Research On Some Issues Of Carrier Transport In Wide Bandgap Semiconductors

In recent years,the material and structure of high electron mobility transistor?HEMT?in ?-nitride core electronic devices follow the trend of further alloying.High voltage devices using AlGaN channel to improve the breakdown voltage,and the high frequency power devices with quaternary alloy barrier layer have been proved to enhance the power performance above Ku band.Under the application of a strong electric field,hot phonon effect?HPE?in nitrides significantly affects the energy and momentum relaxation of hot carriers.Similar to the two-dimensional electron gas?2DEG?in nitride heterojunction,the surface of the hydrogen?H?-terminated diamond indicates a sheet conductance formed by two-dimensional hole gas?2DHG?.In the study of these structures,phenomena and technologies,the lack of systematic research on the transport of carriers is a common problem.To solve the above issues,this paper investigates the two-dimensional transport characteristics of 2DEG in AlInGaN/?Al?GaN heterostructures and 2DHG in H-terminated diamond structures,and explores the influence of HPE on the energy and momentum relaxation time in the nitrides at high electric field.The main research results are as follows:1.The continuous changing principles of the 2DEG mobility limited by alloy disorder?ADO?scattering from AlInGaN barrier layer with Al?In,Ga?N alloy composition are revealed in quaternary AlInGaN/GaN heterostructures.The results show that the magnitude of the mobility is mainly determined by the alloy scattering potential,while the 2DEG density and penetration probability also affect.Finally,the potential of the AlInGaN material as the barrier layer in high-conductance nitride HEMT structure is proved.2.The physical model of the alloy composition fluctuation scattering in the quaternary Al_xIn_yGa_1-x-yN/GaN heterojunctions is established for the first time.Its effect on the 2DEG mobility is analyzed for five possible cases.First,only the aluminum mole fraction fluctuates and the indium mole fraction is constant?noted as S0 case?.Second,only the indium composition fluctuates and the aluminum mole fraction remains unchanged?noted as S1case?.The other cases are that both aluminum and indium composition fluctuate with equivalent amplitudes in opposite directions?noted as S2 case?,with unequal amplitudes in opposite directions?noted as S3 case?and with independent amplitudes in the same directions?noted as S4 case?.The effects of different parameters,alloy composition x and y,alloy composition fluctuation?35?x and?35?y,lateral correlation length L and the thickness of barrier d on mobility are considered and discussed.The results show that in the generally accepted range of L and d with|?35?x,?35?y|?0.05,the mobility is the largest for S4 case,and gradually decreases from the S0 case to the S3 case.3.The electronic transport property of 2DEG in AlInGaN/AlGaN heterostructures for high voltage devices is researched for the first time,and the effects of the alloy disorder scattering from both barrier and channel layer are specifically analyzed.With the comprehensive considerations of 2DEG density,2DEG mobility and sheet resistance,the alloy compositions of the barrier layer in Al_xIn_yGa_1-x-yN/Al_0.2Ga_0.8N heterostructures are of0.58?x?0.76 and 0?y?0.14,which corresponds to the threshold voltage of HFET devices to be-1.55^-6.24 V.Temperature dependence of the mobility in LM AlInGaN/AlGaN heterostructures with Al content of 0.05,0.2 and 0.5,In content of about 0.06 is also investigated,and the results indicate that the reductions of the mobility are 61%,47%and37%,respectively.The main reason is that the temperature independent ADO scattering gets significantly intensified with alloy composition,but the effect of polar optical phonon scattering which is dominant in GaN channel obviously weakens.4.The dependences of the 2DHG mobility on temperature and hole sheet density in H-terminated diamond are quantitatively considered for the first time.The calculation results show that the relatively low 2DHG mobility is insensitive to temperature and decreases with the rise of carrier density,which can be attributed to the surface ionized impurity scattering.On this basis,the 2DHG mobility for H-terminated diamond with stacked gate dielectric layers?high?/oxide layer?is also studied considering the effect of remote interface roughness?RIRS?scattering and remote coulomb?RCS?scattering.The dependences of the mobility on dielectric constant,the thickness of dielectric layer,interface roughness and correlation length are summarized.For practical application,the stacked gate dielectric layers should be optimized to obtain the best device performance.5.Energy and momentum relaxation of the hot electrons in GaN,AlN and InN are investigated and compared considering the nonparabolicity of the conduction band and high electron temperature?>300 K?.The impact of the HPE on the energy relaxation time is estimated by using various optical phonon lifetimes.The calculation results show that the presence of the HPE slows down the power dissipation and increases the energy relaxation time of the hot electrons of the density 1×10¹⁸ cm^-3(or 1×10¹⁹ cm^-3)by around one or two orders of magnitude at an electron temperature within 3000 K.In particular,the saturated energy relaxation times are 0.12 ps in bulk GaN,22 fs in AlN and 26 fs in InN for an electron density of 1×10¹⁸ cm^-3,which implies a rather weak HPE in AlGaN and AlInN alloys.With the detailed calculation of the scattering process between electrons and polar optical phonons,we evaluate the effect of HPE on momentum relaxation.Compared from the energy relaxation time and momentum relaxation time,we find that longer lifetime of the LO phonon mainly resulted in the overheating of electrons rather than the stimulated scattering process by hot phonons.