Synergistic effects of ionization and displacement damage in GaN HEMTs

The effect of ionization damage on displacement damage in AlGaN/GaN high electron mobility transistors (HEMTs) is investigated via sequential proton and electron irradiation experiments. Electrical degradation is characterized following 170-keV proton irradiation and subsequent 1-MeV electron irradiation. A positive threshold voltage shift is observed after proton irradiation at a fluence of 2 × 10 13 cm −2 , while subsequent electron irradiation causes no further change in the threshold voltage, indicating that proton-induced displacement defects are highly stable under electron irradiation. The damage-enhancement effect previously reported for simultaneous proton and electron irradiation is not observed under sequential irradiation. Density functional theory simulations reveal that radiation defects form more readily through atomic collisions when GaN is in an unsteady state. Deep-level transient spectroscopy (DLTS) results show that a defect with an energy level of E C – 0.9 eV, associated with displacement damage, appears after both proton-only and simultaneous proton–electron irradiation. Simultaneous irradiation produces more displacement defects than proton irradiation alone. The ionization-induced reduction in the displacement threshold energy may be one reason for the synergistic effect observed in AlGaN/GaN HEMTs.