First-Principles Study of Radiation-Induced Electron-Hole Pair Generation and Transport in GaN and Al 0.25 Ga 0.75 N/GaN HEMTs

We present a first-principles study of the generation of radiation-induced electron-hole pairs in GaN. To generate the spatial and energetic distributions of the excited primary pairs, we used the energy-loss function calculated using time-dependent density functional theory. We simulated the thermalization of these charge carriers in a full-band Monte Carlo code with rigorous handling of plasmon emission, impact ionization, and phonon scattering. We find that, following relaxation, the total number of thermal pairs in the simulation is effectively equivalent to that predicted using the measured average electron-hole pair creation energy of 8.9 eV. This result suggests that the primary-pair energetic distribution predicted using the energy-loss function is a realistic representation of the true distribution of carriers generated by a 1-MeV proton in GaN. We also fitted empirical activation energies, cross sections, and densities for the activation of defect-precursors at the AlGaN/GaN interface of a Al0.25Ga0.75N/GaN high electron mobility transistor (HEMT). Finally, we simulated a proton-induced ionized particle track of electrons and holes in this HEMT. We calculated the density of defects activated by these carriers at the AlGaN/GaN interface and their effect on the threshold voltage(Vth). When scaled to fluences typical of device testing or space applications, the calculated density of defects activated by proton irradiation is similar to that observed in experiments. We analyze the temporal evolution of the excited carriers. For a device in the off state, we observe in the simulation the temporary formation of the well-known parasitic bipolar junction transistor. The HEMT recovers and returns to a steady state after ∼ 80 ps. This timescale is found to be in reasonable agreement with measured single event transient pulse widths.