Imaging Electronic Degradation in a Stressed GaN HEMT

The standard tool for high-resolution imaging of microelectronics is the transmission electron microscope (TEM), which can resolve atomic-scale features in thin cross-section samples extracted from (pristine or stressed) components. However, cross-sectioning typically leaves such a sample “dead” electronically. Even for an electronically “viable” sample, TEM is insensitive to a device's electronic structure and may be blind to the dynamics responsible for a its function and failure. Here we discuss the preparation, and in situ stressing, of a cross-section extracted from a GaN high electron-mobility transistor (HEMT) using a plasma-focused ion beam (PFIB). Scanning TEM electron beam-induced current (STEM EBIC) imaging is used to confirm the electronic viability of the sample and to reveal bias-induced electronic changes under normal operating values applied in situ. Under in situ stressing, subtle electronic defects appear in STEM EBIC prior to any physical signs of stress. After stressing to the point of failure, indicated by a sudden decrease gate-source resistance, defects form which generate intense STEM EBIC contrast under small bias, but no evidence of the defects are seen with standard TEM-based imaging.