Method of forming semiconductor device

A method of forming a semiconductor device is disclosed. The method includes forming a plurality of isolation regions on a semiconductor substrate, forming a protective layer in a resistor region of the semiconductor substrate, after forming the protective layer, etching a gate dielectric layer to form first and second gate dielectric layers of a transistor in a transistor region of the semiconductor substrate, removing the protective layer, forming first and second dummy gate stacks over the first and second gate dielectric layers, respectively, forming a resistor in the resistor region, forming third and fourth dummy gate stacks over the resistor, and replacing each of the first, second, third, and fourth dummy gate stacks with a conductive material.