Physical Failure Analysis of Stressed and Proton-irradiated RF AlGaN-GaN HEMTs with Ni-Pd-Au Schottky Contacts

GaN power amplifiers based on AlGaN-GaN high electron mobility transistor (HEMT) technology are produced for defense applications that require high voltage, high power, and high efficiency operation. Many groups have reported various failure mechanisms in GaN HEMTs over the last two decades. However, the fact that no dominant failure mode and mechanism has been identified is a major concern for space applications. Earlier our group reported radiation effects of commercial high-power GaN HEMTs exposed to protons and heavy ions as well as failure mechanisms in RF GaN HEMTs fabricated per our design. For the present study, we continued to investigate RF AlGaN-GaN HEMTs grown on SiC substrate with Ni-Pd-Au Schottky contacts. Our GaN HEMTs had a Schottky gate length of 250 nm, a total gate width of 6 x 150.m periphery, and a field plate. Our HEMTs were irradiated with 50 MeV protons at the Lawrence Berkeley National Laboratory, with three different doses of 42, 160, and 889 krad(Si) for this experiment. All our irradiated devices were exposed to protons while they were shorted. First, pre- and post-irradiation DC I-V characteristics were compared, and DC bias- temperature stress tests were performed on unirradiated HEMTs. Some HEMTs were thermally stressed as monitor samples for comparison. Second, we employed focused ion beam (FIB) techniques to prepare TEM specimens from degraded devices for defect and chemical analysis using a high resolution TEM. AFM techniques were also employed to study gate pitting.