| Gallium nitride (GaN) and related materials have emerged as the next generation material for microwave power applications. Their wide energy bandgap allows transistors fabricated on them to deliver unprecedented levels of microwave power density. Potential applications include wireless communications, satellite communications, military and commercial radar systems, etc. As material growth technology progresses at a fast pace, a need arises for device design and processing technology that are targeted towards the aforementioned microwave applications.; This thesis reports the design, fabrication and characterization results of AlGaN/GaN based heterojunction field effect transistors (HFET's) with emphasis on microwave power applications. From understanding the novel polarization characteristics of the material to using such knowledge in device layer structure design, from developing the basic device fabrication technology to optimizing layout design for maximum power performance, this project serves as an important link between GaN material synthesis and GaN circuit realization. Through this research it was determined for 3-to-12 GHz power operation, the optimum device should have approximately 30% aluminum in a 300 Å thick AlGaN barrier structure, and a gate length between 0.3 μm and 0.5 μm. Maximum device size will be limited to around 1 mm, with eight gate fingers, each 125 μm long. Substrate of choice is silicon carbide because of better epitaxial quality as well as much higher thermal conductivity than sapphire. Demonstrated results include state-of-the-art frequency response for devices on both sapphire (f T = 67 GHz, fmax = 140 GHz) and silicon carbide (fT = 74 GHz) substrates. High power added efficiencies (over 70% at 3 GHz) and good output power (over 4 W total power from a 1-mm device at 4 GHz) were also measured.; Although there are still issues to be solved in AlGaN/GaN HFET technology (e.g. current dispersion), it is expected that the continued advancement of device design and processing technology, together with improved material growth, will ultimately enable GaN-based power amplifiers to deliver tens of watts of microwave power with over 50% efficiency. |
