Study Of Polar Optical Phonon Scattering And Density Of Two-dimensional Electron Gas In GaN-based High Electron Mobility Transisitors

GaN based high electron mobility transistors(HEMT)have shown great potential for the high-temperature,high-frequency and high-power applications due to the wide band gap,high electron saturation velocity and high breakdown voltage of GaN material.The density and mobility of two-dimensional electron gas(2DEG),which determine the output current density of GaN-based HEMT,are important parameters to characterize the performance of the device.Studying the analytical model of 2DEG density and mobility is of great significance to the design and performance improvement of GaN-based HEMT devices.Based on the study of the origin of 2DEG and polar optical phonon scattering in GaN-based HEMT,analytical models for 2DEG density and mobility limited by polar optical phonon scattering are established in this thesis.The results are discussed as follow:In view of the polarization effect and surface states of wurtzite GaN materials,based on the uniform distribution of surface states on the surface energy levels,a physics-based model of 2DEG density in GaN HEMT of Al GaN/GaN structure and GaN/Al GaN/GaN structure is presented respectively when the density of surface states is so low that can't pin the Fermi levels.The effect of the surface state density,Al content,Al GaN barrier layer thickness and GaN cap layer thickness on 2DEG density can be accounted for by these two models.Furthermore,the expression of the critical thickness of GaN cap layer producing two-dimensional hole gas(2DHG)in GaN/Al GaN/GaN HEMT is given by the critical conditions for GaN/Al GaN interface to produce 2DHG.Based on the scattering theory and the origin of polar optical phonon scattering,a physics-based model for the mobility limited by polar optical phonon scattering is presented.The mobility variation with temperature,200?400 K,and 2DEG density,6×10¹¹?1×10¹³ cm^-2,has been analyzed.It is found that the mobility limited by polar phonon scattering decreases monotonously with the temperature and its dependence is well approximated with the function of?_PO=AT^-?(?=3.5).Furthermore,absorption phonon is predominant because of high optical phonon energy in GaN.The mobility of polar optical phonon scattering is further studied with the variation of optical phonon energy,which shows that the room temperature mobility of2DEG can be improved by increasing the energy of polar optical phonon.Combining the model for polar optical phonon scattering and model for 2DEG density,the effects of Al GaN barrier layer thickness,Al composition and device structure on the mobility limited by polar optical phonon scattering are investigated.The results show that the increase of Al GaN barrier thickness or the increase of Al composition both will decrease the mobility limited by polar optical phonon scattering.In addition,the calculated results of polar optical phonon scattering in Al GaN/GaN HEMT and GaN/Al GaN/GaN HEMT indicate that the GaN cap will increase the mobility limited by polar optical phonon scattering.