Device reliability is a diverse topic, and influenced by a variety of factors. Device reproducibility which relies heavily on the specific device process, but also depends on local material quality is commonly focused on by industry. Device performance changes under operation and/or under specific environments, such as elevated temperatures, radiation or chemistry (often water-related) may have to be taken into account as well. The investigation of device performance changes can profit from accompanying TEM analysis, but only if the necessary TEM sample preparation and the imaging / spectroscopy technique do not produce their own artifacts. Examples of a device structure characterization include inter-diffusion phenomena at interfaces, diffusion of metal atoms from the device contacts, oxidation / corrosion effects and the study of extended defects as well as their impact on diffusion. AlGaN/GaN:Si HEMTs and GaN-based vertical power device structures are currently studied. This presentation will demonstrate how advanced TEM characterization can significantly improve device performance and reliability, which is enabled through advanced TEM sample preparation and electron dose control during imaging, which is particularly important at locally defective device areas.