave devices.However,their typical large leakage currents and current collapses are the main factors limiting their development.One of the main methods to solve these two problems is to grow an insulation layer on the AlGaN material,that is,to form a metal-oxide(insulator)-semiconductor high electron mobility transistor(MOS(MIS)HEMT).Previous studies have shown that this kind of structures can effectively suppress the current collapse and reduce the gate leakage current.In recent years,a great progress has been made in the researches of MOSHEMTs using Al2O3,SiO2 and HfO2 oxides as insulating gates.On this basis,the electron states and optical phonons in AlGaN/GaN MOS(MIS)HEMTS are investigated,and then the electron mobility under the electron-optical-phonon interaction is disc.ussed.Furthermore,the ?-?output characteristic of the device are analyzed.Our specific research contents,study methods and main conclusions are summarized as follows:(1)The mobility ? of 2DEG under the scattering of optical phonons in Al2O3/AlGaN/GaN MOSHEMTs.Considering the band offsets and the built-in electric fields caused by the polarization,the Schrodinger equation of the single electron is solved by the finite element difference method to obtain the energy levels and wave functions.Using the MREI model,weight method and dielectric continuum model,the optical phonon's structures and electrostatic potentials are discussed considering the two-mode properties of transverse optical phonons in AlGaN.According to the Lei-Ting equilibrium equation theory,the electron mobility in the system under the scattering of each branch of optical phonons is studied.At the same time,the influences of the ternary mixed crystal(TMC)and size effects,temperature and remote interface charge on the electron states and optical phonons,and then on the mobility of 2DEG are analyzed,respectively.The results show that the increase of Al component x in AlxGa1-xN will deepen potential well and enhance the confinement to 2DEG.The potentials of interface(IF)optical phonons increase,while the potentials of half-space(HS)optical phonons decrease,and the electron mobility ? decreases first and then increases with increasing x.Increasing the thickness of AlGaN barrier and the remote fixed electric charges in a certain range can increase the mobility of 2DEG,while increasing the temperature and the thickness of Al2O3 will decrease it.(2)Electron mobility in AlGaN/GaN MOS(MIS)HEMTs with different insulating gates.The electron states,electronic sheet densities and mobility of 2DEG in MOS(MIS)HEMTs with Al2O3,HfO2,SiO2 and Si3N4 as gates are compared respectively.The fixed charge,TMC and size effects in different structures are analyzed.The results show that the potential well is the deepest and the confinement force to the 2DEG is the strongest when HfO2 is used as an insulating gate.When Al component x and thickness of AlxGa1-xN material increase the electronic sheet density n,and the mobility ? of 2DEG reduce consequently,while both of them increase in the systems with the other three materials as gates.The value of ? is the largest in the systems with HfO2 as a gate when x is small;but the system with Al2O3 as a gate can reach the largest ? when x is larger than a certain value.No matter what materials the insulating gates are,the variations of ns and ? with the fixed electron charges and the thicknesses of gates are basically the same.(3)Influences of AIN inserting layer on the electron mobility in Al2O3/AlGaN/GaN MOSHEMTs.The dispersion relation and potential of each branch of optical phonons after introducing the AIN inserting layer are analyzed by the transfer matrix method.Based on content(1),the influences of AIN inserting layer on optical phonons,electron states and electron mobility are discussed.It is found that the scattering on an electron from IF optical phonons can be weakened,and the mobility of 2DEG will increase after introducing the inserting layer.However,the mobility decreases as the inserting layer's thickness increases,so a thinner inserting layer can optimize the performances of devices.(4)?-? output characteristics of Al2O3/AlGaN/GaN MOSHEMTs.Under the assumption of total exhaustion,the output currents are computed for MOSHEMTs by the charge control model.The results show that the threshold voltage of the device can drift negatively with variation of Al component and material's size of each layer.The output current increases with increasing A1 component and the thickness of an insulating gate,whereas the current decreases with increasing the thickness of AlGaN barrier.The TMC and size effects have huge influences on the threshold voltage and?-? output characteristics of the devices.The above results on the electron mobility and ?-? output characteristics of MOS(MIS)HEMT devices can be a theoretical foundation for the fabrication and the performance improvement of MOS(MIS)HEMTs and relevant devices.