# Study of Self-Heating and High-Power Microwave Effects for Enhancement-Mode p-Gate GaN HEMT
## 3 Results and Discussion
### Thermal Characteristics
#### 3.2.2 HPM Effect
Figure 10: (a) Variation of _Tmax_ (maximum temperature, red line, left Y-axis) and _Tave_ (average temperature, blue line, right Y-axis) over time inside the device; (b) _Tmax_ (red line, left Y-axis) and HPM signal (grey line, right Y-axis) change trend within 10 ns. The temperature of the device at the end of the first temperature cycle (point C) is higher than the initial time, indicating that the heat has achieved a net accumulation with the action of the HPM signal. From the end of the first clock cycle to the beginning of the second clock cycle, the device temperature keeps dropping until the HPM signal reaches a certain amplitude (point D), that is, the second threshold temperature rise point, the heat can achieve a net accumulation. Figure 11 shows the thermal distribution under HPM injection which behaves slightly different from that in self-heating mode. It can be seen that the center of the thermal source is located on both sides of the gate close to the source and drain. Taking the source as an example to analyze the detailed damage process due to the symmetry, Figure 12 depicts the distribution of electrons near the damage zone. With the injection of the HPM signal, there is a huge horizontal potential difference between the gate and side electrode which results in a region of maximum electric field strength. In the vertical direction, the free electrons in the barrier layer enter the quantum well of the two-dimensional electron gas layer, leaving a large amount of ionized central charges. The very thin barrier layer also promotes the formation of maximum field strength. The intersection of the maximum of the horizontal field strength and the maximum of the vertical field strength constructs a potential breakdown zone. When the breakdown occurs, a large number of electrons pass directly through the p-GaN side zone which contributes a great current. Joule heat is generated sharply in the area where the breakdown current flows. Figure 11: The internal thermal distribution of the device under HPM effect. Figure 12: Thermal damage mechanism description of the hot spot. Figure 13 shows the electron current distribution near the source at the different moments extracted from the simulation software which fit the above hypothesis well. Before the breakdown, electrons are concentrated in the two-dimensional electron gas layer, where the temperature gradually rises. With the accumulation of heat, the internal energy of electrons and atoms continues to increase, and the probability of breakdown occurring increases with time, and finally reaches the breakdown threshold, forming a large breakdown current across the channel, AlGaN-barrier, and p-GaN region. As shown in Figure 13b, the breakdown zone shares most of the current in the channel region under the gate and makes the channel almost off. The reaction of the breakdown current and resistance in this area generates a large amount of Joule heat, which causes the temperature of the HEMT device to increase until it burns out. To verify the above explanation further, the experiment was performed by directly injecting high-power microwaves into a low-noise amplifier (LNA) containing a pHEMT device [36]. The injected HPM signal is a continuous wave with high-density power, generated by a synthesized sweeper (HP 83630 A) modulated by a pulse generator (HP 81130 A). In the experiment, it is difficult to extract the internal temperature distribution of the device, and the LNA noise figure deterioration of 20 dB is defined as the damage threshold [41]. We observed the internal features with a scanning electron microscope. From the results of the scanning electron microscope, as shown in Figure 14, it can be seen that obvious burnout occurred in the area of the gate close to the source and drain. There are multiple melting pits in this area, which are caused by the high current density. Breakdown makes the device partially melt, and the damage position is verified by the previous analysis of this paper.