A semiconductor device includes a semiconductor fin, a first silicon nitride based layer, a lining oxide layer, a second silicon nitride based layer and a gate oxide layer. The semiconductor fin has a top surface, a first side surface adjacent to the top surface, and a second side surface which is disposed under and adjacent to the first side surface. The first silicon nitride based layer peripherally encloses the second side surface of the semiconductor fin. The lining oxide layer is disposed conformal to the first silicon nitride based layer. The second silicon nitride based layer is disposed conformal to the lining oxide layer. The gate oxide layer is disposed conformal to the top surface and the first side surface of the semiconductor fin.