| Microwave power transistors based on conventional semiconductors, such as Si and GaAs, have already approached their performance limits. With combined merits of high power and high speed, high electron mobility transistors (HEMTs) made of GaN and its alloys are promising for future needs of wireless communication systems. Although previous development efforts resulted in outstanding power transistors at lower bands of the microwave spectrum, some important issues remain for higher frequency devices. Firstly, the notorious high-field-assisted trapping, which causes dispersion between DC and RF currents, becomes more prominent due to restricted room for electric field shaping by conventional field plates. Secondly, the short-channel effect, especially at high drain biases, limits the effectiveness of gate length scaling hence the devices' high frequency performance. Finally, these short-gate-length devices have been plagued with increased parasitic gate leakage current, leading to unwanted power consumption and early device degradation.;This dissertation studies the aforementioned issues and provides solutions to them. Advanced device structures employing recessed V-gate and T-gate were developed to minimize the high-field-assisted trapping with the least compromise of devices' high-frequency performance. Engineering of the devices' buffer layers with the incorporation of an AlGaN buffer successfully improved the channel confinement and reduced the short-channel effect. Understanding and development of a surface plasma treatment technique effectively suppressed the parasitic gate leakage current without causing any adverse side effects. With these advances in epilayer structure, device design, and processing technologies, state-of-the-art power performance at X-band and promising results at Ka-band were demonstrated. |
