## 11 Reliability of GaN-Based Devices  
Using GaN HEMT has the special advantage of better performance under extreme temperatures, as what is reported for the short time operation at 1000 \({}^{\circ}\)C in vacuum; also, high radiation hardness makes it a reliable candidate for aerospace and military applications [1; 64; 141; 142; 143]. Despite excellent features reported for GaN-based HEMTs, some issues still limit their performance, such as dispersion effects, trapping effect, parasitic effects, and reliability for long-term stability [144]. Failure mechanism identified on GaN HEMTs can be classified into thermally-activated mechanisms, mechanisms related to the presence of hot electrons, and GaN devices' specific mechanisms due to its polar and piezoelectric nature [145]. Temperature as an accelerating factor for passivation stability results in gate-metal interconnect degradation, feed metal interconnect degradation, ohmic contact degradation and thermally-induced delamination of passivation [144]. The presence of hot electrons accelerated by the high longitudinal electric field as a common degradation factor in all high-voltage FETs and the dominant failure mechanism in the GaN HEMTs can lead to electron trapping in SiN, crystallographic damage and hot electrons trap generation [144; 145; 146; 147; 148; 149]. Figure 8: Comparison of converter efficiency vs. frequency for CoolMOS and GaN technologies for the 2.5 kW-48 V LLC converter. It has been demonstrated that parasitic effects leading to current collapse can be reduced by passivating surface states using deposited grown SiN and controlling the electric field with field-plates [150; 151]. For commercially available GaN devices at 200 \({}^{\circ}\)C channel temperature, the Median Time To Failure (MTTF) is reported in order of \(10^{7}\), which is an excellent value [152]. Moreover, many different studies on the short circuit characterization of GaN HEMTs have been performed in [153; 154; 155; 156], showing the results of finite element simulations and experimental tests in different conditions. In [153], short circuit test on 650 V GaN HEMTs is conducted when the drain voltage is greater than 350 V. Simulation results showed that in these test conditions, a very high-power density was dissipated in a critical region of the device and it was intensified by significant current focalization, as proved by experimental observations. It has been demonstrated that the temperature in the critical region can reach the maximum allowable temperature for a GaN/AlGaN structure and bring the device to failure. Further details are provided in [154; 155], where two kinds of failure are described for 650 V GaN HEMTs non-destructive short circuit tests. The first one involves a large amount of energy causing the increase of the temperature, which exceeds the melting temperature of the metallization of drain contacts. The second failure mechanism happens at higher drain voltages, but the damaged area is smaller and it is located below the source field plate at the gate edge on the drain side. An analysis of the influence of environmental adaptability on key characteristics of GaN HEMTs has been conducted in [157] by temperature cycling, high temperature storage, and steady-state operation life tests. The drain current of all the GaN HEMTs has risen after the tests, possibly due to the increasing concentration of 2-D electron gas in the channel. Before and after the tests, the threshold voltage for GaN HEMTs had decreased and there was no obvious trend in the gate-to-source leakage current variation for these devices. In assessing the reliability of AlGaN/GaN HEMTs for high voltage applications, the reverse-bias voltage is introduced as the key factor based on High Temperature Reverse Bias (HTRB) stress [158].