The disclosed technology relates generally to semiconductor devices and manufacturing methods thereof, and more particularly to field-effect transistors operating at different voltages and methods for integrating the same. In one aspect, a method of fabricating a semiconductor device comprises: a) providing a substrate and a first hardmask; b) next, providing a second hardmask over a first region of the first hardmask; c) next, forming a first set of hardmask fins in a second region of the first hardmask; d) next, masking the second region; e) next, providing a set of photoresist fins on the second hardmask; f) next, patterning the second hardmask and the first region by using the photoresist fins as a mask; g) next, forming a first set of semiconductor fins of a first height by etching the substrate; h) next, removing the mask provided in step d; i) next, forming a second set of semiconductor fins of a second height in the second region and extending the height of the first set of semiconductor fins to a third height in the first region, by etching the substrate by using the first and second sets of hardmask fins as masks.