Study of the effect of forward gate bias stress on the reliability of AlGaN/GaN HEMTs on SI

• Main Author: Gao, Yu
• Other Authors: Gan Chee Lip
• Format: Thesis-Doctor of Philosophy
• Language: English
• Published: Nanyang Technological University 2021
• Subjects: Engineering::Materials
• Online Access: https://hdl.handle.net/10356/152654
• Institution: Nanyang Technological University

AlGaN/GaN high electron mobility transistors (HEMTs) have shown promising capabilities for high frequency and high-power applications. As 5G technology and autonomous vehicles become more and more attractive, HEMTs will start to play an increasingly important role in the semiconductor industry. While GaN HEMTs are most commonly fabricated on SiC or sapphire substrates, HEMTs on Si is an attractive alternative due to lower processing costs and the ability for monolithic integration with Si-CMOS (Complementary Metal Oxide Semiconductor). However, the wide-scale adoption of GaN HEMTs on Si is still limited by reliability issues because of the greater difference in the coefficients of thermal expansion and higher dislocation densities compared conventional substrates. Previous studies of the reliability of GaN HEMTs on Si have mainly focused on degradation of the drain saturation current, while the origin of gate leakage current degradation has remained poorly understood.In this dissertation, degradation under forward gate bias stressing was investigated. After stressing, the gate leakage current increased dramatically without much degradation on the drain saturation current. As high leakage current could lead to high power consumption and high noise, it is important and necessary to understand why the leakage current increases during stressing.First, the current transport mechanisms of fresh devices were studied. Both electrical measurements and simulations were carried out to investigate the electron transport mechanisms, especially in the reverse bias region. It was found that the leakage current for fresh devices was mainly dominated by Fowler-Nordheim tunneling at room temperature.The devices were then subjected to forward gate bias stressing and the failure criterion was set as the time at which the leakage current reached 1 μA. The degradation was highly accelerated by the stressing voltage. The degradation mainly took place at the Schottky contact, with the barrier height reduced and ideality factor increased over the stressing time.By further analyzing transport mechanism for stressed devices, it was found that the leakage current was dominated by Poole-Frenkel emission. Finally, the degraded devices were examined using different material characterization techniques. The stressed devices showed a highly localized leakage path as detected from photon emission microscopy. Transmission electron microscopy was utilized to investigate structural changes at the high leakage path region. From electron energy loss spectroscopy analysis, a localized high carbon region was found in the gate metal. As carbon has a lower work function than the gate metal, and this is likely to cause of a reduction of the barrier height. The carbon is suspected to come from photoresist residue during the metal lift-off process. During electrical stressing, the electrical field is sufficiently high to trigger carbonization at the gate. Then the carbonized carbon residue lowers the barrier height leading to the high leakage path. Furthermore, as the tested devices are Ga-polar device, the carbon residues might also create CGa or CAl defects and enhance Poole-Frenkel emission after stressing. In summary, high leakage currents were observed after forward gate bias stressing. It was found that carbon residue from photoresist was the main reason leading to the degradation. Thus, this carbon residue is detrimental for device reliability. In order to improve device reliability, a clean surface between the gate metal and semiconductor is essential. This methods developed in this dissertation provide an integrated experimental approach to investigate AlGaN/GaN HEMT device reliability, especially for gate leakage failure. By combining electrical characterization, physical characterization and computer-aided simulation, this approach provides a comprehensive understanding of the failure mechanism for improved device reliability.