A method for accurately electrically measuring a width of a fin of a FinFET, using a semiconductor fin quantum well structure is provided. The semiconductor fin quantum well structure includes a semiconductor substrate and at least one semiconductor fin coupled to the substrate. Each of the semiconductor fin is sandwiched by an electrical isolation layer from a top and a first side and a second side across from the first side, to create a semiconductor fin quantum well. At least one gate material is provided on each side of the electrical isolation layer. A dielectric layer is provided over the top of the electrical isolation layer to further increase the electrical isolation between the gate materials. The width of the semiconductor fin is measured accurately by applying a resonant bias voltage across the fin by applying voltage on the gate materials from either side. The peak tunneling current generated by the applied resonant bias voltage is used to measure width of the fin.