Microwave power transistors made of conventional semiconductors have already approached their performance limit. In order to meet the future needs of wireless communication systems, research efforts are being putting on wide bandgap semiconductors such as SiC and GaN. With combined merits of high power and high speed, high electron mobility transistors (HEMTs) made of AlGaN-GaN materials are the subject of this thesis....[ Read more ]

Microwave power transistors made of conventional semiconductors have already approached their performance limit. In order to meet the future needs of wireless communication systems, research efforts are being putting on wide bandgap semiconductors such as SiC and GaN. With combined merits of high power and high speed, high electron mobility transistors (HEMTs) made of AlGaN-GaN materials are the subject of this thesis.

Electronic properties of AlGaN/GaN epilayers were characterized to assess the suitability for HEMT fabrication. Device processing technologies of the baseline AlGaN-GaN single-channel HEMTs were established in HKUST. HEMTs fabricated with those epilayers and processing technologies were subjected to extensive testing, showing satisfactory DC and RF performance.

Though the AlGaN/GaN HEMTs possess superior performance in many aspects, those devices are usually plagued with current collapse and instability problem, which greatly limits the power performance. Possible reason leading to current collapse is the large amount of defect-related trap states in the currently imperfect AlGaN/GaN materials. Trap states in baseline AlGaN-GaN single-channel HEMTs were characterized and analyzed. Correlation was found between the surface trap states and the current collapse behaviors.

A novel AlGaN-GaN double-channel HEMT design was developed to enhance the device performance and to study the operation mechanism of GaN-based HEMTs. Benefiting from the polarization effect of nitride semiconductors, double-channel HEMTs with optimized structure design exhibit favorable device performance. It was found that the double-channel HEMTs have alleviated current collapse problem, and additional degrees of freedom for linearity engineering.