| New microwave transistor applications continue to present ever increasing demands for output power, and pseudomorphic High Electron Mobility Transistors (pHEMTs) are often the device of choice to fill these needs. The most advantageous way to improve output power is to increase the off-state breakdown voltage, Vbr. This is commonly accomplished through reducing the peak electric field in the transistor since gate leakage and impact ionization, the primary mechanisms behind breakdown, are caused by high electric field strengths. Field plate structures have become a commonly employed method to reduce the peak electric field in the gate region of the transistor, and this strategy has been followed in the fabrication of the devices studied here. Various measurements were conducted on these transistors to better understand the underlying physical mechanisms responsible for breakdown and to learn the breakdown and performance dependencies of the various transistor topologies. It was found that in the sample transistors, breakdown was due to a combination of gate leakage through thermionic field emission and impact ionization. It was also found that a major influence in the breakdown behavior was the quality of the nitride-semiconductor interface in the field plate region of the device. To study this region, a new analytical method utilizing existing current transient Deep Level Transient Spectroscopy (DLTS) was developed to determine defect characteristics and concentrations in the region of this interface. Additionally, computer modeling of the fabricated devices was performed to gain insight into the various performance trade-offs of changing the topology of a field plate transistor. Finally, three novel device concepts are proposed and modeled which promise to extend the maximum operating voltages of field effect transistors. |
