Transmission electron microscopy (TEM) provides sub-angstrom resolution imaging of semiconductor device cross-sections, enabling atomic-level characterization of transistor structures, interfaces, and defects. Operating principle: high-energy electron beam (80-300kV) transmitted through ultra-thin specimen (<100nm), forming images from transmitted and diffracted electrons. Resolution: <0.1nm (sub-angstrom) for aberration-corrected STEM—can resolve individual atomic columns. TEM modes: (1) Conventional TEM (CTEM)—parallel beam illumination, bright/dark field imaging, diffraction patterns; (2) Scanning TEM (STEM)—focused probe scanned across sample, HAADF detector provides Z-contrast (heavier atoms brighter); (3) HR-TEM—high resolution lattice imaging showing crystal structure. Analytical techniques: (1) EDS (Energy Dispersive X-ray Spectroscopy)—elemental composition mapping at nm resolution; (2) EELS (Electron Energy Loss Spectroscopy)—chemical bonding, oxidation state, electronic structure; (3) 4D-STEM—diffraction pattern at each probe position for strain mapping. Sample preparation: FIB lift-out is standard—extract site-specific lamella, thin to <50nm with final low-kV polish to minimize damage. Semiconductor applications: (1) Gate stack analysis—measure high-κ thickness, interface layer, metal gate work function layers; (2) Fin/nanosheet profiling—channel dimensions, shape, crystal quality; (3) Contact/via analysis—barrier conformality, fill quality, voiding; (4) Defect identification—dislocations, stacking faults, precipitates, contamination; (5) Epitaxy quality—SiGe composition, interface abruptness. Limitations: destructive (sample consumed), time-consuming preparation, small field of view. TEM is the ultimate characterization tool for semiconductor process development and failure analysis at the atomic scale.