On-state reliability study of AlGaN/GaN high electron mobility transistor on silicon

• Main Author: Syaranamual, Govindo Joannesha
• Other Authors: Gan Chee Lip
• Format: Theses and Dissertations
• Language: English
• Published: 2018
• Subjects: DRNTU::Engineering::Materials::Microelectronics and semiconductor materials
• Online Access: http://hdl.handle.net/10356/74934
• Institution: Nanyang Technological University

Monolithic integration of AlGaN/GaN high electron mobility transistor (HEMT) into silicon (Si) platform is very attractive as this is a cost-effective solution to extend the capabilities of silicon technology especially for high power and high frequency applications. It is not only that Si substrate is cheaper than other commonly used substrates for GaN epitaxy such as Sapphire and SiC but growing on Si substrate also allows integration with current Si technology in the industry. Unfortunately, this technology is still very limited by its associated reliability issues. This study focused on depletion-mode (D-mode) AlGaN/GaN HEMT devices on Si with negative threshold voltage (VTH) of about −3 V. AlGaN/GaN HEMT reliability can be divided into OFF-state and ON-state reliabilities. AlGaN/GaN HEMT ON-state reliability is not as well reported as OFF-state reliability. Therefore, this thesis aims to fill up the knowledge gap in AlGaN/GaN HEMT ON-state reliability. This study has three main objectives. The first objective is to comprehend the electrical and physical degradation mechanism in AlGaN/GaN HEMT devices stressed under ON-state condition. The second objective is to understand the effects of stressing and process parameters on AlGaN/GaN HEMT ON-state degradation. Finally, this study aims to develop a reliability model based on the degradation mechanism in AlGaN/GaN HEMT stressed under ON-state condition. OFF-state and ON-state degradations were compared in Chapter 4. It was observed that devices stressed under ON-state condition degraded faster than similar devices stressed under OFF-state condition with higher stressing temperature. This is because of the occurrence of electro-chemical oxidation of AlGaN away from the gate edge during ON-state stressing. Dark features containing gallium, aluminum and oxygen were found at the AlGaN/SixN1-x interface away from the gate edge on the drain side of the ON-state-stressed devices. These oxidized portions of AlGaN were etched away during metallization and passivation layer etching leaving behind pits at the drain-gate access region. The total area of pits in the drain-gate access region correlated well with ID-max degradation. It was concluded that 2DEG electrons contributed to the AlGaN oxidation away from the gate edge during ON-state stressing. A qualitative model for ON-state degradation was proposed based on the electro-chemical oxidation mechanism. In Chapter 5, fast and slow degradation modes were observed for devices stressed under ON-state condition. These two degradation modes were characterized by the dominant source of oxygen. In fast degradation mode, the dominant source of oxygen was the pre-existing oxygen at the AlGaN/SixN1-x interface whereas the dominant source of oxygen in slow degradation mode was oxygen from the ambient which diffused through the SixN1-x passivation. In addition, it was established that the maximum distance for pit formation from the gate edge correlated with the stressing current density. In Chapter 6, effect of passivation density on AlGaN/GaN HEMT ON-state degradation was investigated. High passivation density will mitigate the passivation layer degradation and hinder oxygen diffusion from ambient to the AlGaN/SixN1-x interface. Once the passivation degrades, oxygen from the ambient will diffuse through and oxidize AlGaN layer. Mathematical relationship between stressing temperature and ID-max in slow mode degradation was derived based on oxygen diffusion through the SixN1-x passivation. In the final chapter, the thesis was summarized and its implications and limitations were discussed. Finally, possible future works were proposed to further advance the knowledge of AlGaN/GaN HEMT ON-state reliability.