A method includes forming an active fin using a hard mask as an etching mask, wherein the active fin comprises a source region, a drain region, and a channel region, the hard mask remains over the active fin after etching the semiconductive substrate, and the hard mask has a first portion vertically overlapping the source region of the active fin, a second portion vertically overlapping the channel region of the active fin, and a third portion vertically overlapping the drain region of the active fin. A sacrificial gate is formed over the second portion of the hard mask and the channel region of the active fin. The first and third portions of the hard mask are etched. After etching the first and third portions of the hard mask, a gate spacer is formed extending along sidewalls of the sacrificial gate, and the sacrificial gate is replaced with a replacement gate.