Study The Capacitance-voltage Characterization Of AlGaN/GaN Heterostructures

Galliμm nitride (GaN),the present of the third generation of semiconductor materials, has a broad application prospect in the recent years. As the emergence of the commercialized GaN LED and LD, more and more research insititutes and companies were attracted and turned to research and product the GaN semiconductor devices. The AlGaN/GaN HEMT is one of the new type micro wave power devices. In contrast with the traditional micro wave power devices, the AlGaN/GaN HEMT is more adaptable to the high temperatures, and better satisfies the requirements of large voltages and large power, so it will be the core element of the power amplify of the next generation wireless communication system.The good performances depend on the high qualities of the materials. Although the high quality epitaxy GaN layer can be grown on the sapphire and SiC substrate, and the optoelectronic devices such as LED et.al. have been commercialized. But many difficulties still exist in choosing suitable substrate materials, for instance cracks may form in the GaN expitaxial layer deposited on the Si substrate, it is hard to grow a GaN expitaxial layer with thickness rather than 1.5?m, growing AlGaN/GaN HEMT must require the thickness of the GaN expitaxial layer between 2?m to 2.5?m, but it's hard to grow such thick GaN expitaxial layer, and the relationship among the two dimensional electronic gas (2DEG), the thickness of barrier layer and Al component is unclear. In this thesis, we have done some works relative to the mentioned above.1. In order to solve the problem of cracks, AIN buffer layers was inserted on the Si(111) substrate, and the effects of the growing temperatures and the thickness of AIN buffer layers to the GaN epitaxial layer were discussed. 1) We investigated the effects of thickness of AIN buffer layers prepared at the low temperature nearly 800?C on the epitaxial layer. By comparing with the results of HR-XRD,AFM et.al, it was found that the high quality crystal of the GaN epitaxial layer would be obtained with thickness around 30nm. 2) We also studied the similar case with only different temperature at high rang closed to 1060?C, and the results demonstrated that the best epitaxial GaN sample were prepared when the thickness of the HT-AlN buffer was 300nm. 2. Not only the A1N buffer layer should grow on the 2-inch Si(111) substrate , but also three LT-A1N intermediate inserted layers should be introduced to eliminate the stress and reduce the numbers of dislocations, for growing the high quality epitaxial GaN layer which is thicker than 1.5?m and nearly crack-free on the 2-inch Si(111) substrate.3. So as to study the relationships among 2DEG, the thickness of AlGaN barrier layers, Al component in AlGaN barrier and other parameters, the AlGaN/GaN heterojunction was grown on sapphire; for the sake of increasing the surface density of 2DEG, a very thin AIN barrier layer was introduced on the heterogeneous interface, so the 2DEG can be best constrained on the surface and the scatter ratio was reduced. After comparing with the results, we found that 2DEG seem to be more sensitive to Al-content when the barrier thickness exceeds 20 nm. By adjusting the barrier thickness and Al content, suitable pinch-off voltage and threshold voltage can be obtained for the operation of AlGaN/GaN HEMTs. The expeirmental are consistent with theoretical simulations, and can provide a reliable way in performance parameters optimization for AlGaN/GaN based HEMT.