# Temperature-Dependent Dynamic on Resistance in Gamma-Irradiated AlGaN/GaN Power HEMTs
## 3 Results and Discussion
### Dynamic R\({}_{\text{ON}}\)  
The results of the dynamic on-resistance tests are shown in Figure 9. After a stress of V\({}_{\text{DS}}\) = 500 V was applied to the GaN HEMTs in the off state, two pulses were applied (Figure 4) using a double pulse test (DPT) circuit to measure the dynamic on-resistance. Figure 9: The dynamic R\({}_{\text{ON}}\) double pulse measurements in the off state (V\({}_{\text{DS}}\) = 500 V and V\({}_{\text{GS}}\) = \(-\)5 V) and on state (V\({}_{\text{GS}}\) = 3 V and I\({}_{\text{DS}}\) = 2.5 A): (**a**) device G2, which was subjected to drain and gate voltage stress during irradiation at 133 \({}^{\circ}\)C; (**b**) device G3, which was subjected to drain voltage stress during irradiation at 133 \({}^{\circ}\)C; (**c**) device G7, which was subjected to drain and gate voltage stress during irradiation at 53 \({}^{\circ}\)C; (**d**) device G9, which was subjected to drain voltage stress during irradiation at 53 \({}^{\circ}\)C.  
In all of the devices that were subjected to gamma radiation at the higher temperature (133 \({}^{\circ}\)C), regardless of the voltage stress that was applied (Table 1), a degradation of the dynamic R\({}_{\text{ON}}\) was observed, which increased with the total ionising dose and was not recoverable after the two-step annealing process (Figure 9a,b). For the devices that were irradiated at the lower temperature (53 \({}^{\circ}\)C), the dynamic on-resistance increased with the accumulated dose until it reached step 4 (184 \(\text{krad}\)(SiO\({}_{2}\))) when the R\({}_{\text{ON}}\) started to reduce in value and it only increased after the 100 \({}^{\circ}\)C annealing process, which was permanent (Figure 9c,d). In Figure 9, increases in dynamic R\({}_{\text{ON}}\) can be observed that were higher in the second pulse than in the first. This was evidence of a trapping process, which was induced by hot electrons during the semi-on state of the switching events. Under hard-switching conditions, the GaN HEMTs were simultaneously subjected to high drain voltages and a rising drain current. The electrons that were injected from the source to the drain were accelerated by the high electric field (usually called \"hot electrons\"). In order to quantify the evolution of dynamic R\({}_{\text{ON}}\) during the irradiation process, we measured the average value of R\({}_{\text{ON}}\) during an interval of 20 us at the end of the second pulse. Figure 10 shows the evolution of the dynamic R\({}_{\text{ON}}\) values for all of the devices during all of the radiation steps throughout the test campaign. All of the devices that were irradiated at 133 \({}^{\circ}\)C suffered from an increase in R\({}_{\text{ON}}\) that reached saturation at 99 \(\text{krad}\)(SiO\({}_{2}\)) or 132 \(\text{krad}\)(SiO\({}_{2}\)), which was indicative of the saturation of the electric charge traps in the devices that, in turn, limited the total ionising dose (TID) effects. However, in the devices that were irradiated at the lower temperature (53 \({}^{\circ}\)C), the resistance increased with the total accumulated dose and become saturated close to 99 \(\text{krad}\)(SiO\({}_{2}\)) and began to decrease in the range of 132-184 \(\text{krad}\)(SiO\({}_{2}\)). Finally, in the annealing step at 100 \({}^{\circ}\)C, a permanent degradation was observed. It was also mandatory to investigate the behaviour of dynamic R\({}_{\text{ON}}\) when multiple pulses were applied. The multi-pulse test (MPT) was conducted using the circuit setup that was shown in Figure 3, but with a driving signal test sequence of multiple pulses, a stress time of 500 ms, and an on-time of 6 us. The multi-pulse sequence finished once the stationary value of R\({}_{\text{ON}}\) was found, using a mathematical function in the digital oscilloscope that was used to record the drain current and drain voltage waveforms [15]. The results are shown in Figures 11 and 12.