A semiconductor device includes a semiconductor substrate, a gate stack, an air spacer, a first spacer, a second spacer, a sacrificial layer, and a contact plug. The gate stack is on the semiconductor substrate. The air spacer is around the gate stack. The first spacer is around the air spacer. The second spacer is on the air spacer and the first spacer. The sacrificial layer is on the gate stack, and an etching selectivity between the second spacer and the sacrificial layer is in a range from about 10 to about 30. The contact plug lands on the second spacer and the gate stack.