There is an increasing interest in AlGaN/GaN high electron mobility transistors (HEMTs) due to their great potential for high performance at microwave frequencies. However, the performance of these devices is often limited by material reliability issues. Unfortunately, a detailed physical understanding of the degradation mechanisms is still lacking. The objective of this project is to develop that understanding through appropriate testing and failure analysis, so that test methods and models can be developed that will lead to further improvement in the reliability and electrical performance of these devices though optimization their design.

Figure 1: Pits and particles observed at the gate edges of a stressed AlGaN HEMT (top view) ^[1] .

Recent work has focused on the formation of pits at the edge of the gate contact during electrical stressing and performance degradation ^{[1] [2] [3]} . These pits have been observed to form under a variety of stressing conditions and in a range of temperatures.  We have found that in some cases the pits are associated with formation of particles that appear to be an oxide of Ga (Figure 1), and that pit and particle formation is suppressed when samples are properly passivated or when they are stressed in ultra-high vacuum conditions. Also, stressing in the presence of water vapor was found to enhance the rate of degradation ^[1] .  This suggests that this failure mechanism is associated with electrochemically-enhanced oxidation.  We have also observed that the rate of pit formation is affected by temperature, both in isothermal experiments and in experiments in which the temperature within an individual device varies significantly.  This finding indicates that this failure process is thermally activated. We estimate an activation energy of about 0.3eV ^[2] .

Analytical techniques such as scanning electron microscopy (SEM), transmission electron microscopy (TEM), atomic force microscopy (AFM), cathode luminescence (CL), and energy-dispersive X-ray spectroscopy (EDX) will be employed in future studies of this and other degradation processes, with the goal of developing predictive models for failure rates and reliability.

1. P. Makaram, J. Joh, J. A. del Alamo, T. Palacios, and C. V. Thompson, “Evolution of structural defects associated with electrical degradation in AlGaN/GaN high electron mobility transistors,” Appl. Phys. Lett,  Vol. 96, p. 233509, 2010.
2. F. Gao, B. Lu, L. Li, S. Kaun, J. S. Speck, C. V. Thompson, and T. Palacios, “Role of oxygen in the OFF-state degradation of AlGaN/GaN high electron mobility transistors,” Appl. Phys. Lett., vol. 99, p. 223506, 2011.
3. L. Li, J. Joh, J. A. del. Alamo, and C. V. Thompson,“Spatial distribution of structural degradation under high-power stress in AlGaN/GaN HEMTs,” Appl. Phys. Lett., to be published.

Thanks to their excellent electrical performance, AlGaN/GaN high electron mobility transistors (HEMTs) are considered ideal devices for the next generation of high-power and high-frequency electronics ^[1] . However, the limited understanding of their long-term reliability and degradation mechanisms is slowing down the insertion of these devices in actual systems ^[2] .

Recently, we have reported the formation of oxide particles next to the gate edge of GaN HEMTs after OFF-state step-stress degradation experiments ^[3] . Underneath these particles, pits similar to the ones reported in previous papers ^[4] are observed. In this work, we investigate the role of oxygen in the formation of these particles/pits during OFF-state stress and use oxygen diffusion barriers to improve the reliability of AlGaN/GaN HEMTs.

Two different dielectrics have been used: Al₂O₃ deposited by atomic layer deposition and in-situ SiN_x deposited immediately after the growth of the AlGaN/GaN epitaxial layer by metal organic chemical vapor deposition (MOCVD). Step-stress degradation experiments were performed in both samples in air and vacuum. No degradation was found in either sample during the experiments in vacuum, which shows that the oxygen necessary for the particle formation probably comes from air. In contrast, when the samples were stressed in air, a large degradation and particle/pit formation were found in the Al₂O₃-passivated sample, while no structural or electrical degradation was found in the sample with in-situ SiN_x (see Figures 1 and 2). The in-situ SiN_x dielectric is believed to be a much better diffusion barrier for oxygen gas and water vapor than Al₂O₃, which significantly reduces device degradation.

In summary, the in-situ deposition of a SiN_x gate dielectric and passivation layer successfully eliminated the diffusion of oxygen from air and water vapor to the AlGaN surface, which improved the OFF-state reliability of AlGaN/GaN HEMTs.

1. U. K. Mishra, L. Shen, T. E. Kazior, and Y-F Wu, Proceedings of the IEEE, 2008, vol. 96, p. 287.
2. G. Meneghesso, G. Verzellesi, F. Danesin, F. Rampazzo, F. Zanon, A. Tazzoli, M. Meneghini, and E. Zanoni, IEEE Trans. Device Mater. Reliab. vol. 8, p. 332, 2008.
3. F. Gao, B. Lu, L. Li, S. Kaun, J. S. Speck, C. V. Thompson, and T. Palacios, Appl. Phys. Lett. vol. 99, p. 223506, 2011.
4. J. Joh and J. A. del Alamo, IEEE Electron Device Lett. vol. 29, p. 287, 2008.