| Gallium nitride (GaN) has attracted a lot of attention as the next generation of semiconductor material for microwave power application. The unique and superior material properties of GaN and its heterostructure, such as excellent transport property, high breakdown voltage and sheet carrier concentration, and thermal and mechanical stabilities, enable AlGaN/GaN heterostructure field effect transistors (HFETs) to deliver unprecedented levels of microwave power performance. Potential applications include ultra-wide bandwidth communications and radar systems, wireless base stations, and communications satellites.; Tremendous efforts to realize the potential of Al-GaN/GaN HFETs have been made over the last decade focusing on improving microwave power performance via optimizing material growth and semiconductor processing technologies. As the device performance is getting mature, the device's reliability becomes a major concern for manufacturability of commercially available AlGaN/GaN HFETs. However, comprehensive study on the reliability of these devices is still lacking.; This dissertation describes the fabrication, performance and degradation characteristics and mechanism of AlGaN/GaN HFETs. The devices were fabricated with alloyed Ti/Al/Ti/Au ohmic contact and Ni/Au mushroom gate contact using E-beam lithography. The device's microwave performance was significantly improved after SiN passivation due to reduced surface effects. Several degradation modes, primarily a decrease of the output current and microwave output power density, were observed under various electrical stress tests including high current stress, high field stress, and RF overdrive. To further investigate the physical mechanism of observed degradations, SiN passivation, pulsed IV (gate lag), low frequency noise measurements, deep level transient spectroscopy (DLTS), and scanning kelvin probe microscopy (SKPM) have all been employed with hot electron stress testing. The results clearly demonstrated that charge accumulation and trap creation at the semiconductor surface and interface induced by hot electron effects are responsible for observed degradation. |
