An overview is presented of displacement damage (DD) effects, total-ionizing-dose (TID) effects, and single-event effects in AlGaN/GaN high electron mobility transistors (HEMTs). High-fluence proton-induced DD creates point defects and impurity complexes at fluences that are comparable to or higher than those encountered in space applications. Defect and impurity dehydrogenation also contributes significantly to the DD/TID response at fluences typical of realistic space environments. The bias applied during irradiation can affect the DD/TID response strongly. Bias stress before irradiation can lead to enhanced proton-induced degradation of AlGaN/GaN HEMTs. Low-frequency noise measurements and density functional calculations provide insight into defect microstructures and energy levels. GaN-based HEMTs can be quite vulnerable to single-event effects in space. Of particular concern is single-event burnout (SEB). The vulnerabilities of GaN-based devices to SEB at voltages below rated limits and significant device-to-device variations in SEB response lead to significant voltage derating for GaN-based power devices in space systems. Developing an improved understanding of the effects of defects and hydrogen on the radiation response of AlGaN/GaN HEMTs can improve the DD/TID response by reducing threshold-voltage shifts and transconductance degradation. Reducing defect densities may also reduce the variation in SEB response, enabling reliable device operation at higher voltages in future space systems.

The degradation induced by ultra-high total ionizing dose in 65 nm MOS transistors is strongly gate-length dependent. The current drive decreases during irradiation, and the threshold voltage often shifts significantly during irradiation and/or high-temperature annealing, depending on transistor polarity, applied field, and irradiation/annealing temperature. Ionization in the spacer oxide and overlying silicon nitride layers above the LDD extensions leads to charge buildup as well as the ionization and/or release of hydrogen. Charge trapped in the spacer oxide or at its interface modifies the parasitic series resistance, reducing the drive current. The released hydrogen transports as H+ with an activation energy of ~ 0.92 eV. If the direction of the electric field is suitable, the H+ can reach the gate oxide interface and de-passivate Si-H bonds, leading to threshold voltage shifts. Newly created interface traps are most prominent near the source or drain. The resulting transistor responses and defect-energy distributions often vary strongly in space and energy as a result, as demonstrated through current-voltage, chargepumping, and low-frequency noise measurements.

Gate and drain bias dependences of hot carrier degradation are evaluated for AlGaN/GaN HEMTs fabricated via two different process methods. Both positive and negative threshold voltage Vth shifts are observed for each device type, depending on the mode and duration of the stress, indicating the presence of significant densities of donor-like and acceptor-like traps. Worst-case stress bias for transconductance degradation is the “ON” state for both device types. We find that transconductance degradation provides a more effective parameter to monitor defect buildup than Vth shifts, and that a single worst-case stressing bias condition cannot be defined for all varieties of AlGaN/GaN HEMTs. Low-frequency noise measurements versus temperature assist the identification of defects responsi-ble for the observed degradation. Defect dehydrogenation and oxygen impurity centers are found to be particularly significant to the response of these devices.

Trapping characteristics of MOS structures with β-Ga2O3 substrates and Al2O3 gate dielectrics are evaluated via constant-voltage stress and X-ray irradiation. Traps that affect bias-induced charging are located primarily in the Al2O3 dielectric layer, and are distributed broadly in time and/or energy. Stress induced flatband voltage shifts are reduced by N2 annealing. Trap assisted tunneling is shown to be responsible for the observed gate leakage. Hole trapping in the Al2O3 dielectric layer dominates device radiation response. The relatively modest radiation-induced charge trapping observed in these devices is promising for the potential future use of Al2O3/β-Ga2O3 devices in a space environment.

Low-energy ion-induced breakdown and single-event burnout are experimentally observed in β-Ga ₂ O ₃ Schottky diodes with voltages well below those of expected electrical breakdown. Fundamentally different responses were observed among alpha-particle, Cf-252, and heavy-ion irradiation. TCAD simulations suggest that ion-induced burnout can be triggered at high voltages as a result of a single ion strike, leading to impact ionization, voltage-induced charge separation accentuated by the low intrinsic hole mobility in β-Ga ₂ O ₃ , and breakdown. At significantly lower voltages, the cumulative buildup of displacement-damage-induced defects in β-Ga ₂ O ₃ during high-fluence ion irradiation can lead to defect-driven breakdown due to the generation and motion of negatively charged Ga vacancies and O interstitials. First-principles calculations show that defect clusters can be formed that are much less resistive than the surrounding material. These clusters can be driven deeply into the device by the reverse bias, ultimately forming conduction paths that can facilitate the destruction of the device at reduced voltages.

The $1{T}$ polytype of TaS₂ exhibits voltage-triggered threshold switching as a result of a phase transition from nearly commensurate to incommensurate charge density wave states. Threshold switching, persistent above room temperature, can be utilized in a variety of electronic devices, e.g., voltage controlled oscillators. We evaluated the total-ionizing-dose response of thin film $1{T}$ -TaS₂ at doses up to 1 Mrad (SiO₂). The threshold voltage changed by less than 2% after irradiation, with persistent self-sustained oscillations observed through the full irradiation sequence. The radiation hardness is attributed to the high intrinsic carrier concentration of $1{T}$ -TaS₂ in both of the phases that lead to threshold switching. These results suggest that charge density wave devices, implemented with thin films of $1{T}$ -TaS₂, are promising for applications in high radiation environments.