A methodology based on the transistor body effect is used to monitor inversion oxide thicknesses (T inv 's) in high-κ/metal-gate undoped ultrathin-body short-channel SOI FINFETs. The extracted T inv 's are benchmarked to independent capacitance-voltage (C-V ) measurements. For the first time, device simulation is introduced to understand the fundamental difference in T inv values extracted using the two techniques, which is driven by the inversion charge centroid at different bias conditions. Index Terms-Fully depleted SOI (FDSOI), FINFETs, inversion oxide thickness, ultrathin body.