Disclosed are methods of forming a semiconductor structure including a bulk semiconductor substrate and, on the substrate, a fin-type field effect transistor (FINFET) with a uniform channel length and a below-channel buried insulator. In the methods, a semiconductor fin is formed with a sacrificial semiconductor layer between lower and upper semiconductor layers. During processing, the sacrificial semiconductor layer is replaced with dielectric spacer material (i.e., a buried insulator). The buried insulator functions as an etch stop layer when etching source/drain recesses, ensuring that they have vertical sidewalls and, thereby ensuring that the channel region has a uniform length. The buried insulator also provides isolation between channel region and the substrate below and prevents dopant diffusion into the channel region from a punch-through stopper (if present). Optionally, the buried insulator is formed so as to contain an air-gap. Also disclosed are structures resulting from the methods.