Synergistic effects of heating and biasing of AlGaN/GaN high electron mobility transistors: An in-situ transmission electron microscopy study

High temperature adversely affects the reliability of AlGaN/GaN high electron mobility transistors (HEMTs). Degradation studies typically involve post-mortem visualization of the device cross-section to identify failure mechanisms. In this study, we present an in-situ technique by operating the transistor inside the transmission electron microscope (TEM) for real time observation of the defects and failure. A custom-made MEMS chip facilitates the simultaneous biasing and heating capability inside the TEM. The results indicate that the high temperature operation promotes nucleation of new defects in addition to the propagation of existing defects, which degrade the performance of the device even at low biasing conditions. The gate Schottky contact is found to be the most vulnerable region at elevated temperature. The diffusion of gate metals, especially the diffusion of Au at the metal-semiconductor interface initiates the gate degradation process, as confirmed by energy dispersive X-ray spectroscopy (EDS), followed by catastrophic failure with the increase of operation temperature and drain biasing voltage. The high-resolution TEM imaging along with geometric phase analysis reveals the evolution of defect clusters, such as dislocations networks, stacking faults, and amorphized regions, in the AlGaN and GaN layers, which increases the lattice strain leading to catastrophic failure at elevated temperature. The insights obtained from the in-situ study may be useful in improving high temperature HEMT reliability.