Al0.3Ga0.7N/GaN MODFETs grown on sapphire substrates by RF-assisted MBE demonstrate excellent DC as well as high-frequency characteristics. Power characteristics of these transistors measured on-wafer at 8GHz showed excellent correlation with the biasing voltages and currents. Since the use of higher drain bias generally leads to higher output power and the maximum drain bias is limited by the drain-source and/or drain-gate breakdowns these need to be evaluated in order to investigate to what extent they limit the power performance of GaN-based MODFETs. The breakdown characteristics of AlGaN/GaN MODFETs were studied by employing the current-injection technique and the gate-drain and drain-source breakdown voltages were obtained as a function of the drain current and the substrate temperature. At room temperature devices demonstrated drain-source and gate-drain breakdown voltages of 70 and 80V, respectively. Based on the gate bias and temperature dependence of the breakdown voltages, the source of breakdown is attributed to the thermionic emission in the gate-drain diode.
