A method for fabricating a semiconductor device is disclosed. A dummy gate is formed on a semiconductor substrate. The dummy gate has a first sidewall and a second sidewall opposite to the first sidewall. A low-k dielectric layer is formed on the first sidewall of the dummy gate and the semiconductor substrate. A spacer material layer is deposited on the low-k dielectric layer, the second sidewall of the dummy gate, and the semiconductor substrate. The spacer material layer and the low-k dielectric layer are etched to form a first spacer structure on the first sidewall and a second spacer structure on the second sidewall. A drain doping region is formed in the semiconductor substrate adjacent to the first spacer structure. A source doping region is formed in the semiconductor substrate adjacent to the second spacer structure.