Various embodiments disclose a method for fabricating a semiconductor structure. In one embodiment, the method includes forming a masking layer over at least a first portion of a source contact layer formed on a substrate. At least a second portion of the source contact layer is recessed below the first portion of the source contact layer. The mask layer is removed and a first spacer layer, a replacement gate on the first spacer layer, a second spacer layer on the replacement gate, and an insulating layer on the second spacer layer are formed. First and second trenches are then formed. A first channel layer is epitaxially grown within the first trench. A second channel layer is epitaxially grown within the second trench. A length of the second channel layer is greater than a length of the first channel layer.