| Summary: | 碩士 === 國立交通大學 === 影像與生醫光電研究所 === 101 === Due to the characteristics of wide bandgap, high breakdown field, high electron mobility and excellent thermal conductivity, GaN electronic devices have become one of the most promising candidates for next generation high-power applications. Among the GaN based transistors, InAlN/GaN high electron mobility transistors (HEMT) have attracted a lot of attentions in recent years. For the InAlN/GaN HEMT, the InAlN material is lattice matched to GaN when the indium composition is 18%. Besides, the InAlN/GaN structure also has larger conduction band offset and higher spontaneous polarization field. Therefore, it can provide superior electrical performance as compared to conventional InGaN/GaN HEMT with good reliability if the material growth issues can be solved.
In this study, InAlN/AlN/GaN heterostructures were grown on sapphire substrates by MOCVD. In the first part, the study was focused on the influence of epitaxial growth parameters, such as growth pressure and temperature, on the InAlN layer. It is found that the growth pressure and temperature affect the growth rate and Al incorporation efficiency in the InAlN material. The optimization of growth parameters were carried out to improve the electrical properties of the InAlN/AlN/GaN heterosturcture after the growth parameters were fixed, the device structure that affect the electrical properties, such as the InAlN barrier thickness, AlN spacer thickness and the interface between AlN spacer and GaN were discussed. These factors are key to achieve a good interface for the InAlN/AlN/GaN structure. By using pressure state time of 15 sec prior to the AlN spacer layer growth and a 1.5-nm thick spacer layer, HEMT structure with sheet resistance of 519 Ω/☐, carrier mobility of 890 cm2/V•s and sheet carrier density of 1.350×1013 cm−2 has been achieved.
Further, in order to improve the HEMT device breakdown voltage, the effect of the HT-AlN buffer layer thickness was also investigated. By increasing the thickness of AlN buffer layer, the leakage current caused by the out-diffusion of O atoms from the substrate can be effectively reduced. Furthermore, from the X-ray diffraction, surface etching pit density (EPD) method and Hall effect measurements, InAlN/AlN/GaN with better crystal quality and improved electrical properties was obtained when thicker AlN buffer layers were used. By using a 235-nm thick AlN buffer, the sheet resistance, carrier mobility, and sheet carrier density were further improved to 334 Ω/☐, 1190 cm2/V∙s and 1.53×1013 cm−2, respectively. The leakage current of the buffer was only 0.1mA/mm when measured at 200 V by using two-terminal method. Finally, HEMT devices with gate length of 1 μm and source-drain spacing of 7 μm were fabricated. The DC measurement showed that a maximum drain current of 890 mA/mm, a tranconductance of 202 mS/mm and an off-state breakdown voltage of more than 200V have been achieved.
|
|---|
