| In the third-generation semiconductors, GaN-based electronic materials and devices has undergone great development towards higher frequency and higher power. In order to achieve higher current, higher power and higher frequency characteristics, it is necessary to increase the Al composition and thickness of the AlGaN barrier layer for conventional AlGaN/GaN heterojunction. But this will cause misfit dislocations and deteriorate the crystalline quality of AlGaN layer, and sthenthen the inverse piezoelectric polarization effect appearing under strong electric field.All these will lead to a degradation of the device performance. The AlInGaN quaternary alloy material features a flexible change of its band gap and lattice constant, a desirable characteristic for the strain engineering and energy band engineering of GaN HFET heterostructures aimed to obtain outstanding HFET device performance. The GaN HFET with InGaN channel shows better low-frequency noise characteristics and restrained current collapse resulting mainly from significantly enhanced quantum confinement caused by large conduction band offsets between the InGaN channel and the barrier layer. However, it is seldom investigated the transport characteristics of nitride heterojunction with a quaternary AllnGaN barrier or InGaN channel. The major research work and results are as follows:Firstly, it is proposed the model of quaternary alloy disorder scattering for2DEG in AllnGaN/GaN heterojunction based on the virtual crystal approximation.Compared with both AlGaN/GaN and AlInN/GaN heterojunction, the magnitude of2DEG mobility limited by quaternary alloy disorder scattering is in the midst of those of them.Secondly, it is investigated the2DEG mobility and alloy disorder scattering of Al(In,Ga)N/GaN heterojunction as a function of2DEG density (with alloy composition fixed), In composition in AllnGaN (with2DEG density fixed), temperature, the other alloy composition of the barrier layer and AlN interlayer based on a comprehensive mobility model with various scattering considered. The mobility limited by alloy disorder scattering is almost the same for AlGaN, AllnN or AllnGaN barrier layer when Al composition is high. If small strain is introduced in the nearly lattice-matched high Al composition AlInGaN/AlN/GaN heterojunction structure by substituting Ga for In, the2DEG mobility is almost unchanged, and however conductivity can be significantly improved.Third, it is given the momentum relaxation rate of InGaN channel alloy disorder scattering based on Fang-Howard variational wave function. In AlInGaN/InGaN/GaN heterojunction,2DEG experiences the alloy disorder scattering from both the barrier layer and the channel layer, and the resulting scattering effect increases, i.e., the related mobility reduces, with increasing Al composition in the barrier layer, or increases first and then decreases with increasing In composition in the InGaN channel.At last, it is researched the critical factors determing the2DEG mobility and how they work for InGaN channel heteroj unctions with the barrier of AlGaN, AlInN or AlInGaN. The2DEG mobility in InGaN channel is generally lower than that in the GaN channel for low In composition (0<x<0.3), and the critical mechanism is the very strong effects of both alloy scattering and interface roughness scattering. When In composition in InGaN channel increases,2DEG mobility of InGaN channel heteroj unction reduces at first and then rises with a minimum reached at an In composition of0.3, and the critical mechanism is that the change of In composition influences2DEG density and effective electron mass. Compared with AlInGaN/AlN/InGaN heterojunction, AlInN/AlN/lnGaN structure is more likely to suffer from In phase separation or alloy composition fluctuation in the channel. An InGaN layer with low In or low Ga composition is most suitable to be channel material, and especially the channel consisting of InGaN with low Ga composition or pure InN promises superior2DEG mobility. |
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